Question What is the difference between an Applet and an Application? (Applets) The differences between an applet and an application are as follows: 1. Applets can be embedded in HTML pages and downloaded over the Internet whereas Applications have no special support in HTML for embedding or downloading.

Answer

2. Applets can only be executed inside a java compatible container, such as a browser or appletviewer whereas Applications are executed at command line by java.exe or jview.exe. 3. Applets execute under strict security limitations that disallow certain operations (sandbox model security) whereas Applications have no inherent security restrictions. 4. Applets don't have the main() method as in applications. Instead they operate on an entirely different mechanism where they are initialized by init(),started by start(),stopped by stop() or destroyed by destroy().

Question What are the Applet's Life Cycle methods? Explain them? Following are methods in the life cycle of an Applet:

(Applets)

init() method - called when an applet is first loaded. This method is called only once in the entire cycle of an applet. This method usually intialize the variables to be used in the applet start() method - called each time an applet is started Answer

paint() method - called when the applet is minimized or refreshed. This method is used for drawing different strings, figures, and images on the applet window stop() method - called when the browser moves off the applet’s page destroy() method - called when the browser is finished with the applet

Question What is the sequence for calling the methods by AWT for applets? (Applets) Answer When an applet begins, the AWT calls the following methods, in this sequence: init() start() paint()

When an applet is terminated, the following sequence of method calls takes place stop() destroy() Question How are the differences between Applets and Applications? Application:

(Applets)

-Applications are Stand Alone and do not need web-browser. -Execution starts with main(). -May or may not be a GUI. Answer Applets -Needs no explicit installation on local machine. Can be transferred through Internet on to the local machine and may run as part of web-browser. -Execution starts with init() method. Must run within a GUI (Using AWT / Swing) Question How do we pass a parameter from HTML page to Applet? (Applets) [html] [applet code="Launch.class" archive="bak.jar" width=740 height=460] [param name="lang" value="English"] [/applet] Answer [/html] And within the init() of your applet, Use the following line to get the value. String langs = getParameter("lang");

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When an user wants to send an int from HTML what does he needs to do? (Applets) Either the user wants to send an int or a String, it will be the same. Here are steps he might do to get an int

[html] [applet code="Launch.class" archive="bak.jar" width=740 height=460] [param name="mynum" value="098765"] Answer [/applet] [/html] And within the init() of your applet, Use the following line to get the value. String mynum = getParameter("mynum"); int x = Integer.parseInt(mynum);

How can I arrange for different applets on a web page to communicate with each other? (Applets) Name your applets inside the Applet tag and invoke AppletContext’s getApplet() method in your applet code to obtain references to the other Answer applets on the page Question

Question How do I go from my applet to another JSP or HTML page? (Applets) Use AppletContext and invoke showDocument() on that context object. Below is sample code URL targetURL; String URLString = "http://localhost:8080/mypage.jsp"; AppletContext context = getAppletContext(); try Answer { targetURL = new URL(URLString); } catch (MalformedURLException e) { e.printStackTrace(); } context.showDocument(targetURL); Question How do I determine the width and height of my application? (Applets) Use the getSize() method, which the Applet class inherits from the Component class in the Java.awt package. The getSize() method returns the size of the applet as a Dimension object, from which you extract separate width, Answer height fields. The following code snippet explains this: Dimension dim = getSize(); int appletwidth = dim.width(); int appletheight = dim.height(); Question Which classes and interfaces does Applet class consist? (Applets) Applet class consists of a single class, the Applet class and three Answer interfaces: AppletContext, AppletStub, and AudioClip Question What is AppletStub Interface? (Applets) The applet stub interface provides the means by which an applet and the Answer browser communicate. Your code will not typically implement this interface What tags are mandatory when creating HTML to display an applet? (Applets) Answer code, height, width Question

Question What are the methods to retrive information about an applet?

(Applets)

getAppletInfo() : Returns a string describing the applet, its author, copyright information, etc. Answer getParameterInfo( ) method: Returns an array of string describing the applet’s parameters Question If you need to display a String on the applet, what would you do? (Applets) drawString() is used to output a String to an applet. This method is Answer included in the paint() of the Applet

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How do you play audio clips without using applet class? (Applets) You can do this using Java Media Framework. I dont have much context Answer on this API. Question which containers use a border Layout as their default layout? (AWT) The window, Frame and Dialog classes use a border layout as their Answer default layout Question What is the preferred size of a component? (AWT) The preferred size of a component is the minimum component size that Answer will allow the component to display normally Question Which containers use a FlowLayout as their default layout? (AWT) Answer The Panel and Applet classes use the FlowLayout as their default layout Question What is the immediate superclass of the Applet class? Answer Panel

(AWT)

Question Name three Component subclasses that support painting (AWT) Answer The Canvas, Frame, Panel, and Applet classes support painting Question What is the immediate superclass of the Dialog class? Answer Window

(AWT)

Question What is clipping? (AWT) Clipping is the process of confining paint operations to a limited area or Answer shape What is the difference between a MenuItem and a CheckboxMenuItem? (AWT) The CheckboxMenuItem class extends the MenuItem class to support a Answer menu item that may be checked or unchecked Question

Question What class is the top of the AWT event hierarchy?

(AWT)

Answer

The java.awt.AWTEvent class is the highest-level class in the AWT eventclass hierarchy

In which package are most of the AWT events that support the eventdelegation model defined? (AWT) Most of the AWT-related events of the event-delegation model are Answer defined in the java.awt.event package. The AWTEvent class is defined in the java.awt package. Question

Which class is the immediate superclass of the MenuComponent class (AWT) Answer Object Question

Question Which containers may have a MenuBar? Answer Frame

(AWT)

What is the relationship between the Canvas class and the Graphics class? (AWT) A Canvas object provides access to a Graphics object via its paint() Answer method. Question

Question How are the elements of a BorderLayout organized? (AWT) The elements of a BorderLayout are organized at the borders (North, Answer South, East, and West) and the center of a container. Question What is the difference between a Window and a Frame? (AWT) The Frame class extends Window to define a main application window Answer that can have a menu bar. Question What is the difference between the Font and FontMetrics classes? (AWT) The FontMetrics class is used to define implementation-specific properties, Answer such as ascent and descent, of a Font object. Question How are the elements of a CardLayout organized? (AWT) The elements of a CardLayout are stacked, one on top of the other, like a Answer deck of cards. Question What is the relationship between clipping and repainting? (AWT) When a window is repainted by the AWT painting thread, it sets the Answer clipping regions to the area of the window that requires repainting. What is the relationship between an event-listener interface and an event-adapter class? (AWT) Answer An event-listener interface defines the methods that must be implemented by an event handler for a particular kind of event. An event Question

adapter provides a default implementation of an event-listener interface. Question How can a GUI component handle its own events? (AWT) A component can handle its own events by implementing the required Answer event-listener interface and adding itself as its own event listener. Question How are the elements of a GridBagLayout organized? (AWT) The elements of a GridBagLayout are organized according to a grid. However, the elements are of different sizes and may occupy more than Answer one row or column of the grid. In addition, the rows and columns may have different sizes. What advantage do Java's layout managers provide over traditional windowing systems? (AWT) Java uses layout managers to lay out components in a consistent manner across all windowing platforms. Since Java's layout managers aren't tied to Answer absolute sizing and positioning, they are able to accomodate platformspecific differences among windowing systems. Question

Question What is the difference between the paint() and repaint() methods? (AWT) The paint() method supports painting via a Graphics object. The repaint() Answer method is used to cause paint() to be invoked by the AWT painting thread. Question How can the Checkbox class be used to create a radio button? Answer By associating Checkbox objects with a CheckboxGroup

(AWT)

Question What is the difference between a Choice and a List? (AWT) A Choice is displayed in a compact form that requires you to pull it down to see the list of available choices. Only one item may be selected from a Answer Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items. Question What interface is extended by AWT event listeners? (AWT) Answer All AWT event listeners extend the java.util.EventListener interface. Question What is a layout manager? (AWT) A layout manager is an object that is used to organize components in a Answer container Question Which Component subclass is used for drawing and painting? Answer Canvas

(AWT)

What are the problems faced by Java programmers who dont use layout managers? (AWT) Answer Without layout managers, Java programmers are faced with determining Question

how their GUI will be displayed across multiple windowing systems and finding a common sizing and positioning that will work within the constraints imposed by each windowing system Why variables/objects in java bean classes are declared as private? Question by Rajesh (Beans) Since you do not want users to access these objects directly and wanted Answer users to get access to these objects using getter and setter methods. Question

Question Can there be an abstract class with no abstract methods in it? Answer Yes Question Can an Interface be final? Answer No

(CoreJava)

(CoreJava)

Question Can an Interface have an inner class? (CoreJava) Yes public interface abc { static int i=0; void dd(); class a1 { a1() { int j; Answer System.out.println("in interfia"); }; public static void main(String a1[]) { System.out.println("in interfia"); } } } Can we define private and protected modifiers for variables in interfaces? (CoreJava) Answer No Question

Question What is Externalizable? (CoreJava) Externalizable is an Interface that extends Serializable Interface. And Answer sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in) Question What modifiers are allowed for methods in an Interface? (CoreJava) Answer Only public and abstract modifiers are allowed for methods in interfaces. Question What is a local, member and a class variable? (CoreJava) Variables declared within a method are "local" variables. Variables declared within the class i.e not within any methods are Answer "member" variables (global variables). Variables declared within the class i.e not within any methods and are defined as "static" are class variables I made my class Cloneable but I still get 'Can't access protected method clone. Why? (CoreJava) Answer Yeah, some of the Java books, in particular "The Java Programming Language", imply that all you have to do in order to have your class support clone() is implement the Cloneable interface. Not so. Perhaps that Question

was the intent at some point, but that's not the way it works currently. As it stands, you have to implement your own public clone() method, even if it doesn't do anything special and just calls super.clone(). Question What are the different identifier states of a Thread? The different identifiers of a Thread are: R - Running or runnable thread S - Suspended thread Answer CW - Thread waiting on a condition variable MW - Thread waiting on a monitor lock MS - Thread suspended waiting on a monitor lock

(CoreJava)

Question Can we overload main method in java? (CoreJava) Yes. But the main method with String args[] is called when you call the programme (java Test1) Have a look at this demo. public class Test1 { public static void main(String[] args) { System.out.println("in main method"); Answer } public static void main(String args[],String arg) { System.out.println("in overload method"); } } Question What are some alternatives to inheritance? (CoreJava) Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance Answer is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass). Question Why isn't there operator overloading? (CoreJava) Because C++ has proven by example that operator overloading makes code almost impossible to maintain. In fact there very nearly wasn't even method overloading in Java, but it was thought that this was too useful for Answer some very basic methods like print(). Note that some of the classes like DataOutputStream have unoverloaded methods like writeInt() and writeByte(). Question What does it mean that a method or field is "static"? (CoreJava) Answer Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that

variable changes for all instances of that class. Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work. out is a static field in the java.lang.System class. Question Diffrence between JRE And JVM AND JDK (CoreJava) JVM - Java Virtual Machine. This is where all java classes gets executed. JVM converts byte code to machine understandable code. JRE - Java Runtime Environment - Contains all core API class files that are Answer needed to execute a java application JDK - Java Development Kit - Contains tools to use java, e.g. javac, java, javaw, keytool, etc... and JRE (See above for description of JRE) Question Why do threads block on I/O? (CoreJava) Threads block on i/o (that is enters the waiting state) so that other Answer threads may execute while the i/o Operation is performed. Question What is synchronization and why is it important? (CoreJava) With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without Answer synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors. Question Is null a keyword? (CoreJava) Answer The null value is not a keyword. Which characters may be used as the second character of an identifier,but not as the first character of an identifier? (CoreJava) The digits 0 through 9 may not be used as the first character of an Answer identifier but they may be used after the first character of an identifier. Question

What modifiers may be used with an inner class that is a member of an outer class? (CoreJava) A (non-local) inner class may be declared as public, protected, private, Answer static, final, or abstract. Question

How many bits are used to represent Unicode, ASCII, UTF-16, and UTF-8 characters? (CoreJava) Unicode requires 16 bits and ASCII require 7 bits. Although the ASCII character set uses only 7 bits, it is usually represented as 8 bits. UTF-8 Answer represents characters using 8, 16, and 18 bit patterns. UTF-16 uses 16-bit and larger bit patterns. Question

Question What are wrapped classes?

(CoreJava)

Answer

Wrapped classes are classes that allow primitive types to be accessed as objects.

What restrictions are placed on the location of a package statement within a source code file? (CoreJava) A package statement must appear as the first line in a source code file Answer (excluding blank lines and comments). Question

What is the difference between preemptive scheduling and time slicing? (CoreJava) Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into Answer existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors. Question

Question What is a native method? (CoreJava) A native method is a method that is implemented in a language other Answer than Java. What are order of precedence and associativity, and how are they used? (CoreJava) Order of precedence determines the order in which operators are Answer evaluated in expressions. Associatity determines whether an expression is evaluated left-to-right or right-to-left Question

Question What is the catch or declare rule for method declarations? (CoreJava) If a checked exception may be thrown within the body of a method, the Answer method must either catch the exception or declare it in its throws clause. Can an anonymous class be declared as implementing an interface and extending a class? (CoreJava) An anonymous class may implement an interface or extend a superclass, Answer but may not be declared to do both. Question

Question What is the range of the char type? (CoreJava) Answer The range of the char type is 0 to 2^16 - 1. Question What is the purpose of finalization? (CoreJava) The purpose of finalization is to give an unreachable object the Answer opportunity to perform any cleanup processing before the object is garbage collected. Question What is the difference between the Boolean & operator and the &&

operator? (CoreJava) If an expression involving the Boolean & operator is evaluated, both operands are evaluated. Then the & operator is applied to the operand. When an expression involving the && operator is evaluated, the first Answer operand is evaluated. If the first operand returns a value of true then the second operand is evaluated. The && operator is then applied to the first and second operands. If the first operand evaluates to false, the evaluation of the second operand is skipped. How many times may an object's finalize() method be invoked by the garbage collector? (CoreJava) An object's finalize() method may only be invoked once by the garbage Answer collector. Question

What is the purpose of the finally clause of a try-catch-finally statement? (CoreJava) The finally clause is used to provide the capability to execute code no Answer matter whether or not an exception is thrown or caught. Question

Question What is the argument type of a program's main() method? (CoreJava) Answer A program's main() method takes an argument of the String[] type. Question Which Java operator is right associative? Answer The = operator is right associative.

(CoreJava)

Question Can a double value be cast to a byte? (CoreJava) Answer Yes, a double value can be cast to a byte. What is the difference between a break statement and a continue statement? (CoreJava) A break statement results in the termination of the statement to which it Answer applies (switch, for, do, or while). A continue statement is used to end the current loop iteration and return control to the loop statement.

Question

Question What must a class do to implement an interface? (CoreJava) It must provide all of the methods in the interface and identify the Answer interface in its implements clause. What is the advantage of the event-delegation model over the earlier event-inheritance model? (CoreJava) Answer The event-delegation model has two advantages over the eventinheritance model. First, it enables event handling to be handled by objects other than the ones that generate the events (or their containers). This allows a clean separation between a component's design and its use. The other advantage of the event-delegation model is that it performs much better in applications where many events are generated. This performance Question

improvement is due to the fact that the event-delegation model does not have to repeatedly process unhandled events, as is the case of the eventinheritance model. How are commas used in the intialization and iteration parts of a for statement? (CoreJava) Commas are used to separate multiple statements within the initialization Answer and iteration parts of a for statement. Question

Question What is an abstract method? (CoreJava) An abstract method is a method whose implementation is deferred to a Answer subclass. What value does read() return when it has reached the end of a file? (CoreJava) Answer The read() method returns -1 when it has reached the end of a file. Question

Question Can a Byte object be cast to a double value? (CoreJava) Answer No, an object cannot be cast to a primitive value. What is the difference between a static and a non-static inner class? (CoreJava) A non-static inner class may have object instances that are associated Answer with instances of the class's outer class. A static inner class does not have any object instances. Question

If a variable is declared as private, where may the variable be accessed? (CoreJava) A private variable may only be accessed within the class in which it is Answer declared. Question

Question What is an object's lock and which object's have locks? (CoreJava) An object's lock is a mechanism that is used by multiple threads to obtain synchronized access to the object. A thread may execute a synchronized Answer method of an object only after it has acquired the object's lock. All objects and classes have locks. A class's lock is acquired on the class's Class object. Question What is the % operator? (CoreJava) It is referred to as the modulo or remainder operator. It returns the Answer remainder of dividing the first operand by the second operand. Question When can an object reference be cast to an interface reference? (CoreJava) An object reference be cast to an interface reference when the object Answer implements the referenced interface.

Question Which class is extended by all other classes? (CoreJava) Answer The Object class is extended by all other classes. Question Can an object be garbage collected while it is still reachable? (CoreJava) A reachable object cannot be garbage collected. Only unreachable objects Answer may be garbage collected. Question Is the ternary operator written x : y ? z or x ? y : z ? Answer It is written x ? y : z.

(CoreJava)

Question How is rounding performed under integer division? (CoreJava) The fractional part of the result is truncated. This is known as rounding Answer toward zero. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream class hierarchy? (CoreJava) The Reader/Writer class hierarchy is character-oriented, and the Answer InputStream/OutputStream class hierarchy is byte-oriented. Question

Question What classes of exceptions may be caught by a catch clause? (CoreJava) A catch clause can catch any exception that may be assigned to the Answer Throwable type. This includes the Error and Exception types. If a class is declared without any access modifiers, where may the class be accessed? (CoreJava) A class that is declared without any access modifiers is said to have Answer package access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package. Question

Question Does a class inherit the constructors of its superclass? (CoreJava) Answer A class does not inherit constructors from any of its superclasses. Question What is the purpose of the System class? (CoreJava) Answer The purpose of the System class is to provide access to system resources. Question Name the eight primitive Java types. (CoreJava) The eight primitive types are byte, char, short, int, long, float, double, Answer and boolean. Which class should you use to obtain design information about an object? (CoreJava) Answer The Class class is used to obtain information about an object's design. Question

Question Is "abc" a primitive value? (CoreJava) Answer The String literal "abc" is not a primitive value. It is a String object. What restrictions are placed on the values of each case of a switch statement? (CoreJava) During compilation, the values of each case of a switch statement must Answer evaluate to a value that can be promoted to an int value. Question

Question What modifiers may be used with an interface declaration? Answer An interface may be declared as public or abstract. Question Is a class a subclass of itself? Answer A class is a subclass of itself.

(CoreJava)

(CoreJava)

What is the difference between a while statement and a do statement? (CoreJava) A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a Answer loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once. Question

Question What modifiers can be used with a local inner class? Answer A local inner class may be final or abstract.

(CoreJava)

Question What is the purpose of the File class? (CoreJava) The File class is used to create objects that provide access to the files and Answer directories of a local file system. Question Can an exception be rethrown? (CoreJava) Answer Yes, an exception can be rethrown. Question When does the compiler supply a default constructor for a class? The compiler supplies a default constructor for a class if no other Answer constructors are provided.

(CoreJava)

If a method is declared as protected, where may the method be accessed? (CoreJava) A protected method may only be accessed by classes or interfaces of the Answer same package or by subclasses of the class in which it is declared. Question

Which non-Unicode letter characters may be used as the first character of an identifier? (CoreJava) The non-Unicode letter characters $ and _ may appear as the first Answer character of an identifier Question

Question What restrictions are placed on method overloading? (CoreJava) Two methods may not have the same name and argument list but Answer different return types. Question What is casting? (CoreJava) There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is Answer used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference. Question What is the return type of a program's main() method? Answer A program's main() method has a void return type.

(CoreJava)

What class of exceptions are generated by the Java run-time system? (CoreJava) The Java runtime system generates RuntimeException and Error Answer exceptions. Question

Question What class allows you to read objects directly from a stream? (CoreJava) The ObjectInputStream class supports the reading of objects from input Answer streams. What is the difference between a field variable and a local variable? (CoreJava) A field variable is a variable that is declared as a member of a class. A Answer local variable is a variable that is declared local to a method. Question

Question How are this() and super() used with constructors? (CoreJava) Discuss in Detail this() is used to invoke a constructor of the same class. super() is used to Answer invoke a superclass constructor. What is the relationship between a method's throws clause and the exceptions that can be thrown during the method's execution? (CoreJava) A method's throws clause must declare any checked exceptions that are Answer not caught within the body of the method. Question

Question Why are the methods of the Math class static? (CoreJava) Answer So they can be invoked as if they are a mathematical code library. Question What are the legal operands of the instanceof operator? (CoreJava) The left operand is an object reference or null value and the right Answer operand is a class, interface, or array type.

Question What an I/O filter? (CoreJava) An I/O filter is an object that reads from one stream and writes to Answer another, usually altering the data in some way as it is passed from one stream to another. Question If an object is garbage collected, can it become reachable again? (CoreJava) Once an object is garbage collected, it ceases to exist. It can no longer Answer become reachable again. Question What are E and PI? (CoreJava) Answer E is the base of the natural logarithm and PI is mathematical value pi. Question Are true and false keywords? (CoreJava) Answer The values true and false are not keywords. What is the difference between the File and RandomAccessFile classes? (CoreJava) The File class encapsulates the files and directories of the local file Answer system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file. Question

Question What happens when you add a double value to a String? Answer The result is a String object.

(CoreJava)

Question What is your platform's default character encoding? (CoreJava) If you are running Java on English Windows platforms, it is probably Answer Cp1252. If you are running Java on English Solaris platforms, it is most likely 8859_1. Question Which package is always imported by default? (CoreJava) Answer The java.lang package is always imported by default. What interface must an object implement before it can be written to a stream as an object? (CoreJava) An object must implement the Serializable or Externalizable interface Answer before it can be written to a stream as an object. Question

Question Whats the difference between notify() and notifyAll()? (CoreJava) notify() is used to unblock one waiting thread; notifyAll() is used to unblock all of them. Using notify() is preferable (for efficiency) when only one blocked thread can benefit from the change (for example, when Answer freeing a buffer back into a pool). notifyAll() is necessary (for correctness) if multiple threads should resume (for example, when releasing a "writer" lock on a file might permit all "readers" to resume).

Why can't I say just abs() or sin() instead of Math.abs() and Math.sin()? (CoreJava) The import statement does not bring methods into your local name space. It lets you abbreviate class names, but not get rid of them altogether. That's just the way it works, you'll get used to it. It's really a lot safer this way.
However, there is actually a little trick you can use in some cases that gets you what you want. If your top-level class doesn't Answer need to inherit from anything else, make it inherit from java.lang.Math. That *does* bring all the methods into your local name space. But you can't use this trick in an applet, because you have to inherit from java.awt.Applet. And actually, you can't use it on java.lang.Math at all, because Math is a "final" class which means it can't be extended. Question

Question Wha is the output from System.out.println("Hello"+null); Answer Hellonull

(CoreJava)

Question Why are there no global variables in Java? (CoreJava) Global variables are considered bad form for a variety of reasons: · Adding state variables breaks referential transparency (you no longer can understand a statement or expression on its own: you need to understand it in the context of the settings of the global variables). · State variables lessen the cohesion of a program: you need to know more to understand how something works. A major point of Object-Oriented Answer programming is to break up global state into more easily understood collections of local state. · When you add one variable, you limit the use of your program to one instance. What you thought was global, someone else might think of as local: they may want to run two copies of your program at once. For these reasons, Java decided to ban global variables. Question What does it mean that a class or member is final? (CoreJava) A final class can no longer be subclassed. Mostly this is done for security reasons with basic classes like String and Integer. It also allows the compiler to make some optimizations, and makes thread safety a little easier to achieve. Methods may be declared final as well. This means they may not be overridden in a subclass. Answer Fields can be declared final, too. However, this has a completely different meaning. A final field cannot be changed after it's initialized, and it must include an initializer statement where it's declared. For example, public final double c = 2.998; It's also possible to make a static field final to get the effect of C++'s const statement or some uses of C's #define, e.g. public static final double c = 2.998; Question What does it mean that a method or class is abstract? (CoreJava) Answer An abstract class cannot be instantiated. Only its subclasses can be instantiated. You indicate that a class is abstract with the abstract keyword like this:

public abstract class Container extends Component { Abstract classes may contain abstract methods. A method declared abstract is not actually implemented in the current class. It exists only to be overridden in subclasses. It has no body. For example, public abstract float price(); Abstract methods may only be included in abstract classes. However, an abstract class is not required to have any abstract methods, though most of them do. Each subclass of an abstract class must override the abstract methods of its superclasses or itself be declared abstract. Question what is a transient variable? (CoreJava) Answer transient variable is a variable that may not be serialized. Question How are Observer and Observable used? (CoreJava) Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of Answer each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. Question Can a lock be acquired on a class? (CoreJava) Yes, a lock can be acquired on a class. This lock is acquired on the class's Answer Class object. What state does a thread enter when it terminates its processing? (CoreJava) Answer When a thread terminates its processing, it enters the dead state. Question

Question How does Java handle integer overflows and underflows? (CoreJava) It uses those low order bytes of the result that can fit into the size of the Answer type allowed by the operation. Question What is the difference between the >> and >>> operators? (CoreJava) The >> operator carries the sign bit when shifting right. The >>> zeroAnswer fills bits that have been shifted out. Question Is sizeof a keyword? (CoreJava) Answer The sizeof operator is not a keyword. Does garbage collection guarantee that a program will not run out of memory? (CoreJava) Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster Answer than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection Question

Can an object's finalize() method be invoked while it is reachable? (CoreJava) An object's finalize() method cannot be invoked by the garbage collector Answer while the object is still reachable. However, an object's finalize() method may be invoked by other objects. Question

What value does readLine() return when it has reached the end of a file? (CoreJava) Answer The readLine() method returns null when it has reached the end of a file. Question

Question Can a for statement loop indefinitely? (CoreJava) Yes, a for statement can loop indefinitely. For example, consider the Answer following: for(;;) ; To what value is a variable of the String type automatically initialized? (CoreJava) Answer The default value of an String type is null.

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Question What is a task's priority and how is it used in scheduling? (CoreJava) A task's priority is an integer value that identifies the relative order in Answer which it should be executed with respect to other tasks. The scheduler attempts to schedule higher priority tasks before lower priority tasks. Question What is the range of the short type? (CoreJava) Answer The range of the short type is -(2^15) to 2^15 - 1. Question What is the purpose of garbage collection? (CoreJava) The purpose of garbage collection is to identify and discard objects that Answer are no longer needed by a program so that their resources may be reclaimed and reused. Question What do you understand by private, protected and public? (CoreJava) These are accessibility modifiers. Private is the most restrictive, while public is the least restrictive. There is no real difference between protected Answer and the default type (also known as package protected) within the context of the same package, however the protected keyword allows visibility to a derived class in a different package. Question What is Downcasting ? (CoreJava) Downcasting is the casting from a general to a more specific type, i.e. Answer casting down the hierarchy

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Can a method be overloaded based on different return type but same argument type ? (CoreJava)

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No, because the methods can be called without using their return type in which case there is ambiquity for the compiler

What happens to a static var that is defined within a method of a class ? (CoreJava) Answer Can't do it. You'll get a compilation error Question

Question How many static init can you have ? (CoreJava) As many as you want, but the static initializers and class variable initializers are executed in textual order and may not refer to class Answer variables declared in the class whose declarations appear textually after the use, even though these class variables are in scope. What is the difference amongst JVM Spec, JVM Implementation, JVM Runtime ? (CoreJava) The JVM spec is the blueprint for the JVM generated and owned by Sun. The JVM implementation is the actual implementation of the spec by a Answer vendor and the JVM runtime is the actual running instance of a JVM implementation Question

Question Describe what happens when an object is created in Java? (CoreJava) Several things happen in a particular order to ensure the object is constructed properly: 1. Memory is allocated from heap to hold all instance variables and implementation-specific data of the object and its superclasses. Implemenation-specific data includes pointers to class and method data. 2. The instance variables of the objects are initialized to their default values. Answer 3. The constructor for the most derived class is invoked. The first thing a constructor does is call the consctructor for its superclasses. This process continues until the constrcutor for java.lang.Object is called, as java.lang.Object is the base class for all objects in java. 4. Before the body of the constructor is executed, all instance variable initializers and initialization blocks are executed. Then the body of the constructor is executed. Thus, the constructor for the base class completes first and constructor for the most derived class completes last. What does the "final" keyword mean in front of a variable? A method? A class? (CoreJava) FINAL for a variable : value is constant Answer FINAL for a method : cannot be overridden FINAL for a class : cannot be derived Question

Question What is the difference between instanceof and isInstance? (CoreJava) Answer instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class exception. isInstance() Determines if the specified Object is assignment-compatible with the

object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator. The method returns true if the specified Object argument is non-null and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise. Question Why is not recommended to have instance variables in Interface (CoreJava) By Default, All data members and methods in an Interface are public. Answer Having public variables in a class that will be implementing it will be violation of the Encapsulation principles Question What is the diffrence between inner class and nested class? (CoreJava) When a class is defined within a scope od another class, then it becomes inner class. Answer If the access modifier of the inner class is static, then it becomes nested class.

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How the private constructor is called in the main java programme? (CoreJava) Have a look at this demo. public class Test2 { private Test2() { System.out.println("Test2class"); }

class Subclass extends Test2 { public Subclass() { Answer System.out.println("Subclass"); } } public static void main(String[] args) { Subclass s = new Test2().new Subclass(); } } This works because an inner class is allowed to access private members of its enclosing instance, including the private constructor. Which is garbage collected first: Normal variables or static variables? (CoreJava) Answer Normal variables will be collected first. Lets take a simple example: Class A is having a static variable s which is used by obj1, obj2 and obj3 of Class B. Each object of class B is having instance variables a and b (normal variables). Lets say if obj1 is not being in use since long time, then automatically the garbage collector will collect the space occupied by obj1. Question

It will not destroy the static variable S as it is being used by the other two objects obj2 and obj3. Therefore only normal variables will be destroyed first. We can say it in a simple statement that "Variables having less scope will be destroyed first" Question Diffrence between JRE And JVM AND JDK (CoreJava) (CoreJava) The "JDK" is the Java Development Kit. I.e., the JDK is bundle of software that you can use to develop Java based software. The "JRE" is the Java Runtime Environment. I.e., the JRE is an implementation of the Java Answer Virtual Machine which actually executes Java programs. Typically, each JDK contains one (or more) JRE's along with the various development tools like the Java source compilers, bundling and deployment tools, debuggers, development libraries, etc. is it necessary to initialize a final variable at the time of declaretion ? (CoreJava) NO, it's not necessary. Many text books say like this but thats not true. Value of a final variable can be instance specific also, but in this case we have to initialise the variable in all the constructors. If we want to have a common final value of a variable for all the instances Answer then there are two ways. 1. Initialise the variable at class level (at the time of declaration) or 2. just declare variable at class level and initialise it in any one of the instance blocks i.e. A. class A { final int a; {a=5;}} B. class A { final int a = 5;} Question

Question What is a compilation unit? (CoreJava) Answer A compilation unit is a Java source code file. Question What restrictions are placed on method overriding? (CoreJava) Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. Answer The overriding method may not throw any exceptions that may not be thrown by the overridden method. Question How can a dead thread be restarted? Answer A dead thread cannot be restarted.

(CoreJava)

Question What happens if an exception is not caught? (CoreJava) An uncaught exception results in the uncaughtException() method of the Answer thread's ThreadGroup being invoked, which eventually results in the termination of the program in which it is thrown.

Which arithmetic operations can result in the throwing of an ArithmeticException? (CoreJava) Answer Integer / and % can result in the throwing of an ArithmeticException Question

Question Can an abstract class be final? (CoreJava) Answer An abstract class may not be declared as final What happens if a try-catch-finally statement does not have a catch Question clause to handle an exception that is thrown within the body of the try statement? (CoreJava) The exception propagates up to the next higher level try-catch statement Answer (if any) or results in the program's termination Question What is numeric promotion? (CoreJava) Numeric promotion is the conversion of a smaller numeric type to a larger numeric type, so that integer and floating-point operations may take place. Answer In numerical promotion, byte, char, and short values are converted to int values. The int values are also converted to long values, if necessary. The long and float values are converted to double values, as required Question What is the difference between a public and a non-public class? (CoreJava) A public class may be accessed outside of its package. A non-public class Answer may not be accessed outside of its package. To what value is a variable of the boolean type automatically initialized? (CoreJava) Answer The default value of the boolean type is false Question

Question Can try statements be nested? Answer Yes

(CoreJava)

What is the difference between the prefix and postfix forms of the ++ operator? (CoreJava) The prefix form performs the increment operation and returns the value of the increment operation. The postfix form returns the current value all Answer of the expression and then performs the increment operation on that value. Question

Question What is the purpose of a statement block? (CoreJava) A statement block is used to organize a sequence of statements as a Answer single statement group Question What is a Java package and how is it used? (CoreJava) Answer A Java package is a naming context for classes and interfaces. A package is used to create a separate name space for groups of classes and

interfaces. Packages are also used to organize related classes and interfaces into a single API unit and to control accessibility to these classes and interfaces. Question What modifiers may be used with a top-level class? Answer A top-level class may be public, abstract, or final.

(CoreJava)

Question What are the Object and Class classes used for? (CoreJava) The Object class is the highest-level class in the Java class hierarchy. The Answer Class class is used to represent the classes and interfaces that are loaded by a Java program. How does a try statement determine which catch clause should be used to handle an exception? (CoreJava) When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they Answer appear. The first catch clause that is capable of handling the exception is executed. The remaining catch clauses are ignored. Question

Question What are synchronized methods and synchronized statements? (CoreJava) Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are Answer similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement. What is the difference between an if statement and a switch statement? (CoreJava) The if statement is used to select among two alternatives. It uses a boolean expression to decide which alternative should be executed. The Answer switch statement is used to select among multiple alternatives. It uses an int expression to determine which alternative should be executed. Question

Question What gives java it's "write once and run anywhere" nature? (CoreJava) Java is compiled to be a byte code which is the intermediate language between source code and machine code. This byte code is not platorm specific and hence can be fed to any platform. After being fed to the JVM, Answer which is specific to a particular operating system, the code platform specific machine code is generated thus making java platform independent. Question What are the four corner stones of OOP ? (CoreJava) Answer Abstraction, Encapsulation, Polymorphism and Inheritance Question Difference between a Class and an Object ?

(CoreJava)

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A class is a definition or prototype whereas an object is an instance or living representation of the prototype

What is the difference between method overriding and overloading? (CoreJava) Overriding is a method with the same name and arguments as in a Answer parent, whereas overloading is the same method name but different arguments Question

Question What is a "stateless" protocol ? (CoreJava) Without getting into lengthy debates, it is generally accepted that Answer protocols like HTTP are stateless i.e. there is no retention of state between a transaction which is a single request response combination Question What is constructor chaining and how is it achieved in Java ? (CoreJava) A child object constructor always first needs to construct its parent (which Answer in turn calls its parent constructor.). In Java it is done via an implicit call to the no-args constructor as the first statement. Question What is passed by ref and what by value ? (CoreJava) All Java method arguments are passed by value. However, Java does Answer manipulate objects by reference, and all object variables themselves are references You can create a String object as String str = "abc"; Why cant a button object be created as Button bt = "abc";? Explain (CoreJava) The main reason you cannot create a button by Button bt1= "abc"; is because "abc" is a literal string (something slightly different than a String object, by-the-way) and bt1 is a Button object. The only object in Java that Answer can be assigned a literal String is java.lang.String. Important to note that you are NOT calling a java.lang.String constuctor when you type String s = "abc"; Question

What does the "abstract" keyword mean in front of a method? A class? (CoreJava) Abstract keyword declares either a method or a class. If a method has a abstract keyword in front of it,it is called abstract method.Abstract method hs no body.It has only arguments and return type.Abstract methods act as Answer placeholder methods that are implemented in the subclasses. Abstract classes can't be instantiated.If a class is declared as abstract,no objects of that class can be created.If a class contains any abstract method it must be declared as abstract Question

How many methods do u implement if implement the Serializable Interface? (CoreJava) Answer The Serializable interface is just a "marker" interface, with no methods of its own to implement. Other 'marker' interfaces are Question

java.rmi.Remote java.util.EventListener What are the practical benefits, if any, of importing a specific class rather Question than an entire package (e.g. import java.net.* versus import java.net.Socket)? (CoreJava) It makes no difference in the generated class files since only the classes that are actually used are referenced by the generated class file. There is another practical benefit to importing single classes, and this arises when two (or more) packages have classes with the same name. Take java.util.Timer and javax.swing.Timer, for example. If I import java.util.* and javax.swing.* and then try to use "Timer", I get an error while Answer compiling (the class name is ambiguous between both packages). Let's say what you really wanted was the javax.swing.Timer class, and the only classes you plan on using in java.util are Collection and HashMap. In this case, some people will prefer to import java.util.Collection and import java.util.HashMap instead of importing java.util.*. This will now allow them to use Timer, Collection, HashMap, and other javax.swing classes without using fully qualified class names in. What is the difference between logical data independence and physical data independence? (CoreJava) Logical Data Independence - meaning immunity of external schemas to Answer changeds in conceptual schema. Physical Data Independence - meaning immunity of conceptual schema to changes in the internal schema.

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Question What is user defined exception ? (CoreJava) Apart from the exceptions already defined in Java package libraries, user Answer can define his own exception classes by extending Exception class. Question Difference Between Abstraction and Encapsulation (CoreJava) Abstraction is removing some distinctions between objects, so as to show their commonalities. Answer Encapsulation is hiding the details of the implementation of an object so that there are no external dependencies on the particular implementation. Question What are Checked and Un-Checked Exceptions? Explain. (CoreJava) Answer Throwable extends Object (checked) Exception extends Throwable (checked) RuntimeException extends Exception (un-checked) Error extends Throwable (un-checked) So anything that extends Throwable or Exception (except RuntimeException) will be checked. Anything that extends Error or RuntimeException will be un-checked Checked exceptions are problems that arise in correct code and may be due to technical problems such as IO problems or user mistakes such as opening a socket when the remote machine does not exist. Because these problems can occur at anytime, say due to network outage, you must have code that can handle and recover from these. In fact, the Java compiler

checks that you have trapped them, hence checked exceptions. Runtime exceptions are typically bugs in the program. Errors are severe problems such as out of memory and sufficiently rare, that you are not required to handle them as they are usually unrecoverable. Question Can we sort an Hashtable? (CoreJava) Yes. Here is an example Answer http://www.javagalaxy.com:8080/CoreJava/View.jsp?slno=90&tbl=0 What is the exact difference between Abstract classes and Interfaces? (CoreJava) Interfaces provide a form of multiple inheritance -- any number of interfaces can be implemented A class can extend only one other class. Answer Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc. Question

Question Garbage collector thread belongs to which priority? (CoreJava) Answer Garbage collector thread belongs to low priority (MIN_PRIORITY) Can you throw (and re-throw) an exception inside a catch{} clause? (CoreJava) Yes, It will cause the exception to be passed to the handlers at the nextAnswer higher level. All further catch clauses are ignored in the current try block. Question

Question What is the difference between ArrayList and Vector Answer Methods in Vectors are synchronized

(CoreJava)

Question Can we have inner class in the interface ? (CoreJava) Answer Yes.We can have inner class in the interface. Question How to get values from a Vector (CoreJava) Answer Vector v = new Vector(10,2) v.add(3); v.add(4); int i= v.get(1);

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What is meant by Instance Variables and Class Variables

(CoreJava)

instance variables Any item of data that is associated with a particular object. Each instance of a class has its own copy of the instance variables defined in the class. Also called a field. class variables A data item Answer associated with a particular class as a whole--not with particular instances of the class. Class variables are defined in class definitions. Also called a static field Question What is difference between jsp and Servlet? (CoreJava) Answer THe main diff between jsp and servlet is code and content presentation in

jsp, it is not possible in servlet. Does the code in finally block get executed if there is an exception and a return statement in a catch block? (CoreJava) If an exception occurs and there is a return statement in catch block, the finally block is still executed. The finally block will not be executed when Answer the System.exit(1) statement is executed earlier or the system shut down earlier or the memory is used up earlier before the thread goes to finally block. Question

Question What are the Object and Class classes used for? (CoreJava) The Object class is the highest-level class in the Java class hierarchy. The Answer Class class is used to represent the classes and interfaces that are loaded by a Java program. What interface must an object implement before it can be written to a stream as an object? (CoreJava) An object must implement the Serializable or Externalizable interface Answer before it can be written to a stream as an object. Question

Question What is an I/O filter? (CoreJava) An I/O filter is an object that reads from one stream and writes to Answer another, usually altering the data in some way as it is passed from one stream to another. Question What class allows you to read objects directly from a stream? (CoreJava) The ObjectInputStream class supports the reading of objects from input Answer streams. If a class is declared without any access modifiers, where may the class be accessed? (CoreJava) A class that is declared without any access modifiers is said to have Answer package or friendly access. This means that the class can only be accessed by other classes and interfaces that are defined within the same package. Question

If class A does not implement Serializable but a subclass B implements Question Serializable, will the fields of class A be serialized when B is serialized? (CoreJava) Only the fields of Serializable objects are written out and restored. The object may be restored only if it has a no-arg constructor that will initialize Answer the fields of non-serializable supertypes. If the subclass has access to the state of the superclass it can implement writeObject and readObject to save and restore that state. Question When a Serializable object is written with writeObject, then modified and written a second time, why is the modification missing when the stream is

deserialized? (CoreJava) The ObjectOutputStream class keeps track of each object it serializes and sends only the handle if the object is written into the stream a subsequent time. This is the way it deals with graphs of objects. The corresponding ObjectInputStream keeps track of all of the objects it has created and their handles so when the handle is seen again it can return the same object. Answer Both output and input streams keep this state until they are freed. Alternatively, the ObjectOutputStream class implements a reset method that discards the memory of having sent an objecct, so sending an object again will make a copy. Question How do I get the serialVersionUID of a class? (CoreJava) Run the serialver tool, supplying the name of the class, as shown in the example that follows: Answer serialver java.lang.String

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The object serialization classes are stream oriented. How do I write objects to a random access file? (CoreJava) Currently there is no direct way to write objects to a random access file.

You can use the ByteArray I/O streams as an intermediate place to write and read bytes to/from the random access file and create Object I/O streams from the byte streams to write/read the objects. You just have to make sure that you have the entire object in the byte stream or Answer reading/writing the object will fail. For example, java.io.ByteArrayOutputStream can be used to receive the bytes of ObjectOutputStream. From it you can get a byte[] of the result which, in turn, can be used with ByteArrayInputStream as input to ObjectInput. Can I compress the serial representation of my objects using my own zip/ unzip methods? (CoreJava) ObjectOutputStream produces an OutputStream. If your zip object Answer extends the OutputStream class, there is no problem compressing it. Question

When a local object is serialized and passed as a parameter in an RMI call, are the byte codes for the local object's methods also passed? What Question about object coherency, if the remote VM application "keeps" the object handle? (CoreJava) Answer The bytecodes for a local object's methods are not passed directly in the ObjectOutputStream, but the object's class may need to be loaded by the receiver if the class is not already available locally. (The class files themselves are not serialized, just the names of the classes.) All classes must be able to be loaded during deserialization using the normal class loading mechanisms. For applets this means they are loaded by the AppletClassLoader.

There are no conherency guarantees for local objects passed to a remote VM, since such objects are passed by copying their contents (a true passby-value). Question Explain about Singleton Class (CoreJava) In general, you use a singleton to enforce the notion that there will be only one instance of a given class. Singletons should be used in situations where creating more than one of something would be a logical error. For example, a ConnectionPool would be a good place to use a singleton. If clients could arbitrarily create ConnectionPools without regard to what already exists, you would have a waste of resources. So you limit the possible number of connection pools to 1 (per JVM), and you then know that all clients are getting their connections from a single source. Answer Another example of Singleton use is for Object Factories. Say you have a class called FooFactory that is responsible for fetching/saving Foo objects to/from a database. You want to ensure that for each Foo record in the db, there is only one corresponding Foo object floating around your application. By centralizing all the creation logic in a single class, and making that class a Singleton, you eliminate the possibility fo duplicate objects. The code that uses a connection obtained from the connection pool is another matter. If all it does is do a getData() type operation, there is no harm in having more than one of them.

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Strings are immutable, How are we able to perform concatination on String object? (CoreJava) Yes. Strings are immutable. Thats why while concatenating, it always returns a new string object. If we take this example : String s1 = "psn"; s1 = s1.concat("prasad"); // Here you are reassigning the new object to the older reference s1 System.out.println(s1); String s1 = "psn"; String s2 = s1.concat("prasad"); System.out.println(s1); // will remain same . no change. it prints "psn" only System.out.println(s2); // as you have assigned the newly created object to s2

Question It is valid to declare an inherited method as abstract? (CoreJava) Yes,It is valid. However, there is no way to get to behaviour which is Answer located above the abstract method in the hierarchy. In effect, you will block access to parent methods further up the hierarchy.

Question what is j2EE? (CoreJava) J2EE menas Java 2 Enterprise Edition.. it follows certain rules and regulations to develop web technologies. and certain technologies are Answer under comes i this category like EJB/JSP/Servlets/JMS/Web Servers/App Servers/Struts and Oracle-9.0i Question Can an Interface have an inner class? (CoreJava) Yes. Interface can have an inner class. The possible use of this is to provide multiple inheritance in java. A class "abc1" can extend normal Answer class "abc2" and can implement an Interface "abc" having an inner class "abc3". This way class "abc1" can get the functionality of both "abc2" and "abc3" classes. Question what is tunnelling? (CoreJava) Tunnelling is a route to somewhere. For example, RMI tunnelling is a way Answer to make RMI application get through firewall. In CS world, tunnelling means a way to transfer data. What is the difference between the File and RandomAccessFile classes? (CoreJava) The File class encapsulates the files and directories of the local file Answer system. The RandomAccessFile class provides the methods needed to directly access data contained in any part of a file. Question

Question How are this() and super() used with constructors? (CoreJava) this() is used to invoke a constructor of the same class. super() is used to Answer invoke a superclass constructor Question What is casting? (CoreJava) There are two types of casting, casting between primitive numeric types and casting between object references. Casting between numeric types is Answer used to convert larger values, such as double values, to smaller values, such as byte values. Casting between object references is used to refer to an object by a compatible class, interface, or array type reference. Question What is the purpose of the Runtime class? (CoreJava) The purpose of the Runtime class is to provide access to the Java runtime Answer system Question Does object serialization support encryption? (CoreJava) Object serialization does not contain any encryption/decryption in itself. It writes to and reads from Java Streams, so it can be coupled with any Answer available encryption technology. Object serialization can be used in many different ways from simple persistence, writing and read to/from files, or for RMI to communicate across hosts.

Why is OutOfMemoryError thrown after writing a large number of objects into an ObjectOutputStream? (CoreJava) The ObjectOutputStream maintains a table mapping objects written into the stream to a handle. The first time an object is written to a stream, its contents are written into the stream; subsequent writes of the object result in a handle to the object being written into the stream. This table Answer maintains references to objects that might otherwise be unreachable by an application, thus, resulting in an unexpected situation of running out of memory. A call to the ObjectOutputStream.reset() method resets the object/handle table to its initial state, allowing all previously written objects to be elgible for garbage collection. Question

Why is UTFDataFormatException thrown by DataOutputStream.writeUTF() when serializing a String? (CoreJava) DataOutputStream.writeUTF() does not support writing out strings larger than 64K. The first two bytes of a UTF string in the stream are the length Answer of the string. If a java.lang.String can be larger than 64K, it needs to be stored in the stream by an alternative method rather than depending on the default method of storing a String in the stream, writeUTF. Question

How can I create an ObjectInputStream from an ObjectOutputStream without a file in between? (CoreJava) ObjectOutputStream and ObjectInputStream work to/from any stream object. You could use a ByteArrayOutputStream and then get the array and insert it into a ByteArrayInputStream. You could also use the piped stream classes as well. Any java.io class that extends the OutputStream Answer and InputStream classes can be used. Question

Alternatively, the ObjectOutputStream> class implements a reset method that discards the memory of having sent an object, so sending an object again will make a copy. Why can't a file that contains multiple appended ObjectOutputStreams be deserialized by one ObjectInputStream? (CoreJava) Using the default implementation of serialization, there must be a one-toone mapping between ObjectOutputStream construction and ObjectInputStream construction. ObjectOutputStream constructor writes a stream header andObjectInputStream reads this stream header. A Answer workaround is to subclass ObjectOutputStream and override writeStreamHeader(). The overriding writeStreamHeader() should call the super writeStreamHeader method if it is the first write to the file and it should call ObjectOutputStream.reset() if it is appending to a pre-existing ObjectOutputStream within the file. Question

Question What is the difference between Shallow Copy and Deep Copy? (CoreJava) Answer Shallow Copy: If a shallow copy is performed on an object, then it gets copied but its contained objects are not copied. Also any changes made in the cloned object is automatically reflected in the shallowed copy object as well. An

example class Student implements Cloneable { public String name; public String age; public Student(String name,String address) { this.name = name; this.age = age; } public Object clone() throws java.lang.CloneNotSupportedException { return this; } } public class ShallowCloneClient { public ShallowCloneClient() throws java.lang.CloneNotSupportedException { Student st1 = new Student("guddu","22,nagar road"); Student st2 = (Student)st1.clone(); st2.name="new name"; System.out.println(st1.name); } public static void main(String args[]) throws java.lang.CloneNotSupportedException { new ShallowCloneClient(); } } When you execute the programme, the output will be "new name", this shows that both st1 and st2 instances are the same, changing one changes other too. Deep Copy: A deep copy occurs when an object is copied along with the objects to which it refers to are also copied. This occurs only when every object in the tree is serializable. An example import java.io.*; class Student1 implements Cloneable,Serializable { public String name; public String age; public Student1(String name,String address) {

this.name = name; this.age = age; } public Object clone() throws java.lang.CloneNotSupportedException { try{ ByteArrayOutputStream byteArr = new ByteArrayOutputStream(); ObjectOutputStream objOut = new ObjectOutputStream(byteArr); objOut.writeObject(this); ByteArrayInputStream byteArrIn = new ByteArrayInputStream(byteArr.toByteArray()); ObjectInputStream objIn = new ObjectInputStream(byteArrIn); return objIn.readObject(); }catch(Exception ex){ ex.printStackTrace(); return null; } } } public class ShallowCloneClient1 { public ShallowCloneClient1() throws java.lang.CloneNotSupportedException { Student1 st1 = new Student1("guddu","22,nagar road"); Student1 st2 = (Student1)st1.clone(); st2.name="new name"; System.out.println(st1.name); } public static void main(String args[]) throws java.lang.CloneNotSupportedException { new ShallowCloneClient1(); } } When you execute the programme, the output will be "guddu", this shows that st1 and st2 instances are different. Question Can an abstract class have final method? Answer Yes

(CoreJava)

Question Can an abstract class have only final methods? Answer Yes

(CoreJava)

Question Can a final class have abstract method? (CoreJava) Answer No. It gives compile time error saying that the class is not abstract. Question Can you compare if null is equal to null? i.e.

String str = null; String str1 = null; if(str == str1) SOP("true"); (CoreJava)

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Yes, And its going to be true as the objects have not been allocated any memory location. Post-increment or Post-Decrement? Which would give better performance? e.g. for(int x=0; x < 10000; x++) SOP("JG"); for(int y>0; y < 10000; y--) SOP("JG"); (CoreJava)

Post-Decrement would give better performance. This is due to the binary Answer subtraction (1's complement/ 2's complement) Question What's the difference between a queue and a stack? (CoreJava) Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO Answer rule. Can an inner class declared inside of a method access local variables of this method? (CoreJava) Answer It's possible if these variables are final. Question

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What are the different ways in which polymorphism can be achieved in java? (CoreJava) Polymorphism can be acheived two ways overloading - static binding/early binding overriding - dynamic binding/late binding

Answer In case of overloading the method to be called is decided at the compile time based on the method signature. In case of overriding the method to be called is decided at run time and NOT at compile time. This is runtime polymorphism. Question What's the difference between constructors and other methods? (CoreJava) Constructors must have the same name as the class and can not return a Answer value. They are only called once while regular methods could be called many times. Question It is valid to declare an inherited method as abstract? (CoreJava) Answer Yes its valid to declare an inherited method abstract. But it would be of

no use as you need to define the class abstract again. e.g. abstract class AbsTest { abstract void setName(String name); } public class AbsTest1 extends AbsTest //compile error is thrown as there is an abstract method, declare the class abstract { abstract void setName(String name){} // error, abstract method cannot have body } What would you use to compare two String variables - the operator == or the method equals()? (CoreJava) I'd use the method equals() to compare the values of the Strings and the Answer == to check if two variables point at the same instance of a String object. Question

Question How do you know if an explicit object casting is needed? (CoreJava) If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example: Object a; Customer b; b = (Customer) a; Answer When you assign a subclass to a variable having a supeclass type, the casting is performed automatically. Question What will the output of the following programme be? Look at the programme carefully, we have an instance method and static method (class method) defined in both Animal class and Cat class. public class Animal { public static void hide() { System.out.format("The hide method in Animal.%n"); } public void override() { System.out.format("The override method in Animal.%n"); } } public class Cat extends Animal { public static void hide() { System.out.format("The hide method in Cat.%n"); } public void override() {

}

System.out.format("The override method in Cat.%n");

public static void main(String[] args) { Cat myCat = new Cat(); Animal myAnimal = myCat; //myAnimal.hide(); //BAD STYLE Animal.hide(); //Better! myAnimal.override(); } } (CoreJava)

The output of the programme will be.. The hide method in Animal. The override method in Cat. For class methods, the runtime system invokes the method defined in the compile-time type of the reference on which the method is called. In the example, the compile-time type of myAnimal is Answer Animal. Thus, the runtime system invokes the hide method defined in Animal. For instance methods, the runtime system invokes the method defined in the runtime type of the reference on which the method is called. In the example, the runtime type of myAnimal is Cat. Thus, the runtime system invokes the override method defined in Cat. An instance method cannot override a static method, and a static method cannot hide an instance method. Have a look at the following code. How do I execute the method d() in class B without creating another instance in the main method. class A { void c() { System.out.println("In A class"); } } Question public class B extends A { void d() { System.out.println("In B class"); } public static void main(String args[]) { A a = new B(); } } (CoreJava)

Answer

Question

you will need to typecast the object to ((B)a).d(); Why do we require public static void main(String args[]) method in Java programme? (CoreJava) Following are few reasons why there is public static void main(String args[]) a. public: The method can be accessed outside the class / package b. static: You need not have an instance of the class to access the method c. void: Your application need not return a value, as the JVM launcher would return the value when it exits d. main(): This is the entry point for the application

Answer

If the main() was not static, you would require an instance of the class in order to execute the method. If this is the case, what would create the instance of the class? What if your class did not have a public constructor? java Test would get converted to Test.main() there by invoking the main() Assume you have an ArrayList with 7 objects in it. And there is an int[] as

int[] intArray = new int[3]; intArray[0] = 2; Question intArray[1] = 4; intArray[2] = 6; How do you go about deleting objects from ArrayList based on the values present in int[]? (CoreJava) Answer This looks to be very simple question as we can make a for loop and remove objects from ArrayList. But remember that for the first time when you try to remove an object at index 2 from ArrayList, you would end up with an ArrayList size reduced from 7 to 6. And your code would be throwing ArrayIndexOutOfBoundsException. Have a look at this piece of code public void testArrayList() { int[] intArray = new int[3]; intArray[0] = 2; intArray[1] = 4; intArray[2] = 6; ArrayList al = new ArrayList();

for(int i=0; i < 7; i++) { al.add("test "+i); } System.out.println(al); for(int x=0; x < intArray.length; x++) { System.out.println(al.remove(intArray[x])); } } The above code doesn't work as the ArrayList size would shrink when you try to remove the first element. Have a look at this working code below public void testArrayList() { int[] intArray = new int[3]; intArray[0] = 2; intArray[1] = 4; intArray[2] = 6; ArrayList al = new ArrayList(); for(int i=0; i < 7; i++) { al.add("test "+i); } System.out.println(al); for(int x = intArray.length-1; (x != -1); x--) { System.out.println(al.remove(intArray[x])); } } You should try to delete from the bottom of the ArrayList. So that the size of the ArrayList doesn't effect the index of the objects that we are trying to delete. How does a try statement determine which catch clause should be used to handle an exception? (CoreJava) When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they Answer appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored Question

Question What is Tight Encapsulation? (CoreJava) Answer Encapsulation is for member variables in a classes that may not be

accessed by any other classes. Below are few examples Example 1: class Test { private String name; } This class is tightly encapsulated as you can't access the member "name". Example 2: class Test2 { private String name; public void setName(String name) { if(name.equals("test2") { this.name = name; } else { //throw user exception } } public String getName() { return name; } } The standard way to protect the data is to make it private, so that no other class can get direct access to it, and then write public methods to get the data and set the data. The method that sets the data should carry out appropriate checks to make sure the incoming data is valid. In Example 2 we are validating the incoming data with "test2". If its test2 we are allowing the data to be set, else we are throwing an exception. Tight encapsulation will not only protect direct access to data members, but will also prevent those members from being set to improper values. Question What is the maximum size of Integer wrapper class? (CoreJava) 2 TO THE POWER OF 31 - 1 or 2147483647 int value. Its little hard to Answer remember all these stuff :( Can main() of one java program be invoked in another java program's main()? (CoreJava) Answer Yes, This is possible. Have a look at the below code

Question

package corejava; public class A { public static void main(String args[]) { System.out.println("in A"); } }

package corejava; public class B { public static void main(String args[]) { System.out.println("in B"); String str = "one,two"; String[] strArray = str.split(str); A.main(strArray); } } Question Can we write a static keyword before the main class? Answer No

Question

(CoreJava)

int[] array = new int[5]; int[] array1 = {1,2,3,4,5}; Why not new is used in second statement? (CoreJava)

In the first line we are creating an int array with size 5 but in the second line we are creating an int array with 5 elements in it. Answer

In short you are initializing an int array in line one and you are creating an int array with elements in line two.

Question Can an exception be re-thrown? (CoreJava) Yes, the exception can be rethrown. And it will be rethrown to the caller method to handle the exception. This process continues till a method that handles this exception is called. Answer Its better to handle the exception using "Exception" class as its the parent class that is being extended by all other exception classes. Question What are the different inner classes available in java? Explain each inner

class with an example. (CoreJava) Answer There are four types of inner classes in java 1. 2. 3. 4.

Member class Static member class Local class Anonymous class

1. Member class A member class is defined as a member of a class. The member class is instance specific and has access to any and all methods and members, even the parent's "this" reference. All public, protected, default, and private members are visible to instances of member class. You must provide an instance of the enclosing class when you create a new instance of member class. public class EnclosingClass { private int instVar = 1; public class MemberClass { public void innerMethod () { instVar++; } } public MemberClass createMember () { return this.new MemberClass (); } } If you need to create an instance of member class outside of the scope of the enclosing class, you need to use an instance of the enclosing class to create the member instance: EnclosingClass ec = new EnclosingClass (); EnclosingClass.MemberClass mc = ec.new MemberClass (); or EnclosingClass.MemberClass mc = new EnclosingClass ().new MemberClass (); member classes can be declared as abstract and final. The access specifiers, public, protected, default and private can be used within the class.

2. Static member class A static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class. These classes can use instance variables and methods only through an object reference. Only public, final, and static are permitted as modifier inside a static member class. E.g. public class EnclosingClass { private static int static_var = 0; // Has got access public int instance_var = 0; // Has got no access public static class StaticInnerClass { } } Because the inner class is static, it can access only the static_var variable, even though it is private It cannot access the instance_var variable because it is not static, regardless of the fact that it is public The fully qualified class name for the inner class is EnclosingClass.StaticInnerClass These classes can be declared as public, abstract and final. public class Outer { public String name = "Outer"; public static void main (String argv[]) { Inner i = new Inner (); i.showName (); } //End of main

}

private static class Inner { String name = new String ("Inner"); void showName () { System.out.println (name); } } //End of Inner class

3. Local class

Local classes are declared within a block of code and are visible only within that block, just as any other method variable. Local classes are good way to maintain Encapsulation. Local classes, like local variables, cannot be declared public, protected, private, or static. Local classes cannot have static members. Local classes can only access final local variables and method arguments of the enclosing method. Local inner classes can be declared as abstract and final. Example public class AnyClass { void localClassDemo() { // a function class LocalClass { // a class inside a function definition void func() { System.out.println( “in Local class”); } } LocalClass local = new LocalClass(); local.func() ; } }

4. Anonymous class An anonymous class is a local class that has no name. An anonymous class is implicitly final. Anonymous classes cannot be public, protected, private, or static. The syntax for anonymous inner classes does not allow for any modifiers to be used. An anonymous inner class can extend a superclass or it can implement an interface. But not both. Example public class SomeGUI extends JFrame { protected void buildGUI() { button1 = new JButton(); button2 = new JButton(); button1.addActionListener( new java.awt.event.ActionListener() { public void actionPerformed(java.awt.event.ActionEvent e) { // do something }

}

});

} Question What is the disadvantage of using an inner class? (CoreJava) Java bytecode has no concept of inner classes, so the compiler translates inner classes into ordinary classes that are accessible to any code in the same package. An inner class gets access to the fields of the enclosing outer class-even if these fields are declared private and the inner class is translated into a Answer separate class. To let this separate class access the fields of the outer class, the compiler silently changes these fields' scope from private to package. As a result, when you use inner classes, you not only have the inner class exposed, but you also have the compiler silently overruling your decision to make some fields private. Question Does java support pointers? (CoreJava) Strictly speaking, in Java, a pointer is a reference that is guaranteed not to be null. However, when people use the term pointer, they usually mean C++ style direct hardware address pointers. Answer

How can you write serious code without pointers? Java does not have raw pointers like C or C++. It has something almost as powerful, but many times safer called references (Java refers to them as pointers in one place, the NullPointerException). They are like pointers, except that the dangerous features are removed.

Question Does java support global variables? (CoreJava) Though java doesn't support global variables, you can achieve this by creating variables public and static. Example public class Global { public static int x = 37; public static String s = "test"; } Answer

Such members can be accessed by saying: public class test { public static void main(String args[]) { Global.x = Global.x + 100; Global.s = "bbb"; } }

Question

Can an interface have variables defined in it? How do you use them? (CoreJava) Yes we can have variables defined in an Interface. Have a look at the class below to know how to use those variables

public interface MyInterface { String STR1 = "STR1"; String STR2 = "STR2"; Answer } public class MyTestClass implements MyInterface { public static void main(String args[]) { System.out.println(STR1); } } In a scenario, where you have to write an application which navigates between different pre-defined states of the Question application, what should I be using, an Abstract class or an Interface? (CoreJava) I would suggest Abstract class as your application is navigating through pre-defined states of the application. Answer

Question What is the differences between inheritance and composition? Answer Inheritance:

(CoreJava)

Inheritance is the ability to derive one class from another; the derived class (also called the subclass) inherits all of the methods and data members of its superclass. class Fruit { //... } class Apple extends Fruit { //... } In the above example, class Apple is related to class Fruit by inheritance, because Apple extends Fruit. In this example Fruit is superclass and Apple is subclass. Composition: Composition (called "has-a") is a relationship between classes where one

class has a data member that is an instance of the other class. class Fruit { //... } class Apple { private Fruit fruit = new Fruit(); //... } In the example above, class Apple is related to class Fruit by composition, because Apple has an instance variable that holds a reference to Fruit object. In this example, Apple is what I will call front-end class and Fruit is what I will call back-end class. In a composition relationship, the front-end class holds a reference in one of its instance variables to a back-end class. Question What is meant by upcasting? (CoreJava) Upcasting is where a derived object reference is cast to one of its base objects reference. class Base { public void show() { System.out.println("In Base class"); } } class Derived extends Base { public void show() Answer { System.out.println("In Derived class"); } public static void main(String args[]) { //upcasting Base base = new Derived(); base.show(); } } When an object is upcast it becomes a base object for the purpose of the cast, therefore any new fields and methods declared in the derived class are not accessible.

Question

Answer

Why Java is not complete Object Oriented Programming language? (CoreJava) I am not sure what the interviewer has in his mind, but I can think of only one reason, "Java doesn't support multiple inheritance". Do you guys have anthing to add?

Write a program in java to get in output as follows. 1 12 123 1 2 3 4. Question

Write the program in java to get in output as follows. 1 23 456 7 8 9 10

Provide a solution (CoreJava) Answer Below is the sample programme that gives you this output. public class A { public static void main(String args[]) throws Exception { /** * outputs the following format *1 *23 *345 *4567 */ for (int i = 1; i <= 4; i++) { for (int j = 0; j < i; j++) { System.out.print(i + j +" "); } System.out.println(""); } System.out.println("*****************"); /** * outputs the following format *1 *12 *123 *1234

*/ for (int i = 1; i <= 4; i++) { for (int j = 1; j <= i; j++) { System.out.print(j +" "); } System.out.println(""); } System.out.println("*****************"); /** * outputs the following format *1 *23 *456 * 7 8 9 10 */ int k=1; int y = 1; for (int i = 1; i <= 4; i++) { for (int j = 0; j < k; j++) { System.out.print(y++ +" "); } k++; System.out.println(""); } } }

Question

What is the difference between break, continue and return statements? (CoreJava)

Answer break: breaks the current loop and moves the cursor to next line after the loop e.g. for(int i=0; i < 10; i++) { System.out.println("i"+i); if(i == 5) break; } continue: Use continue when you have to skip few lines of code in a loop when a condition is satisfied e.g. for(int i=0; i < 10; i++) { System.out.println("i:"+i); if(i == 5) continue;

}

System.out.println("ii:"+i);

return: When a condition is satisfied you may want to return from the method itself. e.g. public int getValue() { int retValue = 0; for(int i=0; i < 10; i++) { retValue = i; System.out.println("i:"+i); if(i == 5) return retValue; System.out.println("ii:"+i); } return retValue; } Question What are dynamic class loaders? (CoreJava) This is a mechanism of loading classes at runtime. They have following characteristics, a. lazyloading - load classes only when they are required. This helps in memory management. b. type safety linkate - Dynamic loading of a class should not require additional run-time checks in order to guarantee type safety. Adds link-time checks by replacing run-time checks. They are performed only once. c. User defined class loaders - Programmers have the control to load classes in their applications. A user can Answer define a class loader to load classes from remote location. d. Multiple namespaces - Class loaders provide separate namespaces for different software components. For example an applet running in a browser loads classes from different jar files. Assume that the classes loaded from different jar files have same name but they are treated as distinct types by JVM E.g. Class cla = Class.forName( "com.javagalaxy.util.MyClass" ); MyClass tCla = (MyClass)cla.newInstance( ); tCla.doSomething(); What is the difference between local inner class and non-local inner class? (CoreJava) Answer Local inner classes may only access the local variables or method Question

parameters of the code block in which they are defined. Local inner classes may only access final variables which have been assigned a value. A non-local inner class has access to all variables declared within the encapsulating class

Question

What is meant by Virtual function in Java? Does Java supports Virtual function? (CoreJava) Java supports Virtual functions, all functions in Java are virtual by default. Virtual functions or virtual methods are functions or methods that will be redefined in derived classes. Pure Virtual functions from C++ could be abstract functions without body. Make your class abstract, define the pure virtual methods you want subclasseses to provide implementation for (using the abstract keyword in their definition) and you should be good.

Answer

public abstract class MyClass { public void concreteInitThing() { // your code here } }

public abstract void specificImplementation(); //Virtual method

public class ConcreteImplementationOfMyClass extends MyClass { public void specificImplementation() { // Your code here } } Question Why does main() in java taking String[] as argument? (CoreJava) Since main(String[]) is the starting point for an application if you need to Answer pass any arguments to application at the startup time, we can send the parameters as Strings Question Why java is called as PLATFORM independent? (CoreJava) Answer Java applications consist of byte-code which may be interpreted by a virtual engine. Thus, the applications are able to run on any hardware for which a virtual engine exists. Interpretation by a virtual engine means a lower processing speed, compared to compiled software. To counter this disadvantage, improvements have been developed, like just-in-time compilation (JIT), which translates program instructions of the virtual engine into instructions for the physical machine. The result in this case is an aligned program in memory, which can be executed rapidly without

interpretation. Aditional analysis of the runtime behavior with Hotspottechnology results in additional improvements. Since Object class is the super class of every class in java and suppose if I Question write class x extends y and this class x also extending Object, Is this multiple Inheritance ? (CoreJava) When you write "public class x" which doesn't extend any other class by default x is extending Object. Answer When you write "public class x extends y" x is extending y and y is extending Object class Your class can extend only one class. How do you uniquely identify one instance of a class among 10 instances of the same class? (CoreJava) Answer One way would be to check an objects hashCode. Question

Question

What is the difference between inheritance and decorator design pattern? (DesignPatterns) Decorators represent a powerful alternative to inheritance. Inheritance lets you add functionality to classes at compile time, decorators let you add functionality to objects at runtime. For example 1. FileReader frdr = new FileReader(filename); 2. LineNumberReader lrdr = new LineNumberReader(frdr);

Answer Line 1 creates a file reader (frdr), and line 2 adds line-number tracking. At runtime, decorators forward method calls to the objects they decorate. For example, in the code above, the line number reader, lrdr, forwards method calls to the file reader, frdr. Decorators add functionality either before or after forwarding to the object they decorate; for example, our line number reader tracks the current line number as it reads from an input stream. What are the difference between DAO (Data Access Object ) and DAC (Data Access Component)? (DesignPatterns) Answer The advantage of using data access objects is that any business object (which contains application or operation specific details) does not require direct knowledge of the final destination for the information it manipulates. As a result, if it is necessary to change where or how that data is stored that modification can be made without needing to change the main application. Question

Data Access Objects can be used in Java to insulate an application from the underlying Java persistence technology, which could be JDBC, JDO, EJB, Hibernate, or any one of a range of technologies. Using Data Access Objects means the underlying technology can be upgraded or swapped without changing other parts of the application. Question Are enterprise beans allowed to use Thread.sleep()? (EJB) Enterprise beans make use of the services provided by the EJB container, such as life-cycle management. To avoid conflicts with these services, Answer enterprise beans are restricted from performing certain operations: Managing or synchronizing threads Is it possible to write two EJB's that share the same Remote and Home Question interfaces, and have different bean classes? if so, what are the advantages/disadvantages? (EJB) It's certainly possible. In fact, there's an example that ships with the Inprise Application Server of an Account interface with separate Answer implementations for CheckingAccount and SavingsAccount, one of which was CMP and one of which was BMP. Is it possible to specify multiple JNDI names when deploying an EJB? (EJB) No. To achieve this you have to deploy your EJB multiple times each Answer specifying a different JNDI name. Question

Is there any way to force an Entity Bean to store itself to the db? I don't Question wanna wait for the container to update the db, I want to do it NOW! Is it possible? (EJB) Specify the transaction attribute of the bean as RequiresNew. Then as per section 11.6.2.4 of the EJB v 1.1 spec EJB container automatically starts a Answer new transaction before the method call. The container also performs the commit protocol before the method result is sent to the client. I am developing a BMP Entity bean. I have noticed that whenever the create method is invoked, the ejbLoad() and the ejbStore() methods are Question also invoked. I feel that once my database insert is done, having to do a select and update SQL queries is major overhead. is this behavior typical of all EJB containers? Is there any way to suppress these invocations? (EJB) This is the default behaviour for EJB. The specification states that ejbLoad() will be called before every transaction and ejbStore() after every Answer transaction. Each Vendor has optimizations, which are proprietary for this scenario. Question Can an EJB send asynchronous notifications to its clients? (EJB) Answer Asynchronous notification is a known hole in the first versions of the EJB spec. The recommended solution to this is to use JMS, which is becoming available in J2EE-compliant servers. The other option, of course, is to use

client-side threads and polling. This is not an ideal solution, but it's workable for many scenarios. Question How can I access EJB from ASP? (EJB) You can use the Java 2 Platform, Enterprise Edition Client Access Services Answer (J2EETM CAS) COM Bridge 1.0, currently downloadable from http://developer.java.sun.com/developer/earlyAccess/j2eecas/ Question Is there a guarantee of uniqueness for entity beans? (EJB) There is no such guarantee. The server (or servers) can instantiate as many instances of the same underlying Entity Bean (with the same PK) as it wants. However, each instance is guaranteed to have up-to-date data Answer values, and be transactionally consistent, so uniqueness is not required. This allows the server to scale the system to support multiple threads, multiple concurrent requests, and multiple hosts. How do the six transaction attributes map to isolation levels like "dirty Question read"? Will an attribute like "Required" lock out other readers until I'm finished updating? (EJB) The Transaction Attributes in EJB do not map to the Transaction Isolation levels used in JDBC. This is a common misconception. Transaction Attributes specify to the container when a Transaction should be started, Answer suspended(paused) and committed between method invocations on Enterprise JavaBeans. For more details and a summary of Transaction Attributes refer to section 11.6 of the EJB 1.1 specification. I have created a remote reference to an EJB in FirstServlet. Can I put the reference in a servlet session and use that in SecondServlet? (EJB) Yes. The EJB client (in this case your servlet) acquires a remote reference to an EJB from the Home Interface; that reference is serializable and can Answer be passed from servlet to servlet. If it is a session bean, then the EJB server will consider your web client's servlet session to correspond to a single EJB session, which is usually (but not always) what you want. Question

Can the primary key in the entity bean be a Java primitive type such as int? (EJB) The primary key can't be a primitive type--use the primitive wrapper Answer classes, instead. For example, you can use java.lang.Integer as the primary key class, but not int (it has to be a class, not a primitive) Question

Question What's new in the EJB 2.0 specification? (EJB) Following are the main features supported in EJB 2.0 * Integration of EJB with JMS * Message Driven Beans * Implement additional Business Answer methods in Home interface which are not specific for bean instance. * EJB QL. Question What is the need of Remote and Home interface. Why cant it be in

one? (EJB) In a few words, I would say that the main reason is because there is a clear division of roles and responsibilities between the two interfaces. The home interface is your way to communicate with the container, that is who is responsible of creating, locating even removing one or more beans. Answer The remote interface is your link to the bean, that will allow you to remotely access to all its methods and members. As you can see there are two distinct elements (the container and the beans) and you need two different interfaces for accessing to both of them. Question What is the difference between Java Beans and EJB?s? (EJB) Java Beans are client-side objects and EJBs are server side object, and Answer they have completely different development, lifecycle, purpose. Question With regard to Entity Beans, what happens if both my EJB Question Server and Database crash, what will happen to unsaved changes? Is there any transactional log file used? (EJB) Actually, if your EJB server crashes, you will not even be able to make a connection to the server to perform a bean lookup, as the server will no longer be listening on the port for incoming JNDI lookup requests. You will lose any data that wasn't committed prior to the crash. This is where you should start looking into clustering your EJB server. Another Answer Hi, Any unsaved and uncommited changes are lost the moment your EJB Server crashes. If your database also crashes, then all the saved changes Answer are also lost unless you have some backup or some recovery mechanism to retrieve the data. So consider database replication and EJB Clustering for such scenarios, though the occurence of such a thing is very very rare. Thx, Uma All databse have the concept of log files(for exampe oracle have redo log files concept). So if data bases crashes then on starting up they fill look up the log files to perform all pending jobs. But is EJB crashes, It depend upon the container how frequenlty it passivates or how frequesntly it refreshes the data with Database. Question Question Can you control when passivation occurs? (EJB) The developer, according to the specification, cannot directly control when passivation occurs. Although for Stateful Session Beans, the container cannot passivate an instance that is inside a transaction. So using transactions can be a a strategy to control passivation. The ejbPassivate() method is called during passivation, so the developer has control over what to do during this exercise and can implement the require optimized logic. Answer Some EJB containers, such as BEA WebLogic, provide the ability to tune the container to minimize passivation calls. Taken from the WebLogic 6.0 DTD "The passivation-strategy can be either "default" or "transaction". With the default setting the container will attempt to keep a working set of beans in the cache. With the "transaction" setting, the container will passivate the bean after every transaction (or method call for a non-transactional invocation)."

Does EJB 1.1 support mandate the support for RMI-IIOP ? What is the Question meaning of "the client API must support the Java RMI-IIOP programming model for portability, but the underlying protocol can be anything" ? (EJB) EJB1.1 does mandate the support of RMI-IIOP. OK, to answer the second question: There are 2 types of implementations that an EJB Server might provide: CORBA-based EJB Servers and Proprietry EJB Servers. Both support the RMI-IIOP API but how that API is implemented is a different story. (NB: By API we mean the interface provided to the client by the stub or proxy). A CORBA-based EJB Server actually implements its EJB Objects as CORBA Answer Objects (it therefore encorporates an ORB and this means that EJB's can be contacted by CORBA clients (as well as RMI-IIOP clients) A proprietry EJB still implements the RMI-IIOP API (in the client's stub) but the underlying protocol can be anything. Therefore your EJB's CANNOT be contacted by CORBA clients. The difference is that in both cases, your clients see the same API (hence, your client portability) BUT how the stubs communicate with the server is different. The EJB specification says that we cannot use Bean Managed Transaction in Entity Beans. Why? (EJB) The short, practical answer is... because it makes your entity beans useless as a reusable component. Also, transaction management is best left to the application server - that's what they're there for. It's all about Answer atomic operations on your data. If an operation updates more than one entity then you want the whole thing to succeed or the whole thing to fail, nothing in between. If you put commits in the entity beans then it's very difficult to rollback if an error occurs at some point late in the operation. Question

Question Can I invoke Runtime.gc() in an EJB? (EJB) You shouldn't. What will happen depends on the implementation, but the call will most Answer likely be ignored. You should leave system level management like garbage collection for the container to deal with. After all, that's part of the benefit of using EJBs, you don't have to manage resources yourself. What is clustering? What are the different algorithms used for clustering? (EJB) Clustering is grouping machines together to transparantly provide enterprise services.The client does not now the difference between Answer approaching one server or approaching a cluster of servers.Clusters provide two benefits: scalability and high availability. Further information can be found in the JavaWorld article J2EE Clustering. Question

What is the advantage of using Entity bean for database operations, over Question directly using JDBC API to do database operations? When would I use one over the other? (EJB) Answer Entity Beans actually represents the data in a database. It is not that Entity Beans replaces JDBC API. There are two types of Entity Beans

Container Managed and Bean Mananged. In Container Managed Entity Bean - Whenever the instance of the bean is created the container automatically retrieves the data from the DB/Persistance storage and assigns to the object variables in bean for user to manipulate or use them. For this the developer needs to map the fields in the database to the variables in deployment descriptor files (which varies for each vendor). In the Bean Managed Entity Bean - The developer has to specifically make connection, retrive values, assign them to the objects in the ejbLoad() which will be called by the container when it instatiates a bean object. Similarly in the ejbStore() the container saves the object values back the the persistance storage. ejbLoad and ejbStore are callback methods and can be only invoked by the container. Apart from this, when you use Entity beans you dont need to worry about database transaction handling, database connection pooling etc. which are taken care by the ejb container. But in case of JDBC you have to explicitly do the above features. what suresh told is exactly perfect. ofcourse, this comes under the database transations, but i want to add this. the great thing about the entity beans of container managed, whenever the connection is failed during the transaction processing, the database consistancy is mantained automatically. the container writes the data stored at persistant storage of the entity beans to the database again to provide the database consistancy. where as in jdbc api, we, developers has to do manually. Question What is the role of serialization in EJB? (EJB) A big part of EJB is that it is a framework for underlying RMI: remote method invocation. You're invoking methods remotely from JVM space 'A' on objects which are in JVM space 'B' -- possibly running on another machine on the network. To make this happen, all arguments of each method call must have their current state plucked out of JVM 'A' memory, flattened into a byte stream Answer which can be sent over a TCP/IP network connection, and then deserialized for reincarnation on the other end in JVM 'B' where the actual method call takes place. If the method has a return value, it is serialized up for streaming back to JVM A. Thus the requirement that all EJB methods arguments and return values must be serializable. The easiest way to do this is to make sure all your classes implement java.io.Serializable. Question What is EJB QL? (EJB) EJB QL is a Query Language provided for navigation across a network of enterprise beans and dependent objects defined by means of container managed persistence. EJB QL is introduced in the EJB 2.0 specification. The EJB QL query language defines finder methods for entity beans with container managed persistenceand is portable across containers and persistence managers. EJB QL is used for queries of two types of finder Answer methods: Finder methods that are defined in the home interface of an entity bean and which return entity objects. Select methods, which are not exposed to the client, but which are used by the Bean Provider to select persistent values that are maintained by the Persistence Manager or to select entity objects that are related to the entity bean on which the query is defined.

Is is possible for an EJB client to marshall an object of class java.lang.Class to an EJB? (EJB) Technically yes, spec. compliant NO! - The enterprise bean must not attempt to query a class to obtain information about the declared members Answer that are not otherwise accessible to the enterprise bean because of the security rules of the Java language. Question

Question Is it legal to have static initializer blocks in EJB? (EJB) Although technically it is legal, static initializer blocks are used to execute some piece of code before executing any constructor or method while instantiating a class. Static initializer blocks are also typically used to Answer initialize static fields - which may be illegal in EJB if they are read/write In EJB this can be achieved by including the code in either the ejbCreate(), setSessionContext() or setEntityContext() methods. Is it possible to stop the execution of a method before completion in a SessionBean? (EJB) Stopping the execution of a method inside a Session Bean is not possible Answer without writing code inside the Session Bean. This is because you are not allowed to access Threads inside an EJB.

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Question What is the default transaction attribute for an EJB? (EJB) There is no default transaction attribute for an EJB. Section 11.5 of EJB v1.1 spec says that the deployer must specify a value for the transaction Answer attribute for those methods having container managed transaction. In weblogic, the default transaction attribute for EJB is SUPPORTS. What is the difference between session and entity beans? When should I use one or the other? (EJB) An entity bean represents persistent global data from the database; a session bean represents transient user-specific data that will die when the Answer user disconnects (ends his session). Generally, the session beans implement business methods (e.g. Bank.transferFunds) that call entity beans (e.g. Account.deposit, Account.withdraw) Question

Is there any default cache management system with Entity beans ? In Question other words whether a cache of the data in database will be maintained in EJB ? (EJB) Caching data from a database inside the Application Server are what Entity EJB's are used for.The ejbLoad() and ejbStore() methods are used to synchronize the Entity Bean state with the persistent storage(database). Transactions also play an important role in this scenario. If data is Answer removed from the database, via an external application - your Entity Bean can still be "alive" the EJB container. When the transaction commits, ejbStore() is called and the row will not be found, and the transcation rolled back.

Question Why is ejbFindByPrimaryKey mandatory? (EJB) An Entity Bean represents persistent data that is stored outside of the EJB Container/Server. The ejbFindByPrimaryKey is a method used to locate and load an Entity Bean into the container, similar to a SELECT statement in SQL. By making this method mandatory, the client programmer can be Answer assured that if they have the primary key of the Entity Bean, then they can retrieve the bean without having to create a new bean each time - which would mean creating duplications of persistent data and break the integrity of EJB. Question Why do we have a remove method in both EJBHome and EJBObject? (EJB) With the EJBHome version of the remove, you are able to delete an entity bean without first instantiating it (you can provide a PrimaryKey object as a parameter to the remove method). The home version only works for Answer entity beans. On the other hand, the Remote interface version works on an entity bean that you have already instantiated. In addition, the remote version also works on session beans (stateless and statefull) to inform the container of your loss of interest in this bean. Question How can I call one EJB from inside of another EJB? (EJB) EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Answer Home Interface of the other bean, then acquire an instance reference, and so forth. What is the difference between a Server, a Container, and a Connector? (EJB) An EJB server is an application, usually a product such as BEA WebLogic, that provides (or should provide) for concurrent client connections and manages system resources such as threads, processes, memory, database connections, network connections, etc. An EJB container runs inside (or within) an EJB server, and provides deployed EJB beans with transaction and security management, etc. The Answer EJB container insulates an EJB bean from the specifics of an underlying EJB server by providing a simple, standard API between the EJB bean and its container. A Connector provides the ability for any Enterprise Information System (EIS) to plug into any EJB server which supports the Connector architecture. See http://java.sun.com/j2ee/connector/ for more indepth information on Connectors. Question

Question How is persistence implemented in enterprise beans? (EJB) Answer Persistence in EJB is taken care of in two ways, depending on how you implement your beans: container managed persistence (CMP) or bean managed persistence (BMP) For CMP, the EJB container which your beans run under takes care of the persistence of the fields you have declared to be persisted with the database - this declaration is in the deployment descriptor. So, anytime you modify a field in a CMP bean, as soon as the method you have executed is finished, the new data is persisted to the database by the

container. For BMP, the EJB bean developer is responsible for defining the persistence routines in the proper places in the bean, for instance, the ejbCreate(), ejbStore(), ejbRemove() methods would be developed by the bean developer to make calls to the database. The container is responsible, in BMP, to call the appropriate method on the bean. So, if the bean is being looked up, when the create() method is called on the Home interface, then the container is responsible for calling the ejbCreate() method in the bean, which should have functionality inside for going to the database and looking up the data. Question What is an EJB Context? (EJB) EJBContext is an interface that is implemented by the container, and it is also a part of the bean-container contract. Entity beans use a subclass of EJBContext called EntityContext. Session beans use a subclass called Answer SessionContext. These EJBContext objects provide the bean class with information about its container, the client using the bean and the bean itself. They also provide other functions. See the API docs and the spec for more details. Question Is method overloading allowed in EJB? Answer Yes you can overload methods

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Question Should synchronization primitives be used on bean methods? (EJB) No. The EJB specification specifically states that the enterprise bean is Answer not allowed to use thread primitives. The container is responsible for managing concurrent access to beans at runtime Question Are we allowed to change the transaction isolation property in middle of a transaction? (EJB) No. You cannot change the transaction isolation level in the middle of Answer transaction. Question

Question For Entity Beans, What happens to an instance field not mapped to any persistent storage,when the bean is passivated? (EJB) The specification infers that the container never serializes an instance of an Entity bean (unlike stateful session beans). Thus passivation simply involves moving the bean from the "ready" to the "pooled" bin. So what happens to the contents of an instance variable is controlled by the programmer. Remember that when an entity bean is passivated the Answer instance gets logically disassociated from it's remote object. Be careful here, as the functionality of passivation/activation for Stateless Session, Stateful Session and Entity beans is completely different. For entity beans the ejbPassivate method notifies the entity bean that it is being disassociated with a particular entity prior to reuse or for dereferenc. Question

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Question What is a Message Driven Bean, What functions does a message driven bean have and how do they work in collaboration with JMS? (EJB)

Message driven beans are the latest addition to the family of component bean types defined by the EJB specification. The original bean types include session beans, which contain business logic and maintain a state associated with client sessions, and entity beans, which map objects to persistent data. Message driven beans will provide asynchrony to EJB based applications by acting as JMS message consumers. A message bean is associated with a JMS topic or queue and receives JMS messages sent by EJB clients or other beans. Unlike entity beans and session beans, message beans do not have home or remote interfaces. Instead, message driven beans are instantiated by the container as required. Like stateless session beans, message beans maintain no client-specific state, allowing the container to optimally manage a pool of message-bean instances. Answer Clients send JMS messages to message beans in exactly the same manner as they would send messages to any other JMS destination. This similarity is a fundamental design goal of the JMS capabilities of the new specification. To receive JMS messages, message driven beans implement the javax.jms.MessageListener interface, which defines a single "onMessage()" method. When a message arrives, the container ensures that a message bean corresponding to the message topic/queue exists (instantiating it if necessary), and calls its onMessage method passing the client's message as the single argument. The message bean's implementation of this method contains the business logic required to process the message. Note that session beans and entity beans are not allowed to function as message beans. The EJB container implements the EJBHome and EJBObject classes. For Question every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes? (EJB) The EJB container maintains an instance pool. The container uses these instances for the EJB Home reference irrespective of the client request. while refering the EJB Object classes the container creates a separate instance for each client request. Another Answer Answer The instance pool maintainence is up to the implementation of the container. If the container provides one, it is available otherwise it is not mandatory for the provider to implement it. Having said that, yes most of the container providers implement the pooling functionality to increase the performance of the app server. How it is implemented, it is again up to the implementer. What is the advantage of putting an Entity Bean instance from the "Ready State" to "Pooled state"? (EJB) Answer The idea of the "Pooled State" is to allow a container to maintain a pool of entity beans that has been created, but has not been yet "synchronized" or assigned to an EJBObject. This mean that the instances do represent entity beans, but they can be used only for serving Home methods (create or findBy), since those methods do not relay on the specific values of the Question

bean. All these instances are, in fact, exactly the same, so, they do not have meaningful state. Jon Thorarinsson has also added: It can be looked at it this way: If no client is using an entity bean of a particular type there is no need for cachig it (the data is persisted in the database). Therefore, in such cases, the container will, after some time, move the entity bean from the "Ready State" to the "Pooled state" to save memory. Then, to save additional memory, the container may begin moving entity beans from the "Pooled State" to the "Does Not Exist State", because even though the bean's cache has been cleared, the bean still takes up some memory just being in the "Pooled State". Question Can a Session Bean be defined without ejbCreate() method? (EJB) The ejbCreate() methods is part of the bean's lifecycle, so, the compiler will not return an error because there is no ejbCreate() method. However, the J2EE spec is explicit: the home interface of a Stateless Session Bean must have a single create() method with no arguments, while the session bean class must contain exactly one ejbCreate() method, also without arguments. Answer Stateful Session Beans can have arguments (more than one create method) stateful beans can contain multiple ejbCreate() as long as they match with the home interface definition You need a reference to your EJBObject to startwith. For that Sun insists on putting a method for creating that reference (create method in the home interface). The EJBObject does matter here. Not the actual bean. Is it possible to share an HttpSession between a JSP and EJB? What Question happens when I change a value in the HttpSession from inside an EJB? (EJB) You can pass the HttpSession as parameter to an EJB method, only if all objects in session are serializable.This has to be consider as "passed-byvalue", that means that it's read-only in the EJB. If anything is altered from inside the EJB, it won't be reflected back to the HttpSession of the Servlet Container.The "pass-by-reference" can be used between EJBs Remote Interfaces, as they are remote references. While it IS possible to pass an HttpSession as a parameter to an EJB object, it is considered to be "bad practice (1)" in terms of object oriented design. This is because you Answer are creating an unnecessary coupling between back-end objects (ejbs) and front-end objects (HttpSession). Create a higher-level of abstraction for your ejb's api. Rather than passing the whole, fat, HttpSession (which carries with it a bunch of http semantics), create a class that acts as a value object (or structure) that holds all the data you need to pass back and forth between front-end/back-end. Consider the case where your ejb needs to support a non-http-based client. This higher level of abstraction will be flexible enough to support it. (1) Core J2EE design patterns (2001) Question Is there any way to read values from an entity bean without locking it for the rest of the transaction (e.g. read-only transactions)? We have a keyvalue map bean which deadlocks during some concurrent reads. Isolation levels seem to affect the database only, and we need to work within a

transaction. (EJB) The only thing that comes to (my) mind is that you could write a 'group accessor' - a method that returns a single object containing all of your entity bean's attributes (or all interesting attributes). This method could then be placed in a 'Requires New' transaction. This way, the current Answer transaction would be suspended for the duration of the call to the entity bean and the entity bean's fetch/operate/commit cycle will be in a separate transaction and any locks should be released immediately. Depending on the granularity of what you need to pull out of the map, the group accessor might be overkill. What is the difference between a "Coarse Grained" Entity Bean and a "Fine Grained" Entity Bean? (EJB) A 'fine grained' entity bean is pretty much directly mapped to one relational table, in third normal form. A 'coarse grained' entity bean is larger and more complex, either because its attributes include values or lists from other tables, or because it 'owns' one or more sets of dependent objects. Note that the coarse grained bean might be mapped to a single table or flat file, but that single table is going to be pretty ugly, with data copied from other tables, repeated field groups, columns that are dependent on non-key fields, etc. Answer Fine grained entities are generally considered a liability in large systems because they will tend to increase the load on several of the EJB server's subsystems (there will be more objects exported through the distribution layer, more objects participating in transactions, more skeletons in memory, more EJB Objects in memory, etc.) The other side of the coin is that the 1.1 spec doesn't mandate CMP Error! No index entries found.support for dependent objects (or even indicate how they should be supported), which makes it more difficult to do coarse grained objects with CMP. The EJB 2.0 specification improves this in a huge way.

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Question What is EJBDoclet? (EJB) EJBDoclet is an open source JavaDoc doclet that generates a lot of the Answer EJB related source files from custom JavaDoc comments tags embedded in the EJB source file. Question What are the main benefits of J2EE? (EJB) Answer J2EE provides the following: Faster solutions delivery time to market. J2EE uses "containers" to simplify development. J2EE containers provide for the separation of business logic from resource and lifecycle management, which means that developers can focus on writing business logic -- their value add -- rather than writing enterprise infrastructure. For example, the Enterprise JavaBeansTM (EJBTM) container (implemented by J2EE technology vendors) handles distributed communication, threading, scaling, transaction management, etc. Similarly, Java Servlets simplify web development by providing infrastructure for component, communication, and session management in a web container that is integrated with a web server. Freedom of choice. J2EE technology is a set of standards that many vendors can implement. The vendors are free to compete on

implementations but not on standards or APIs. Sun supplies a comprehensive J2EE Compatibility Test Suite (CTS) to J2EE licensees. The J2EE CTS helps ensure compatibility among the application vendors which helps ensure portability for the applications and components written for J2EE. J2EE brings Write Once, Run AnywhereTM (WORATM) to the server. Simplified connectivity. J2EE technology makes it easier to connect the applications and systems you already have and bring those capabilities to the web, to cell phones, and to devices. J2EE offers Java Message Service for integrating diverse applications in a loosely coupled, asynchronous way. J2EE also offers CORBA support for tightly linking systems through remote method calls. In addition, J2EE 1.3 adds J2EE Connectors for linking to enterprise information systems such as ERP systems, packaged financial applications, and CRM applications. By offering one platform with faster solution delivery time to market, freedom of choice, and simplified connectivity, J2EE helps IT by reducing TCO and simultaneously avoiding single-source for their enterprise software needs. Question Name a few Design Patterns used in J2ee applications (EJB) Answer MVC, Front Controller, Session Facade, Data Access Object What is the deployment order for the deployed server components in WebLogic server? (EJB) § JDBC Connection Pools § JDBC Multi Pools § JDCB Data Sources § JDBC Tx Data Sources § JMS Connection Factories Answer § JMS Servers § Connector Components § EJB Components § Web App Components --- An examination of the log file .,., Question

Why do the create() or find() method return the remote reference or a primary key only? (EJB) The EJB Specification prohibits this behavior, and the weblogic.ejbc compiler checks for this behavior and prohibits any polymorphic type of response from a create() or find() method. The reason the create() and find() methods cannot return any object or primitive type is similar to the reason that regular constructors can be cast Answer into the class itself or any of it?s super classes. For example A a = new A() or A b = new B() where B is a child of A. You cannot do, for example Vector v = new A(); Question

Question Which XML parser comes with WebLogic Server 6.1? (EJB) Answer We bundle a parser, based on Apache's Xerces 1.3.1 parser, in WebLogic

Server 6.1. In addition, we include a WebLogic proprietary highperformance non-validating parser that you can use for small to medium sized XML documents. The WebLogic XML Registry allows you to configure the parser you want to use for specific document types. Can I use the getAttribute() and setAttribute() methods of Version 2.2 of the Java Servlet API to parse XML documents? (EJB) Yes. Use the setAttribute() method for SAX mode parsing and the getAttribute() method for DOM mode parsing. Using these methods in a Answer Servlet, however, is a WebLogic-specific feature. This means that the Servlet may not be fully portable to other Servlet engines, so use the feature with caution. Question

How can I run multiple instances of the same servlet class in the same WebLogic Server instance? (EJB) If you want to run multiple instances, your servlet will have to implement the SingleThreadModel interface. An instance of a class that implements the SingleThreadModel interface is guaranteed not to be invoked by multiple threads simultaneously. Multiple instances of a SingleThreadModel interface are used to service simultaneous requests, each running in a single thread. Answer When designing your servlet, consider how you use shared resources outside of the servlet class such as file and database access. Because there are multiple instances of servlets that are identical, and may use exactly the same resources, there are still synchronization and sharing issues that must be resolved, even if you do implement the SingleThreadModel interface. Question

Question What technologies are included in J2EE? (EJB) The primary technologies in J2EE are: Enterprise JavaBeansTM (EJBsTM), JavaServer PagesTM (JSPsTM), Java Servlets, the Java Naming and Answer Directory InterfaceTM (JNDITM), the Java Transaction API (JTA), CORBA, and the JDBCTM data access API. What is the Java Authentication and Authorization Service (JAAS) 1.0? (EJB) The Java Authentication and Authorization Service (JAAS) provides a way for a J2EE application to authenticate and authorize a specific user or group of users to run it. JAAS is a Java programing language version of the Answer standard Pluggable Authentication Module (PAM) framework that extends the Java 2 platform security architecture to support user-based authorization.

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Must my bean-managed persistence mechanism use the WebLogic JTS driver? (EJB) BEA recommend that you use the TxDataSource for bean-managed Answer persistence. Question

Question Must EJBs be homogeneously deployed across a cluster? Why? (EJB) Yes. Beginning with WebLogic Server version 6.0, EJBs must be homogeneously deployed across a cluster for the following reasons: n To keep clustering EJBs simple n To avoid cross server calls which results in more efficiency. If EJBs are not deployed on all servers, cross server calls are much more likely. n To ensure that every EJB is available locally Answer n To ensure that all classes are loaded in an undeployable way Every server must have access to each EJB's classes so that it can be bound into the local JNDI tree. If only a subset of the servers deploys the bean, the other servers will have to load the bean's classes in their respective system classpaths which makes it impossible to undeploy the beans. Question Is an XSLT processor bundled in WebLogic Server? (EJB) Yes, we bundle an XSLT processor, based on Apache's Xalan 2.0.1 Answer processor, in WebLogic Server 6.1. I plugged in a version of Apache Xalan that I downloaded from the Question Apache Web site, and now I get errors when I try to transform documents. What is the problem? (EJB) You must ensure that the version of Apache Xalan you download from the Apache Web site is compatible with Apache Xerces version 1.3.1. Because you cannot plug in a different version of Apache Xerces (see the preceding question), the only version of Apache Xerces that is compatible with Answer WebLogic Server 6.1 is 1.3.1. The built-in parser (based on version 1.3.1 of Apache Xerces) and transformer (based on version 2.0.1 of Apache Xalan) have been modified by BEA to be compatible with each other. Question How do I increase WebLogic Server memory? (EJB) Increase the allocation of Java heap memory for WebLogic Server. (Set the minimum and the maximum to the same size.) Start WebLogic Server with the -ms32m option to increase the allocation, as in this example: Answer $ java ... -ms32m -mx32m ... This allocates 32 megabytes of Java heap memory to WebLogic Server, which improves performance and allows WebLogic Server to handle more simultaneous connections. You can increase this value if necessary. Question What causes Java.io exceptions in the log file of WebLogic Server? (EJB) Answer You may see messages like these in the log file: (Windows NT) java.io.IOException Connection Reset by Peer java.io.EOFException Connection Reset by Peer (Solaris) java.io.Exception: Broken pipe These messages occur when you are using servlets. A client initiates an HTTP request, and then performs a series of actions on the browser: 1. Click Stop or enter equivalent command or keystrokes

2. Click Refresh or enter equivalent command or keystrokes 3. Send a new HTTP request. The messages indicate that WebLogic Server has detected and recovered from an interrupted HTTP request. Question What is the function of T3 in WebLogic Server? (EJB) T3 provides a framework for WebLogic Server messages that support for enhancements. These enhancements include abbreviations and features, such as object replacement, that work in the context of WebLogic Server clusters and HTTP and other product tunneling. T3 predates Java Object Serialization and RMI, while closely tracking and leveraging these specifications. T3 is a superset of Java Object. Serialization or RMI; anything you can do in Java Object Serialization and Answer RMI can be done over T3. T3 is mandated between WebLogic Servers and between programmatic clients and a WebLogic Server cluster. HTTP and IIOP are optional protocols that can be used to communicate between other processes and WebLogic Server. It depends on what you want to do. For example, when you want to communicate between n A browser and WebLogic Server-use HTTP n An ORB and WebLogic Server-IIOP. What are the enhancements in EJB 2.0 specification with respect to Asynchronous communication? (EJB) EJB 2.0 mandates integration between JMS and EJB. We have specified the integration of Enterprise JavaBeans with the Java Message Service, and have introduced message-driven beans. A messagedriven bean is a stateless component that is invoked by the container as a Answer result of the arrival of a JMS message. The goal of the message-driven bean model is to make developing an enterprise bean that is asynchronously invoked to handle the processing of incoming JMS messages as simple as developing the same functionality in any other JMS MessageListener. Question

Question What are the enhancements in EJB 2.0 with respect to CMP? (EJB) EJB 2.0 extends CMP to include far more robust modeling capability, with support for declarative management of relationships between entity EJBs. Developers no longer need to re-establish relationships between the various beans that make up their application -- the container will restore the connections automatically as beans are loaded, allowing bean Answer developers to navigate between beans much as they would between any standard Java objects. EJB 2.0 also introduces for the first time a portable query language, based on the abstract schema, not on the more complex database schema. This provides a database and vendor-independent way to find entity beans at run time, based on a wide variety of search criteria. Question Can you briefly describe about local interfaces? (EJB) Answer EJB was originally designed around remote invocation using the Java

Remote Method Invocation (RMI) mechanism, and later extended to support to standard CORBA transport for these calls using RMI/IIOP. This design allowed for maximum flexibility in developing applications without consideration for the deployment scenario, and was a strong feature in support of a goal of component reuse in J2EE. Many developers are using EJBs locally -- that is, some or all of their EJB calls are between beans in a single container. With this feedback in mind, the EJB 2.0 expert group has created a local interface mechanism. The local interface may be defined for a bean during development, to allow streamlined calls to the bean if a caller is in the same container. This does not involve the overhead involved with RMI like marshalling etc. This facility will thus improve the performance of applications in which co-location is planned. Local interfaces also provide the foundation for container-managed relationships among entity beans with container-managed persistence. What are the special design care that must be taken when you work with local interfaces? (EJB) It is important to understand that the calling semantics of local interfaces are different from those of remote interfaces. For example, remote interfaces pass parameters using call-by-value semantics, while local interfaces use call-by-reference. This means that in order to use local interfaces safely, application developers need to carefully consider potential deployment scenarios up Answer front, then decide which interfaces can be local and which remote, and finally, develop the application code with these choices in mind. While EJB 2.0 local interfaces are extremely useful in some situations, the long-term costs of these choices, especially when changing requirements and component reuse are taken into account, need to be factored into the design decision. Question

Question What happens if remove( ) is never invoked on a session bean? (EJB) In case of a stateless session bean it may not matter if we call or not as in both cases nothing is done. The number of beans in cache is managed by the container. Answer In case of stateful session bean, the bean may be kept in cache till either the session times out, in which case the bean is removed or when there is a requirement for memory in which case the data is cached and the bean is sent to free pool. What is the difference between creating a distributed application using RMI and using a EJB architecture? (EJB) Answer It is possible to create the same application using RMI and EJB. But in case of EJB the container provides the requisite services to the component if we use the proper syntax. It thus helps in easier development and lesser error and use of proven code and methodology. But the investment on application server is mandatory in that case. But this investment is warranted because it results in less complex and maintainable code to the client, which is what the end client wants. Almost all the leading application servers provide load balancing and performance tuning Question

techniques. In case of RMI we have to code the services and include in the program the way to invoke these services. Can the bean class implement the EJBObject class directly? If not why? (EJB) It is better not to do it will make the Bean class a remote object and its methods can be accessed without the containers? security, and transaction Answer implementations if our code by mistake passed it in one of its parameters. Its just a good design practice. Question

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What does isIdentical() method return in case of different type of beans? (EJB) Stateless ? true always Stateful ? depends whether the references point to the same session object Entity ? Depends whether the primary key is the same and the home is same

Question How should you type cast a remote object? Why? (EJB) A client program that is intended to be interoperable with all compliant EJB Container implementations must use the javax.rmi.PortableRemoteObject.narrow(...) method to perform typeAnswer narrowing of the client-side representations of the remote home and remote interfaces. Programs using the cast operator for narrowing the remote and remote home interfaces are likely to fail if the Container implementation uses RMI-IIOP as the underlying communication transport. Question What should you do in a passivate method? (EJB) You try to make all nontransient variables, which are not one of the following to null. For the given list the container takes care of serializing and restoring the object when activated. Serializable objects, null, UserTransaction, SessionContext, JNDI contexts Answer in the beans context, reference to other beans, references to connection pools. Things that must be handled explicitly are like a open database connection etc. These must be closed and set to null and retrieved back in the activate method. What is the relationship between local interfaces and container-managed relationships? (EJB) Entity beans that have container-managed relationships with other entity beans, must be accessed in the same local scope as those related beans, Answer and therefore typically provide a local client view. In order to be the target of a container-managed relationship, an entity bean with containermanaged persistence must provide a local interface. Question

Question What does a remove method do for different cases of beans? (EJB) Answer Stateless Session : Does not do anything to the bean as moving the bean

from free pool to cache are managed by the container depending on load. Stateful Session: Removes the bean from the cache. Entity Bean: Deletes the bean (data) from persistent storage Question How does a container-managed relationship work? (EJB) An entity bean accesses related entity beans by means of the accessor methods for its container-managed relationship fields, which are specified by the cmr-field elements of its abstract persistence schema defined in the deployment descriptor. Entity bean relationships are defined in terms of Answer the local interfaces of the related beans, and the view an entity bean presents to its related beans is defined by its local home and local interfaces. Thus, an entity bean can be the target of a relationship from another entity bean only if it has a local interface. What is the new basic requirement for a CMP entity bean class in 2.0 from that of ejb 1.1? (EJB) It must be abstract class. The container extends it and implements Answer methods which are required for managing the relationships Question

Question What are the basic classes required in the client for invoking an EJB? (EJB) The home and the remote interfaces, the implementation of the Naming Context Factory, the stubs and skeletons. Answer In some App servers the stubs and the skeletons can be dynamically downloaded from the server What is the difference between Message Driven Beans and Stateless Session beans? (EJB) In several ways, the dynamic creation and allocation of message-driven bean instances mimics the behavior of stateless session EJB instances, which exist only for the duration of a particular method call. However, message-driven beans are different from stateless session EJBs (and other types of EJBs) in several significant ways: § Message-driven beans process multiple JMS messages asynchronously, rather than processing a serialized sequence of method calls. § Message-driven beans have no home or remote interface, and therefore Answer cannot be directly accessed by internal or external clients. Clients interact with message-driven beans only indirectly, by sending a message to a JMS Queue or Topic. Note: Only the container directly interacts with a message-driven bean by creating bean instances and passing JMS messages to those instances as necessary. § The Container maintains the entire lifecycle of a message-driven bean; instances cannot be created or removed as a result of client requests or other API calls. Question

Question What is the need for Clustering? (EJB) To scale the application so that it Answer 1. Is Highly Available 2. Has High Throughput.

Question What are the benefits of Clustering (Workload Management)? (EJB) They are 1. It balances client processing requests, allowing incoming work requests to be distributed according to a configured Workload Management selection policy. 2. It provides fail over capability by redirecting client requests to a running server when one or more servers are unavailable. This improves the Answer availability of applications and administrative services. 3. It enables systems to be scaled up to serve a higher client load than provided by the basic configuration. With server groups and clones additional instances of servers can easily be added to the configuration. 4. It enables servers to be transparently maintained and upgraded while applications remain available for users. 5. It centralizes administration of application servers and other objects. Question What are the types of Scaling? There are two types of scaling Answer 1. Vertical Scaling 2. Horizontal Scaling.

(EJB)

Question What is Vertical Scaling? (EJB) When multiple server clones of an application server are defined on the Answer same physical m/c, it is called Vertical Scaling. The objective is to use the processing power of that m/c more efficiently. Question What is Horizontal Scaling? (EJB) When Clones of an application server are defined on multiple physical Answer m/c, it is called Horizontal Scaling. The objective is to use more than one less powerful m/c more efficiently. Question What is a Server Group? (EJB) A server group is a template of an Application Server(and its contents) i.e, it is a logical representation of the application server. It has the same Answer structure and attributes as the real Application Server, but it is not associated with any node, and does not correspond to any real server process running on any node. Question What is a Clone? (EJB) The copies of a server group are called Clones. But unlike a Server Group Answer Clones are associated with a node and are real server process running in that node. Question What is Ripple Effect? (EJB) The process of propagating the changes in the properties of a server Answer group during runtime to all the associated clones is called Ripple Effect.

Question What level of Load Balancing is possible with EJBs? (EJB) The workload management service provides load balancing for the following types of enterprise beans Answer 1. Homes of entity or session beans 2. Instances of entity beans 3. Instances of stateless session beans What is the basic requirement for in-memory replication in Weblogic? (EJB) 1. The data in session should consist only of Serialized objects. Answer 2. Only setAttribute function should be used to set objects in session Question

Question How JDBC services can be used in clustered environment? (EJB) Identical DataSource has to be created in each clustered server instances Answer and configure to use different connection pools. What are the services that should not be used in a Clustered Environment? (EJB) Non-clustered services: 1. File Services Answer 2. Time services 3. Weblogic events 4. Weblogic Workspaces (In WebLogic 5.1) Question

Question Mention some tools to cluster Web Servers? Web Servers can be clustered using Answer 1. Edge Server. 2. DNS

(EJB)

Question What is in-memory replication? (EJB) The process by which the contents in the memory of one physical m/c are Answer replicated in all the m/c in the cluster is called in-memory replication. Question What cannot be or is recommended not to be done using EJB? (EJB) Answer · Enterprise Bean must not use read/write static fields. Using read-only static fields is allowed. Therefore, it is recommended that all static fields in enterprise bean class be declared as final. · Enterprise Bean must not use thread synchronization primitives to synchronize execution of multiple instances. · Enterprise bean must not attempt to manage threads. Enterprise bean must not attempt to start, stop, suspend, or resume a thread; or to change a thread's priority or name. The enterprise bean must not attempt to manage thread groups. · Enterprise Bean must not use the AWT functionality to attempt to output information to a display, or to input information from a keyboard. · Enterprise bean must not use java.io package to attempt to access files and directories in file system. · Enterprise bean must not attempt to directly read or write a file

descriptor. · Enterprise bean must not attempt to listen on a socket, accept connections on a socket, or use a socket for multicast. · Enterprise bean must not attempt to set the socket factory used by ServerSocket, Socket, or the stream handler factory used by URL. · Enterprise bean must not attempt to query a class to obtain information about the declared members that are not otherwise accessible to the enterprise bean because of the security rules of the Java language. · The enterprise bean must not attempt to use the Reflection API to access information that the security rules of the Java programming language make unavailable. · Enterprise bean must not attempt to create a class loader; obtain the current class loader; set the context class loader; set security manager; create a new security manager; stop the JVM; or change the input, output, and error streams. · Enterprise bean must not attempt to gain access to packages and classes that the usual rules of the Java programming language make unavailable to the enterprise bean. · Enterprise bean must not attempt to define a class in a package. · Enterprise bean must not attempt to use the subclass and object substitution features of the Java Serialization Protocol. · Enterprise bean must not attempt to obtain the security policy information for a particular code source. · Enterprise bean must not attempt to access or modify the security configuration objects (Policy, Security, Provider, Signer, and Identity). · Enterprise bean must not attempt to pass this as an argument or method result. The enterprise bean must pass the result of SessionContext.getEJBObject() or EntityContext. getEJBObject() instead. Enterprise bean must not attempt to load a native library. Question What is t3 protocol in weblogic? (EJB) T3 is a physical layer (layer 1) protocol, and definitely you don't expect Answer any browser to recognize it (e.g. TCP is a layer 4 protocol, and HTTP is running on top of TCP port 80). It's main use is for EJB's communicating. Question What design pattern would you use to reduce JNDI lookups? (EJB) Answer Use ServiceLocator/EJBHomeFactory Pattern to reduce expensive JNDI lookup process. How it works is to cache those service objects when the client performs JNDI lookup first time and reuse that service object from the cache second time onwards for other clients. This technique maintains a cache of service objects and looks up the JNDI only first time for a service object. This technique reduces redundant and expensive JNDI lookup process thus increasing performance significantly. Service Locator Pattern implements this technique by having a class to cache service objects, methods for JNDI lookup and methods for getting service objects from the

cache. ServiceLocator acts as interceptor between client and JNDI. Question What are the different XML files used in J2EE? (EJB) While working with an .ear deployment, you would generally be having

Answer

web.xml - (part of .war file) ejb-jar.xml - (part of .jar file) application.xml - (part of .ear file) weblogic-ejb-jar.xml - (part of .jar file and while using weblogic server)

Question What is a design pattern? (General) A design pattern systematically names, motivates, and explains a general Answer design that addresses a recurring design problem. Question Describe the visitor design pattern (General) Represents an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates. Answer The root of a class hierarchy defines an abstract method to accept a visitor. Subclasses implement this method with visitor.visit(this). The Visitor interface has visit methods for all subclasses of the baseclass in the hierarchy.

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Answer

What is the difference between Internationalization and Localization? (General) Internationalization (i18n) is the up front work of building a system in such a way that it can be adapted to multiple locales. Internationalization involves writing and designing an application so that it can be used with different languages, date, time, currency and other values without software modification. Localization (l10n) is the process of actually adapting internationalized software to the needs of users in a particular geographical or cultural area. Localization involves writing and designing an application capable of dealing with a specific region, country or language.

What is the difference between a Portal server and an Application server? (General) Answer Before understanding portal server, we will have to understand portal. As we know, portal in nothing but a web application. In simple words, a portal server is an application having a portal and its management capabilities. In traditional web application, we have a separate management section for Question

managing the web application. Portal server provides the application as well as that management section. In other words, a portal server is an application deployed inside application server. Whereas, an application server is a system that provides the execution environment that is at the core of network computing or web-based architectures, providing a full set of services.

Question

What are the basic configurations that needs to be taken care before an application is deployed in weblogic server 8.1? (General) I remember of the following configurations

Answer -

Setting up connection pool Setting up Datasource Setting up a new server instance Clustering (if required) JMS Queue's and Topics (if required)

Question What is the Collections API? (JavaUtil) The Collections API is a set of classes and interfaces that support Answer operations on collections of objects Question What is the List interface? (JavaUtil) Answer The List interface provides support for ordered collections of objects.

Question What is the Vector class? (JavaUtil) The Vector class provides the capability to implement a growable array of Answer objects Question What is an Iterator interface? (JavaUtil) Answer The Iterator interface is used to step through the elements of a Collection Question Which java.util classes and interfaces support event handling? (JavaUtil) The EventObject class and the EventListener interface support event Answer processing Question What is the GregorianCalendar class? (JavaUtil) Answer The GregorianCalendar provides support for traditional Western calendars Question What is the Locale class? (JavaUtil) The Locale class is used to tailor program output to the conventions of a Answer particular geographic, political, or cultural region

Question What is the SimpleTimeZone class? (JavaUtil) Answer The SimpleTimeZone class provides support for a Gregorian calendar Question What is the Map interface? (JavaUtil) The Map interface replaces the JDK 1.1 Dictionary class and is used Answer associate keys with values What is the highest-level event class of the event-delegation model? (JavaUtil) The java.util.EventObject class is the highest-level class in the eventAnswer delegation class hierarchy Question

Question What is the Collection interface? (JavaUtil) The Collection interface provides support for the implementation of a Answer mathematical bag - an unordered collection of objects that may contain duplicates Question What is the Set interface? (JavaUtil) The Set interface provides methods for accessing the elements of a finite Answer mathematical set. Sets do not allow duplicate elements Question What is the purpose of the enableEvents() method? (JavaUtil) The enableEvents() method is used to enable an event for a particular object. Normally, an event is enabled when a listener is added to an object Answer for a particular event. The enableEvents() method is used by objects that handle events by overriding their event-dispatch methods. Question What is the ResourceBundle class? (JavaUtil) The ResourceBundle class is used to store locale-specific resources that Answer can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run. Question What is the default size of the vector? (JavaUtil) Vector v = new Vector(); Answer Constructs an empty vector so that its internal data array has size 10 and its standard capacity increment is zero. Question How does an included file in jsp look in the compiled .java file? (JavaUtil) Answer If you are using jsp action include as <jsp:include page="a.jsp"/> then you see the following line of code in the service method of the .java file (servlet) org.apache.jasper.runtime.JspRuntimeLibrary.include(request, response, "a.jsp", out, false); If you are using directive include as *lt;%@ include file="a.jsp" %> then

you see the following line of code within the static block as static { _jspx_dependants = new java.util.Vector(1); _jspx_dependants.add("/a.jsp"); } Question What is the Properties class? (JavaUtil) The properties class is a subclass of Hashtable that can be read from or Answer written to a stream. It also provides the capability to specify a set of default values to be used. Question What is the difference between ArrayList and Vector? (JavaUtil) a.Internally, both the ArrayList and Vector hold onto their contents using an Array. A Vector defaults to doubling the size of its array, while the ArrayList increases its array size by 50 percent. Answer b.ArrayList doesn't have a constructor for specifying the incremental capacity, where as Vector has a constructor to specify the initial capacity and incremental capacity. c.Vector is synchronized where as ArrayList is not synchronized Question What is the difference between Iterator and Enumeration? (JavaUtil) Iterator takes the place of Enumeration in the Java collections framework. Iterators differ from enumerations in two ways: Iterators allow the caller to Answer remove elements from the underlying collection during the iteration with well-defined semantics. Method names have been improved. Question What is the ResourceBundle class? (JavaUtil) The ResourceBundle class is used to store locale-specific resources that Answer can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

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How would you guarantee sorting of objects in a class that implements Map interface? (JavaUtil) TreeMap class implements SortedMap interface and provides the guarantee of sorted objects in ascending key order. TreeMap treeMap = new TreeMap(); treeMap.put("abc", "xyz"); treeMap.put("xyz", "abc"); treeMap.put("def","xyz"); System.out.println("treeMap::"+treeMap);

How can u avoid the conflict between java.util.Date and java.sql.Date? (JavaUtil) Answer If your code has to use both java.util.Date and java.sql.Date in the same class, Question

Depending on number of times you are using these classes you can import

any one of the package. e.g. Assume you are using java.util.Date for 5 times in your code and you are using java.sql.Date only for one time then import java.util.* and for using java.sql.Date use java.sql.Date sDate = new java.sql.Date(); Question What is the difference b/w HashSet & LinkedHashSet? (JavaUtil) Comment from Gyula, From the javadoc the LinkedHashSet differs from HashSet in that it maintains a doubly-linked list running through all of its Answer entries. By this it preservs the order of elements when somebody iterates over them (the elements will be retrieved in a predictable order - the order of inserting). What is the query used to display all tables names in SQL Server (Query analyzer)? (JDBC) Answer select * from information_schema.tables Question

Question How many types of JDBC Drivers are present and what are they? There are 4 types of JDBC Drivers Type 1: JDBC-ODBC Bridge Driver Answer Type 2: Native API Partly Java Driver Type 3: Network protocol Driver Type 4: JDBC Net pure Java Driver

(JDBC)

Question What is the fastest type of JDBC driver? (JDBC) JDBC driver performance will depend on a number of issues: (a) the quality of the driver code, (b) the size of the driver code, (c) the database server and its load, (d) network topology, Answer (e) the number of times your request is translated to a different API. In general, all things being equal, you can assume that the more your request and response change hands, the slower it will be. This means that Type 1 and Type 3 drivers will be slower than Type 2 drivers (the database calls are make at least three translations versus two), and Type 4 drivers are the fastest (only one translation). Question What Class.forName will do while loading drivers? (JDBC) It is used to create an instance of a driver and register it with the Answer DriverManager. When you have loaded a driver, it is available for making a connection with a DBMS. Question How to Retrieve Warnings? (JDBC) Answer SQLWarning objects are a subclass of SQLException that deal with database access warnings. Warnings do not stop the execution of an application, as exceptions do; they simply alert the user that something

did not happen as planned. A warning can be reported on a Connection object, a Statement object (including PreparedStatement and CallableStatement objects), or a ResultSet object. Each of these classes has a getWarnings method, which you must invoke in order to see the first warning reported on the calling object E.g. SQLWarning warning = stmt.getWarnings(); if (warning != null) { while (warning != null) { System.out.println("Message: " + warning.getMessage()); System.out.println("SQLState: " + warning.getSQLState()); System.out.print("Vendor error code: "); System.out.println(warning.getErrorCode()); warning = warning.getNextWarning(); } } Question what are stored procedures? How is it useful? (JDBC) A stored procedure is a set of statements/commands which reside in the database. The stored procedure is precompiled and saves the database the effort of parsing and compiling sql statements everytime a query is run. Each Database has it's own stored procedure language, usually a variant of C with a SQL preproceesor. Newer versions of db's support writing stored procs in Java and Perl too. Answer Before the advent of 3-tier/n-tier architecture it was pretty common for stored procs to implement the business logic( A lot of systems still do it). The biggest advantage is of course speed. Also certain kind of data manipulations are not achieved in SQL. Stored procs provide a mechanism to do these manipulations. Stored procs are also useful when you want to do Batch updates/exports/houseKeeping kind of stuff on the db. The overhead of a JDBC Connection may be significant in these cases. Question How to call a Stored Procedure from JDBC? (JDBC) The first step is to create a CallableStatement object. As with Statement an and PreparedStatement objects, this is done with an open Connection object. A CallableStatement object contains a call to a stored procedure. Answer E.g. CallableStatement cs = con.prepareCall("{call SHOW_SUPPLIERS}"); ResultSet rs = cs.executeQuery(); Question Is the JDBC-ODBC Bridge multi-threaded? (JDBC) No. The JDBC-ODBC Bridge does not support concurrent access from different threads. The JDBC-ODBC Bridge uses synchronized methods to Answer serialize all of the calls that it makes to ODBC. Multi-threaded Java programs may use the Bridge, but they won't get the advantages of multithreading.

Question

Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection? (JDBC)

Answer

No. You can open only one Statement object per connection when you are using the JDBC-ODBC Bridge.

Question What is cold backup, hot backup, warm backup recovery? (JDBC) a. cold backup - All these files must be backed up at the same time, before the databaseis restarted. Answer b. hot backup - official name is 'online backup' ? is a backup taken of each tablespace while the database is running and is being accessed by the users. Question When we will Denormalize data? (JDBC) Data denormalization is reverse procedure, carried out purely for reasons Answer of improving performance.It maybe efficient for a high-throughput system to replicate data for certain data. Question What is the advantage of using PreparedStatement? (JDBC) If we are using PreparedStatement the execution time will be less.The PreparedStatement object contains not just an SQL statement,but the SQL Answer statement that has been precompiled.This means that when the PreparedStatement is executed,the RDBMS can just run the PreparedStatement's Sql statement without having to compile it first. Question What is a "dirty read"? (JDBC) Quite often in database processing, we come across the situation wherein one transaction can change a value, and a second transaction can read this value before the original change has been committed or rolled back. This is known as a dirty read scenario because there is always the possibility that Answer the first transaction may rollback the change, resulting in the second transaction having read an invalid value. While you can easily command a database to disallow dirty reads, this usually degrades the performance of your application due to the increased locking overhead. Disallowing dirty reads also leads to decreased system concurrency. Question What is Metadata and why should I use it? (JDBC) Metadata ('data about data') is information about one of two things: Database information (java.sql.DatabaseMetaData), or Information about a specific ResultSet (java.sql.ResultSetMetaData). Answer Use DatabaseMetaData to find information about your database, such as its capabilities and structure. Use ResultSetMetaData to find information about the results of an SQL query, such as size and types of columns Question Different types of Transaction Isolation Levels? (JDBC) Answer The isolation level describes the degree to which the data being updated is visible to other transactions. This is important when two transactions are trying to read the same row of a table. Imagine two transactions A & B. Three types of inconsistencies can occur: · Dirty-read: A has changed a row, but has not committed the changes. B reads the uncommitted data but his view of the data may be wrong if A

rolls back his changes and updates his own changes to the database. · Non-repeatable read: B performs a read, but A modifies or deletes that data later. If B reads the same row again, he will get different data. · Phantoms: A does a query on a set of rows to perform an operation. B modifies the table such that a query of A would have given a different result. The table may be inconsistent. TRANSACTION_READ_UNCOMMITTED : DIRTY READS, NON-REPEATABLE READ AND PHANTOMS CAN OCCUR. TRANSACTION_READ_COMMITTED : DIRTY READS ARE PREVENTED, NONREPEATABLE READ AND PHANTOMS CAN OCCUR. TRANSACTION_REPEATABLE_READ : DIRTY READS , NON-REPEATABLE READ ARE PREVENTED AND PHANTOMS CAN OCCUR. TRANSACTION_SERIALIZABLE : DIRTY READS, NON-REPEATABLE READ AND PHANTOMS ARE PREVENTED. Question What is 2 phase commit? (JDBC) A 2-phase commit is an algorithm used to ensure the integrity of a committing transactionIn Phase 1, the transaction coordinator contacts potential participants in the transaction. The participants all agree to make the results of the transaction permanent but do not do so immediately. The participants log information to disk to ensure they can complete Phase 2. If Answer all the participants agree to commit, the coordinator logs that agreement and the outcome is decided. The recording of this agreement in the log ends Phase In Phase 2, the coordinator informs each participant of the decision, and they permanently update their resources. Question How do you handle your own transaction ? (JDBC) Connection Object has a method called setAutocommit ( Boolean istrue) Answer - Default is true Set the Parameter to false , and begin your transaction What is the normal procedure followed by a java client to access the db.? (JDBC) The database connection is created in 3 steps: 1.Find a proper database URL (see FAQ on JDBC URL) 2.Load the database driver 3.Ask the Java DriverManager class to open a connection to your database In java code, the steps are realized in code as follows: Answer 1.Create a properly formatted JDBR URL for your database. (See FAQ on JDBC URL for more information). A JDBC URL has the form jdbc:someSubProtocol://myDatabaseServer/theDatabaseName 2. Class.forName("my.database.driver"); 3 . Connection conn = DriverManager.getConnection("a.JDBC.URL", "databaseLogin","databasePassword"); Question

Question What is a data source? (JDBC) Answer A DataSource class brings another level of abstraction than directly using a connection object. Data source can be referenced by JNDI. Data Source

may point to RDBMS, file System , any DBMS etc. Question What are collection pools? What are the advantages? (JDBC) A connection pool is a cache of database connections that is maintained in Answer memory, so that the connections may be reused How do you get Column names only for a table (SQL Server)? Write the Query. (JDBC) select name from syscolumns where id=(select id from sysobjects where Answer name='user_hdr') order by colid --user_hdr is the table name Question

What is the difference between cached rowset, jdbrowset and webrowset? (JDBC) A CachedRowSet is a disconnected, serializable, scrollable container for tabular data. A primary purpose of the CachedRowSet class is to provide a representation of a JDBC ResultSet that can be passed between different components of a distributed application. For example, a CachedResultSet can be used to send the result of a query executed by an Enterprise JavaBeans component running in a server environment over a network to a client running in a web browser. A second use for CachedRowSets is to provide scrolling and updating for ResultSets that don't provide these capabilities themselves. A CachedRowSet can be used to augment the capabilities of a JDBC driver that doesn't have full support for scrolling and updating. Finally, a CachedRowSet can be used to provide Java Answer applications with access to tabular data in an environment such as a thin client or PDA, where it would be inappropriate to use a JDBC driver due to resource limitations or security considerations. The CachedRowSet class provides a means to "get rows in" and "get changed rows out" without the need to implement the full JDBC API. A JdbcRowSet is a connected rowset that wraps a ResultSet object. The main use of JdbcRowSet is to wrap a ResultSet and make it appear as a JavaBeans(tm) component. The WebRowSet class extends CachedRowSet with the ability to write out the state of the the RowSet as an XML document. The format of the XML document is described by the DTD 'RowSet.dtd'. Question

Question Is thin driver provided by Oracle a type 4 driver? Answer YES

(JDBC)

What are different types of isolation levels in JDBC and explain where you can use them? (JDBC) Answer If the application needs only committed records, then TRANSACTION_READ_COMMITED isolation is the good choice. If the application needs to read a row exclusively till you finish your work, then TRANSACTION_REPEATABLE_READ is the best choice. If the application needs to control all of the transaction problems(dirty read, phantom read and unrepeatable read), you can choose TRANSACTION_SERIALIZABLE for maximum safety. Performance issues have to be taken care with this. E.g Banking applications. Question

If the application don't have to deal with concurrent transactions, then the best choice is TRANSACTION_NONE to improve performance. If the application is searching for records from the database then you can easily choose TRANSACTION_READ_UNCOMMITED because you need not worry about other programmes that are inserting records at the same time. It improves performance.

Question

What is the difference between Statement, PreparedStatement and CallableStatemen? (JDBC) Statement is used for static SQL statement with no input and output parameters, PreparedStatement is used for dynamic SQL statement with input parameters and CallableStatement is used for dynamic SQL satement with both input and output parameters, but PreparedStatement and CallableStatement can be used for static SQL statements as well. CallableStatement is mainly meant for stored procedures.

PreparedStatement gives better performance when compared to Statement because it is pre-parsed and pre-compiled by the database once for the first time and then onwards it reuses the parsed and compiled statement. Answer Because of this feature, it significantly improves performance when a statement executes repeatedly, It reduces the overload incurred by parsing and compiling. CallableStatement gives better performance when compared to PreparedStatement and Statement when there is a requirement for single request to process multiple complex statements. It parses and stores the stored procedures in the database and does all the work at database itself that in turn improves performance. But we loose java portability and we have to depend up on database specific stored procedures.

Who implements the methods of JDBC (java.sql.*) interfaces? e.g.resultSet.next() is a method in ResultSet interface and Sun doesn't Question provide any implementation for this method. Who actually provides the implementation. (JDBC) Its the JDBC Driver vendor. Java provides the specifications for the Answer interfaces and its the vendor who has to implement those methods. Consider the following code: Class.forName("mypackage.MyClass"); Connection con = DriverManager.getConnection("some connection Question string"); What Exceptions can be thrown in the above statements? (JDBC)

Answer

The above statements can throw any or all of the following exceptions

LinkageError - if the linkage fails ExceptionInInitializerError - if the initialization provoked by this method fails ClassNotFoundException - if the class cannot be located SQLException - if a database access error occurs

Question

Answer

What actually does Class.forName("mypackage.MyDriver"); method do? (JDBC) Class.forName("..."); initializes the provided class and returns the Class object associated with the class or interface with the given string name. For example, the following code fragment returns the runtime Class descriptor for the class named java.lang.Thread: Class t = Class.forName("java.lang.Thread")

If your SQL gets truncated in the process of execution, How would you Question know how much of data is trnasfered and how much of data is left over? (JDBC) This can be known using the class DataTruncation. DataTruncation is an exception that reports a DataTruncation warning (on reads) or throws a DataTruncation exception (on writes) when JDBC unexpectedly truncates (meaning that less information was read or written than requested) a data value. So all we should do is write our code Answer using the getDataSize() and getTransferSize() methods of this class in our catch block trapping this SQLException. The getDataSize() returns the number of bytes of data that should have been transferred while the getTransferSize() method returns the number of bytes of data actually transferred. The SQLstate for a DataTruncation is 01004. Question What is the difference between different JDBC drivers? (JDBC) Answer Type 1: A JDBC-ODBC bridge provides JDBC API access via one or more ODBC drivers. Note that some ODBC native code and in many cases native database client code must be loaded on each client machine that uses this type of driver. Hence, this kind of driver is generally most appropriate when automatic installation and downloading of a Java technology application is not important. Type 2: A native-API partly Java technology-enabled driver converts JDBC calls into calls on the client API for Oracle, Sybase, Informix, DB2, or other DBMS. Note that, like the bridge driver, this style of driver requires that some binary code be

loaded on each client machine. Type 3: A net-protocol fully Java technology-enabled driver translates JDBC API calls into a DBMS-independent net protocol which is then translated to a DBMS protocol by a server. This net server middleware is able to connect all of its Java technology-based clients to many different databases. The specific protocol used depends on the vendor. In general, this is the most flexible JDBC API alternative. It is likely that all vendors of this solution will provide products suitable for Intranet use. In order for these products to also support Internet access they must handle the additional requirements for security, access through firewalls, etc., that the Web imposes. Several vendors are adding JDBC technology-based drivers to their existing database middleware products. Type 4: A native-protocol fully Java technology-enabled driver converts JDBC technology calls into the network protocol used by DBMSs directly. This allows a direct call from the client machine to the DBMS server and is a practical solution for Intranet access. Since many of these protocols are proprietary the database vendors themselves will be the primary source for this style of driver. Several database vendors have these in progress. Question How may messaging models do JMS provide for and what are they? (JMS) JMS provide for two messaging models, publish-and-subscribe and pointAnswer to-point queuing Question What is messaging? (JMS) Messaging is a mechanism by which data can be passed from one Answer application to another application. Question What is point-to-point messaging? (JMS) With point-to-point message passing the sending application/client establishes a named message queue in the JMS broker/server and sends Answer messages to this queue. The receiving client registers with the broker to receive messages posted to this queue. There is a one-to-one relationship between the sending and receiving clients. Can two different JMS services talk to each other? For instance, if A and B are two different JMS providers, can Provider A send messages directly to Question Provider B? If not, then can a subscriber to Provider A act as a publisher to Provider B? (JMS) Answer The answers are no to the first question and yes to the second. The JMS

specification does not require that one JMS provider be able to send messages directly to another provider. However, the specification does require that a JMS client must be able to accept a message created by a different JMS provider, so a message received by a subscriber to Provider A can then be published to Provider B. One caveat is that the publisher to Provider B is not required to handle a JMSReplyTo header that refers to a destination that is specific to Provider A. What is the advantage of persistent message delivery compared to nonpersistent delivery? (JMS) If the JMS server experiences a failure, for example, a power outage, any message that it is holding in primary storage potentially could be lost. With persistent storage, the JMS server logs every message to secondary Answer storage. (The logging occurs on the front end, that is, as part of handling the send operation from the message producing client.) The logged message is removed from secondary storage only after it has been successfully delivered to all consuming clients Question

Question How is a java object message delivered to a non-java Client? (JMS) It is according to the specification that the message sent should be received in the same format. A non-java client cannot receive a message in Answer the form of java object. The provider in between handles the conversion of the data type and the message is transferred to the other end. Question What is MDB and What is the special feature of that? (JMS) MDB is Message driven bean, which very much resembles the Stateless session bean. The incoming and out going messages can be handled by the Answer Message driven bean. The ability to communicate asynchronously is the special feature about the Message driven bean. Question Give an example of using the publish/subscribe model. (JMS) JMS can be used to broadcast shutdown messages to clients connected to the Weblogic server on a module wise basis. If an application has six Answer modules, each module behaves like a subscriber to a named topic on the server. Question What is the difference between the Mailing and Messaging? (JMS) Java Mailing is the set of APIs that primarily concerns with the sending of Mail messages through the standard mail protocols. Messaging is the way Answer of communicating to the remote machines using Message Oriented Middlewares. Message Oriented Middlewares do not use mailing internally for communication. They create their own channels for communication. Question What are the types of messaging? (JMS) Answer There are two kinds of Messaging. Synchronous Messaging: Synchronous messaging involves a client that waits for the server to respond to a message.

Asynchronous Messaging: Asynchronous messaging involves a client that does not wait for a message from the server. An event is used to trigger a message from a server. Question What is publish/subscribe messaging? (JMS) With publish/subscribe message passing the sending application/client establishes a named topic in the JMS broker/server and publishes messages to this queue. The receiving clients register (specifically, Answer subscribe) via the broker to messages by topic; every subscriber to a topic receives each message published to that topic. There is a one-to-many relationship between the publishing client and the subscribing clients. Why doesn't the JMS API provide end-to-end synchronous message delivery and notification of delivery? (JMS) Some messaging systems provide synchronous delivery to destinations as a mechanism for implementing reliable applications. Some systems provide clients with various forms of delivery notification so that the clients can detect dropped or ignored messages. This is not the model defined by the JMS API. JMS API messaging provides guaranteed delivery via the once-and-onlyonce delivery semantics of PERSISTENT messages. In addition, message Answer consumers can insure reliable processing of messages by using either CLIENT_ACKNOWLEDGE mode or transacted sessions. This achieves reliable delivery with minimum synchronization and is the enterprise messaging model most vendors and developers prefer. The JMS API does not define a schema of systems messages (such as delivery notifications). If an application requires acknowledgment of message receipt, it can define an application-level acknowledgment message. Question

What are the core JMS-related objects required for each JMS-enabled application? (JMS) Each JMS-enabled client must establish the following: · A connection object provided by the JMS server (the message broker) · Within a connection, one or more sessions, which provide a context for message sending and receiving · Within a session, either a queue or topic object representing the Answer destination (the message staging area) within the message broker · Within a session, the appropriate sender or publisher or receiver or subscriber object (depending on whether the client is a message producer or consumer and uses a point-to-point or publish/subscribe strategy, respectively) Within a session, a message object (to send or to receive) Question

Question What are the various message types supported by JMS? Answer Stream Messages ? Group of Java Primitives Map Messages ? Name Value Pairs. Name being a string Value being a java primitive

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Text Messages ? String messages (since being widely used a separate messaging Type has been supported) Object Messages ? Group of serialize able java object Bytes Message ? Stream of uninterrupted bytes Question What is the Role of the JMS Provider? (JMS) The JMS provider handles security of the messages, data conversion and the client triggering. The JMS provider specifies the level of encryption and Answer the security level of the message, the best data type for the non-JMS client. Question How does a typical client perform the communication? (JMS) 1. Use JNDI to locate administrative objects. 1a. Locate a single ConnectionFactory object. 1b. Locate one or more Destination objects. 2. Use the ConnectionFactory to create a JMS Connection. Answer 3. Use the Connection to create one or more Session(s). 4. Use a Session and the Destinations to create the MessageProducers and MessageConsumers needed. 5. Perform your communication. Question Give an example of using the point-to-point model. (JMS) The point-to-point model is used when the information is specific to a single client. For example, a client can send a message for a print out, and Answer the server can send information back to this client after completion of the print job. Question How does the Application server handle the JMS Connection? (JMS) - App server creates the server session and stores them in a pool - Connection consumer uses the server session to put messages in the session of the JMS. Answer - Server session is the one that spawns the JMS session. - Applications written by Application programmers creates the message listener Question What protocols does JNDI provide an interface to? (JNDI) JNDI itself is independent of any specific directory access protocol. Individual service providers determine the protocols to support. JNDI supports popular protocols, such as LDAP (Light weight Directory Access Answer Protocol) , NDS(Netscape Directory Service), DNS(Domain Naming Service), and NIS(Network Information Service), supplied by different vendors. Question What is Context and InitialContext? (JNDI) Answer A context represents a set of bindings within a naming service. A Context object provides the methods for binding names to objects and unbinding names from objects, for renaming objects, and for listing the bindings. JNDI performs all naming operations relative to a context.

The JNDI specification defines an InitialContext class. This class is instantiated with properties that define the type of naming service in use (such as provider URL, security, ID and password to use when connecting). What's the difference between JNDI lookup(), list(), listBindings(), and search()? (JNDI) lookup() attempts to find the specified object in the given context. I.e., it looks for a single, specific object and either finds it in the current context or it fails. list() attempts to return an enumeration of all of the NameClassPair's of all of the objects in the current context. I.e., it's a listing of all of the objects in the current context but only returns the object's name and the name of the class to which the object belongs. listBindings() attempts to return an enumeration of the Binding's of all of Answer the objects in the current context. I.e., it's a listing of all of the objects in the current context with the object's name, its class name, and a reference to the object itself. search() attempts to return an enumeration of all of the objects matching a given set of search criteria. It can search across multiple contexts (or not). It can return whatever attributes of the objects that you desire. Etc. It's by far the most complex and powerful of these options but is also the most expensive. Question

Question Components of JNDI (JNDI) Naming Interface- The naming interface organizes information hierarchically and maps human-friendly names to addresses or objects that are machine-friendly. It allows access to named objects through multiple namespaces. Answer Directory Interface- JNDI includes a directory service interface that provides access to directory objects, which can contain attributes, thereby providing attribute-based searching and schema support Service Provider Interface- JNDI comes with the SPI, which supports the protocols provided by third parties. Question Is JNDI a protocol? Where is it used? (JNDI) Yes.HotJava Views 1.1 is using JNDI to access LDAP. Enterprise APIs such as Enterprise JavaBeans, Java Message Service, JDBC 2.0 make use of Answer JNDI to for their naming and directory needs. RMI over IIOP applications can use JNDI to access the CORBA (COS) naming service. Question What are native methods? How do you use them? (JNI) Native methods are methods written in other languages like C, C++, or Answer even assembly language. You can call native methods from Java using JNI Question Can we implement an interface in a JSP? Answer No

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Question What is the difference between ServletContext and PageContext? ServletContext: Gives the information about the container Answer PageContext: Gives the information about the Request

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What is the difference in using request.getRequestDispatcher() and context.getRequestDispatcher()? (JSP) request.getRequestDispatcher(path): In order to create it we need to give the relative path of the resource Answer context.getRequestDispatcher(path): In order to create it we need to give the absolute path of the resource. Question

Question How to pass information from JSP to included JSP? Answer Using <%jsp:param> tag.

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Question What is the difference between directive include and jsp include? (JSP) <%@ include> : Used to include static resources during translation time. Answer : Used to include dynamic content or static content during runtime. Question What is the difference between RequestDispatcher and sendRedirect? (JSP) RequestDispatcher: server-side redirect with request and response Answer objects. sendRedirect : Client-side redirect with new request and response objects. Question How does JSP handle runtime exceptions? (JSP) Using errorPage attribute of page directive and also we need to specify Answer isErrorPage=true if the current page is intended to URL redirecting of a JSP. Question How do you delete a Cookie within a JSP?

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Cookie mycook = new Cookie("name","value"); response.addCookie(mycook); Answer Cookie killmycook = new Cookie("mycook","value"); killmycook.setMaxAge(0); killmycook.setPath("/"); killmycook.addCookie(killmycook); Question How do I mix JSP and SSI #include? (JSP) Answer If you're just including raw HTML, use the #include directive as usual inside your .jsp file. But it's a little trickier if you want the server to evaluate any JSP code that's inside the included file. Ronel Sumibcay ([email protected]) says: If your data.inc file contains jsp code you will have to use <%@ vinclude="data.inc" %>

The is used for including non-JSP files. Question What is the difference between Model 1 and Model 2 architecture? (JSP) Answer In Model 1 there is no Controller and in Model 2 there is a Controller How can my application get to know when a HttpSession is removed? (JSP) Define a Class HttpSessionNotifier which implements HttpSessionBindingListener and implement the functionality what you need Answer in valueUnbound() method. Create an instance of that class and put that instance in HttpSession. Question

Question How can I implement a thread-safe JSP page? (JSP) You can make your JSPs thread-safe by having them implement the Answer SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" % > within your JSP page. Question How many JSP scripting elements are there and what are they? There are three scripting language elements: declarations Answer scriptlets expressions

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In the Servlet 2.4 specification SingleThreadModel has been deprecates, why? (JSP) Because it is not practical to have such model. Whether you set isThreadSafe to true or false, you should take care of concurrent client Answer requests to the JSP page by synchronizing access to any shared objects defined at the page level. Question

Question How do I include static files within a JSP page? (JSP) Static resources should always be included using the JSP include directive. This way, the inclusion is performed just once during the translation phase. The following example shows the syntax: Do note that Answer you should always supply a relative URL for the file attribute. Although you can also include static resources using the action, this is not advisable as the inclusion is then performed for each and every request. Question How do I mix JSP and SSI #include? (JSP) If you're just including raw HTML, use the #include directive as usual inside your .jsp file. But it's a little trickier if you want the server to evaluate any JSP code that's Answer inside the included file. If your data.inc file contains jsp code you will have to use <%@ vinclude="data.inc" %> The is used for including non-JSP files.

Question Can a JSP page process HTML FORM data? (JSP) Yes. However, unlike servlets, you are not required to implement HTTPprotocol specific methods like doGet() or doPost() within your JSP page. You can obtain the data for the FORM input elements via the request implicit object within a scriptlet or expression as: <% Answer String item = request.getParameter("item"); int howMany = new Integer(request.getParameter("units")).intValue(); %> or <%= request.getParameter("item") %> Question What JSP lifecycle methods can I override? (JSP) You cannot override the _jspService() method within a JSP page. You can however, override the jspInit() and jspDestroy() methods within a JSP page. jspInit() can be useful for allocating resources like database connections, network connections, and so forth for the JSP page. It is good programming practice to free any allocated resources within jspDestroy(). The jspInit() and jspDestroy() methods are each executed just once during the lifecycle of a JSP page and are typically declared as JSP declarations: <%! public void jspInit() { Answer ... } %> <%! public void jspDestroy() { ... } %> Question How do I include static files within a JSP page? (JSP) Static resources should always be included using the JSP include directive. This way, the inclusion is performed just once during the translation phase. The following example shows the syntax: Answer <%@ include file="copyright.html" %> Do note that you should always supply a relative URL for the file attribute. Although you can also include static resources using the action, this is not advisable as the inclusion is then performed for each and every request. Question How do I perform browser redirection from a JSP page? (JSP) Answer You can use the response implicit object to redirect the browser to a different resource, as: response.sendRedirect("http://www.foo.com/path/error.html"); You can also physically alter the Location HTTP header attribute, as shown below: <% response.setStatus(HttpServletResponse.SC_MOVED_PERMANENTLY);

String newLocn = "/newpath/index.html"; response.setHeader("Location",newLocn); %> You can also use the: <jsp:forward page="/newpage.jsp" /> Also note that you can only use this before any output has been sent to the client. I beleve this is the case with the response.sendRedirect() method as well. If you want to pass any paramateres then you can pass using <jsp:forward page="/servlet/login"> <jsp:param name="username" value="jsmith" /> > Question Can a JSP page instantiate a serialized bean? (JSP) No problem! The useBean action specifies the beanName attribute, which can be used for indicating a serialized bean. For example: <jsp:useBean id="shop" type="shopping.CD" beanName="CD" /> <jsp:getProperty name="shop" property="album" /> Answer A couple of important points to note. Although you would have to name your serialized file "filename.ser", you only indicate "filename" as the value for the beanName attribute. Also, you will have to place your serialized file within the WEB-INF\jsp\beans directory for it to be located by the JSP engine. Can you make use of a ServletOutputStream object from within a JSP page? (JSP) No. You are supposed to make use of only a JSPWriter object (given to you in the form of the implicit object out) for replying to clients. A JSPWriter can be viewed as a buffered version of the stream object Answer returned by response.getWriter(), although from an implementational perspective, it is not. A page author can always disable the default buffering for any page using a page directive as: <%@ page buffer="none" %> Question

What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization? (JSP) Although the SingleThreadModel technique is easy to use, and works well for low volume sites, it does not scale well. If you anticipate your users to increase in the future, you may be better off implementing explicit synchronization for your shared data. The key however, is to effectively minimize the amount of code that is synchronzied so that you take maximum advantage of multithreading. Answer Also, note that SingleThreadModel is pretty resource intensive from the server's perspective. The most serious issue however is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the unserviced requests are queued until something becomes free - which results in poor performance. Since the usage is non-deterministic, it may not help much even if you did add more memory and increased the size of the instance pool. Question

Question Can I stop JSP execution while in the midst of processing a request? (JSP) Answer Yes. Preemptive termination of request processing on an error condition

is a good way to maximize the throughput of a high-volume JSP engine. The trick (asuming Java is your scripting language) is to use the return statement when you want to terminate further processing. For example, consider: <% if (request.getParameter("foo") != null) { // generate some html or update bean property } else { /* output some error message or provide redirection back to the input form after creating a memento bean updated with the 'valid' form elements that were input. this bean can now be used by the previous form to initialize the input elements that were valid then, return from the body of the _jspService() method to terminate further processing */ return; } %> How can I get to view any compilation/parsing errors at the client while developing JSP pages? (JSP) With JSWDK 1.0, set the following servlet initialization property within the \WEB-INF\servlets.properties file for your application: Answer jsp.initparams=sendErrToClient=true This will cause any compilation/parsing errors to be sent as part of the response to the client. Question

Question Is there a way to reference the "this" variable within a JSP page? (JSP) Yes, there is. Under JSP 1.0, the page implicit object is equivalent to Answer "this", and returns a reference to the servlet generated by the JSP page. How do I instantiate a bean whose constructor accepts parameters using the useBean tag? (JSP) Answer Consider the following bean: package bar; public class FooBean { public FooBean(SomeObj arg) { ... } //getters and setters here } The only way you can instantiate this bean within your JSP page is to use a scriptlet. For example, the following snippet creates the bean with session scope: &l;% SomeObj x = new SomeObj(...); bar.FooBean foobar = new FooBean(x); session.putValue("foobar",foobar); %> You can now access this bean within any other page that is part of the same session as: &l;% bar.FooBean foobar = session.getValue("foobar"); %> To give the bean "application scope", you will have to place it within the ServletContext as: &l;% Question

application.setAttribute("foobar",foobar); %> To give the bean "request scope", you will have to place it within the request object as: &l;% request.setAttribute("foobar",foobar); %> If you do not place the bean within the request, session or application scope, the bean can be accessed only within the current JSP page (page scope). Once the bean is instantiated, it can be accessed in the usual way: jsp:getProperty name="foobar" property="someProperty"/ jsp:setProperty name="foobar" property="someProperty" value="someValue"/ Question Can I invoke a JSP error page from a servlet? (JSP) Yes, you can invoke the JSP error page and pass the exception object to it from within a servlet. The trick is to create a request dispatcher for the JSP error page, and pass the exception object as a javax.servlet.jsp.jspException request attribute. However, note that you can do this from only within controller servlets. If your servlet opens an OutputStream or PrintWriter, the JSP engine will throw the following translation error: java.lang.IllegalStateException: Cannot forward as OutputStream or Writer has already been obtained The following code snippet demonstrates the invocation of a JSP error page from within a controller servlet: protected void sendErrorRedirect(HttpServletRequest request, HttpServletResponse response, String errorPageURL, Throwable e) throws ServletException, IOException { request.setAttribute ("javax.servlet.jsp.jspException", e); Answer getServletConfig().getServletContext(). getRequestDispatcher(errorPageURL).forward(request, response); } public void doPost(HttpServletRequest request, HttpServletResponse response) { try { // do something } catch (Exception ex) { try { sendErrorRedirect(request,response,"/jsp/MyErrorPage.jsp",ex); } catch (Exception e) { e.printStackTrace(); } } } Question How can you store international / Unicode characters into a cookie? One way is, before storing the cookie URLEncode it. Answer URLEnocder.encoder(str); And use URLDecoder.decode(str) when you get the stored cookie.

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Question What are the implicit objects? (JSP) Implicit objects are objects that are created by the web container and contain information related to a particular request, page, or application. They are: request response pageContext Answer session application out config page exception Question Is JSP technology extensible? (JSP) YES. JSP technology is extensible through the development of custom Answer actions, or tags, which are encapsulated in tag libraries How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it? (JSP) You can make your JSPs thread-safe by having them implement the SingleThreadModel interface. This is done by adding the directive <%@ page isThreadSafe="false" %> within your JSP page. With this, instead of a single instance of the servlet generated for your JSP page loaded in memory, you will have N instances of the servlet loaded and initialized, with the service method of each instance effectively synchronized. You can typically control the number of instances (N) that are instantiated for all servlets implementing SingleThreadModel through Answer the admin screen for your JSP engine. More importantly, avoid using the <%! DECLARE %> tag for variables. If you do use this tag, then you should set isThreadSafe to true, as mentioned above. Otherwise, all requests to that page will access those variables, causing a nasty race condition. SingleThreadModel is not recommended for normal use. There are many pitfalls, including the example above of not being able to use <%! %>. You should try really hard to make them thread-safe the old fashioned way: by making them thread-safe Question

Question How does JSP handle run-time exceptions? (JSP) Answer You can use the errorPage attribute of the page directive to have uncaught run-time exceptions automatically forwarded to an error processing page. For example: <%@ page errorPage="error.jsp" %> redirects the browser to the JSP page error.jsp if an uncaught exception is encountered during request processing. Within error.jsp, if you indicate that it is an error-processing page, via the directive: <%@ page isErrorPage="true" %> the Throwable object describing the exception may be accessed within the error page via the exception implicit object. Note: You must always use a relative URL as the value for the errorPage

attribute. How do I prevent the output of my JSP or Servlet pages from being cached by the browser? (JSP) You will need to set the appropriate HTTP header attributes to prevent the dynamic content output by the JSP page from being cached by the browser. Just execute the following scriptlet at the beginning of your JSP pages to prevent them from being cached at the browser. You need both the statements to take care of some of the older browser versions. Answer <% response.setHeader("Cache-Control","no-store"); //HTTP 1.1 response.setHeader("Pragma","no-cache"); //HTTP 1.0 response.setDateHeader ("Expires", 0); //prevents caching at the proxy server %> Question

Question How do I use comments within a JSP page? (JSP) You can use "JSP-style" comments to selectively block out code while debugging or simply to comment your scriptlets. JSP comments are not visible at the client. For example: <%-- the scriptlet is now commented out <% out.println("Hello World"); %> --%> You can also use HTML-style comments anywhere within your JSP page. These comments are visible at the client. For example: Answer Of course, you can also use comments supported by your JSP scripting language within your scriptlets. For example, assuming Java is the scripting language, you can have: <% //some comment /** yet another comment **/ %> Question Response has already been commited error. What does it mean? (JSP) Answer This error show only when you try to redirect a page after you already have written something in your page. This happens because HTTP specification force the header to be set up before the lay out of the page can be shown (to make sure of how it should be displayed...contenttype="text/html" or "text/xml" or "plain-text" or "image/jpg", etc...) When you try to send a redirect status (Number is line_status_402), your HTTP server cannot send it right now if it hasn't finished to set up the header. If not starter to set up the header, there are no problems, but if it 's already begin to set up the header, then your HTTP server expects these headers to be finished setting up and it cannot be the case if the stream of the page is not over... In this last case it's like you have a file started with
Tag> <Some Headers> some output (like testing your variables...) ... and before you indicate that the file is over (and before the size of the page can be setted up in the header), you try to send a redirect status... It s simply impossible due to the specification of HTTP 1.0 and 1.1 Question How do I use a scriptlet to initialize a newly instantiated bean? (JSP) A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated. Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone. The following example shows the "today" property of the Foo bean initialized to the current date when it is instantiated. Note that here, we Answer make use of a JSP expression within the jsp:setProperty action. <jsp:useBean id="foo" class="com.Bar.Foo" > <jsp:setProperty name="foo" property="today" value="< %=java.text.DateFormat.getDateInstance().format(new java.util.Date()) %>"/ > <%-- scriptlets calling bean setter methods go here --%> How can I enable session tracking for JSP pages if the browser has disabled cookies? (JSP) Answer We know that session tracking uses cookies by default to associate a session identifier with a unique user. If the browser does not support cookies, or if cookies are disabled, you can still enable session tracking using URL rewriting. URL rewriting essentially includes the session ID within the link itself as a name/value pair. However, for this to be effective, you need to append the session ID for each and every link that is part of your servlet response. Adding the session ID to a link is greatly simplified by means of of a couple of methods: response.encodeURL() associates a session ID with a given URL, and if you are using redirection, response.encodeRedirectURL() can be used by giving the redirected URL as input. Both encodeURL() and encodeRedirectedURL() first determine whether cookies are supported by the browser; if so, the input URL is returned unchanged since the session ID will be persisted as a cookie. Consider the following example, in which two JSP files, say hello1.jsp and hello2.jsp, interact with each other. Basically, we create a new session within hello1.jsp and place an object within this session. The user can then traverse to hello2.jsp by clicking on the link present within the page.Within hello2.jsp, we simply extract the object that was earlier placed in the session and display its contents. Notice that we invoke the encodeURL() within hello1.jsp on the link used to invoke hello2.jsp; if cookies are disabled, the session ID is automatically appended to the URL, allowing hello2.jsp to still retrieve the session object. Try this example first with cookies enabled. Then disable cookie support, restart the brower, and try again. Each time you should see the maintenance of the session across pages. Do note that to get this example to work with cookies disabled at the browser, your JSP engine has to support URL rewriting. Question

hello1.jsp <%@ page session="true" %> <% Integer num = new Integer(100); session.putValue("num",num); String url =response.encodeURL("hello2.jsp"); %> hello2.jsp hello2.jsp <%@ page session="true" %> <% Integer i= (Integer )session.getValue("num"); out.println("Num value in session is "+i.intValue()); Question How can I declare methods within my JSP page? (JSP) You can declare methods for use within your JSP page as declarations. The methods can then be invoked within any other methods you declare, or within JSP scriptlets and expressions. Do note that you do not have direct access to any of the JSP implicit objects like request, response, session and so forth from within JSP methods. However, you should be able to pass any of the implicit JSP variables as parameters to the methods you declare. For example: <%! public String whereFrom(HttpServletRequest req) { HttpSession ses = req.getSession(); ... return req.getRemoteHost(); } %> <% out.print("Hi there, I see that you are coming in from "); %> Answer <%= whereFrom(request) %> Another Example file1.jsp: <%@page contentType="text/html"%> <%! public void test(JspWriter writer) throws IOException{ writer.println("Hello!"); } %> file2.jsp <%@include file="file1.jsp"%> <%test(out);% > Question Is there a way I can set the inactivity lease period on a per-session basis?

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Typically, a default inactivity lease period for all sessions is set within your JSP engine admin screen or associated properties file. However, if your JSP engine supports the Servlet 2.1 API, you can manage the inactivity lease period on a per-session basis. This is done by invoking the HttpSession.setMaxInactiveInterval() method, right after the session has Answer been created. For example: <% session.setMaxInactiveInterval(300); %> would reset the inactivity period for this session to 5 minutes. The inactivity interval is set in seconds. Question How can I set a cookie and delete a cookie from within a JSP page? A cookie, mycookie, can be deleted using the following scriptlet: <% //creating a cookie Cookie mycookie = new Cookie("aName","aValue"); response.addCookie(mycookie); Answer //delete a cookie Cookie killMyCookie = new Cookie("mycookie", null); killMyCookie.setMaxAge(0); killMyCookie.setPath("/"); response.addCookie(killMyCookie); %>

(JSP)

Question How does a servlet communicate with a JSP page? (JSP) Answer The following code snippet shows how a servlet instantiates a bean and initializes it with FORM data posted by a browser. The bean is then placed into the request, and the call is then forwarded to the JSP page, Bean1.jsp, by means of a request dispatcher for downstream processing. public void doPost (HttpServletRequest request, HttpServletResponse response) { try { govi.FormBean f = new govi.FormBean(); String id = request.getParameter("id"); f.setName(request.getParameter("name")); f.setAddr(request.getParameter("addr")); f.setAge(request.getParameter("age")); //use the id to compute //additional bean properties like info //maybe perform a db query, etc. // . . . f.setPersonalizationInfo(info); request.setAttribute("fBean",f); getServletConfig().getServletContext().getRequestDispatcher ("/jsp/Bean1.jsp").forward(request, response); } catch (Exception ex) { ... } }

The JSP page Bean1.jsp can then process fBean, after first extracting it from the default request scope via the useBean action. jsp:useBean id="fBean" class="govi.FormBean" scope="request"/ jsp:getProperty name="fBean" property="name" / jsp:getProperty name="fBean" property="addr" / jsp:getProperty name="fBean" property="age" / jsp:getProperty name="fBean" property="personalizationInfo" / How do I have the JSP-generated servlet subclass my own custom servlet class, instead of the default? (JSP) One should be very careful when having JSP pages extend custom servlet classes as opposed to the default one generated by the JSP engine. In doing so, you may lose out on any advanced optimization that may be provided by the JSP engine. In any case, your new superclass has to fulfill the contract with the JSP engine by: Implementing the HttpJspPage interface, if the protocol used is HTTP, or implementing JspPage otherwise Ensuring that all the methods in the Servlet interface are declared final Additionally, your servlet superclass Answer also needs to do the following: The service() method has to invoke the _jspService() method The init() method has to invoke the jspInit() method The destroy() method has to invoke jspDestroy() If any of the above conditions are not satisfied, the JSP engine may throw a translation error. Once the superclass has been developed, you can have your JSP extend it as follows: <%@ page extends="packageName.ServletName" %< Question

How can I prevent the word "null" from appearing in my HTML input text fields when I populate them with a resultset that has null values? (JSP) You could make a simple wrapper function, like <%! String blanknull(String s) { return (s == null) ? "" : s; Answer } %> then use it inside your JSP form, like Question

How can I get to print the stacktrace for an exception occuring within my JSP page? (JSP) Answer By printing out the exception's stack trace, you can usually diagonse a problem better when debugging JSP pages. By looking at a stack trace, a programmer should be able to discern which method threw the exception and which method called that method. However, you cannot print the stacktrace using the JSP out implicit variable, which is of type JspWriter. You will have to use a PrintWriter object instead. The following snippet demonstrates how you can print a stacktrace from within a JSP error page: Question

<%@ page isErrorPage="true" %> <% out.println(" "); PrintWriter pw = response.getWriter(); exception.printStackTrace(pw); out.println(" "); %> Question How do you pass an InitParameter to a JSP? (JSP) The JspPage interface defines the jspInit() and jspDestroy() method which the page writer can use in their pages and are invoked in much the same manner as the init() and destory() methods of a servlet. The example page below enumerates through all the parameters and prints them to the console. <%@ page import="java.util.*" %> <%! ServletConfig cfg =null; public void jspInit(){ Answer ServletConfig cfg=getServletConfig(); for (Enumeration e=cfg.getInitParameterNames(); e.hasMoreElements();) { String name=(String)e.nextElement(); String value = cfg.getInitParameter(name); System.out.println(name+"="+value); } } %> Question How can my JSP page communicate with an EJB Session Bean? (JSP) Answer The following is a code snippet that demonstrates how a JSP page can interact with an EJB session bean: <%@ page import="javax.naming.*, javax.rmi.PortableRemoteObject, foo.AccountHome, foo.Account" %> <%! //declare a "global" reference to an instance of the home interface of the session bean AccountHome accHome=null; public void jspInit() { //obtain an instance of the home interface InitialContext cntxt = new InitialContext( ); Object ref= cntxt.lookup("java:comp/env/ejb/AccountEJB"); accHome = (AccountHome)PortableRemoteObject.narrow(ref,AccountHome.class); } %> <%

//instantiate the session bean Account acct = accHome.create(); //invoke the remote methods acct.doWhatever(...); // etc etc... %> How do you prevent the Creation of a Session in a JSP Page and why? (JSP) By default, a JSP page will automatically create a session for the request if one does not exist. However, sessions consume resources and if it is not necessary to maintain a session, one should not be created. For example, a marketing campaign may suggest the reader visit a web page for more information. If it is anticipated that a lot of traffic will hit that page, you Answer may want to optimize the load on the machine by not creating useless sessions. Question

The page directive is used to prevent a JSP page from automatically creating a session: <%@ page session="false"> Question What is the difference between page and request scopes? (JSP) If you specify scope as Page then its life span is until page is displayed or control is forwarded to a new page. Accessibility is current page only. Answer But if u specify as Request, its life span is until the request has been completely processed and the response has been sent back to the user. Accessibility is current page and any included or for-warded pages What is the difference between a tag handler and a tag handler class? (JSP) The only difference between a tag handler and a tag handler class is that Answer a tag handler is written in JSP syntax, while a tag handler class is written in the Java language. Question

Question How do you precompile a jsp page? (JSP) To precompile a JSP page, access the page with a query string of ? jsp_precompile Answer http://www.javagalaxy.com:8080/RMI/index.jsp?jsp_precompile the jsp page will not get executed by this action. How do you print the contents of a.jsp in b.jsp. How do you include the file? (JSP) Answer a.jsp <%! public int i=10; %> Question

b.jsp <%@ include file="a.jsp" %>

<% out.println(i); %> do not use as this will include the file at runtime where as the above includes (directive include) the file at compile time a.jsp <%! int x = 10; %> b.jsp <%@ include file="a.jsp" %> Question <% int x = 20; out.println("x:"+x); %> When the above programme is invoked as http://localhost:8080/b.jsp, what is the output? (JSP) Compiler error as x is already initialised. When a.jsp file is included in b.jsp, all the variables of a.jsp are also Answer invoked in b.jsp. The following are the contents of test.jsp

This example demonstrates the output of test.jsp

Question <%! String name=request.getParameter(``name``); %> Name is <%=name %> If the user types http://localhost:8080/test.jsp?name=JavaGalaxy, What will the output be? (JSP) Compilation error as all the implict objects (request,session,out,...) are not available in declarative part.Since we are trying to declare Answer name=request.getParameter("name") it gives us compilation error as stating undefined variable or class name : request How can we access a Vector from one jsp page to another jsp page.? You Question also need to restrict a third jsp page from having access to this Vector. (JSP) Answer If you use session.setAttribute("key","value"); then the object that is put into the session is available till the session is timed out. In this case if you need to restrict the vector in third jsp page, put it in request scope

e.g. request.setAttribute("key", "value"); By this way your object is available till your request has been processed. Question Why would a client application use JTA transactions? (JTA) One possible example would be a scenario in which a client needs to employ two (or more) session beans, where each session bean is deployed on a different EJB server and each bean performs operations against external resources (for example, a database) and/or is managing one or more entity beans. In this scenario, the client's logic could required an allor-nothing guarantee for the operations performed by the session beans; hence, the session bean usage could be bundled together with a JTA UserTransaction object. In the previous scenario, however, the client application developer should Answer address the question of whether or not it would be better to encapsulate these operations in yet another session bean, and allow the session bean to handle the transactions via the EJB container. In general, lightweight clients are easier to maintain than heavyweight clients. Also, EJB environments are ideally suited for transaction management. ... Context c = new InitialContext(); UserTransaction ut = (UserTransaction) c.lookup("java:comp/UserTransaction"); ut.begin(); // perform multiple operations... ut.commit() ... How do I convert a numeric IP address like 192.18.97.39 into a hostname like java.sun.com? (Networking) String hostname = Answer InetAddress.getByName("192.18.97.39").getHostName(); Question

Question What information is needed to create a TCP Socket? (Networking) The Local System?s IP Address and Port Number. And the Remote Answer System's IPAddress and Port Number. What is the difference between URL instance and URLConnection instance? (Networking) A URL instance represents the location of a resource, and a Answer URLConnection instance represents a link for accessing or communicating with the resource at the location. Question

Question What are the two important TCP Socket classes? (Networking) Socket and ServerSocket. ServerSocket is used for normal two-way socket communication. Socket class allows us to read and write through Answer the sockets. getInputStream() and getOutputStream() are the two methods available in Socket class.

Question

Considering a=4 and b=10. Can you swap the values without using any temp variable? The final output should be a=10 and b=4. (Other)

Answer

a = a + b // 4 + 10 = 14 b = a - b // 14 - 10 = 4 a = a - b // 14 - 4 = 10 b=4 a = 10

Write a query for getting the second maximum marks of students from students table? (Other) select max(marks) from students where marks <(select max(marks) Answer from students) Question

Question how to make default checkbox selecte(in struts) Answer useing the set and get methods it can be done.

Question

Answer

(Other)

Considering a=4 and b=10. Can you swap the values without using any temp variable? The final output should be a=10 and b=4. (Other) a = a + b // 4 + 10 = 14 b = a - b // 14 - 10 = 4 a = a - b // 14 - 4 = 10 b=4 a = 10

Question How many types of protocol implementations does RMI have? (RMI) RMI has at least three protocol implementations: Java Remote Method Protocol(JRMP), Internet Inter ORB Protocol(IIOP), and Jini Extensible Answer Remote Invocation(JERI). These are alternatives, not part of the same thing, All three are indeed layer 6 protocols for those who are still speaking OSI reference model. Question Does RMI-IIOP support dynamic downloading of classes? (RMI) No, RMI-IIOP doesn't support dynamic downloading of the classes as it is done with CORBA in DII (Dynamic Interface Invocation).Actually RMI-IIOP combines the usability of Java Remote Method Invocation (RMI) with the Answer interoperability of the Internet Inter-ORB Protocol (IIOP).So in order to attain this interoperability between RMI and CORBA,some of the features that are supported by RMI but not CORBA and vice versa are eliminated from the RMI-IIOP specification. Does RMI-IIOP support code downloading for Java objects sent by value Question across an IIOP connection in the same way as RMI does across a JRMP connection? (RMI) Answer Yes. The JDK 1.2 support the dynamic class loading. Question Can RMI and Corba based applications interact ? (RMI) Answer Yes they can. RMI is available with IIOP as the transport protocol instead

of JRMP. Assume an application needs security permissions granted before the application can talk to the database. How Question do you go about setting the permissions? Give an example of setting the permission in Tomcat and stand alone java programme. (Security) In Tomcat: Add security permission to catalina.policy file located under /conf/ Add the following lines grant codeBase "file:${catalina.home}/webapps/MyContext/-" { permission java.security.AllPermission; }; Answer

In stand alone java application: Create a .policy file that points to the code base where the application resides. grant codebase "file:c:/source/java/-" { permission java.security.AllPermission; }; java -Djava.security.policy=.policy com.test.MyClass

Request parameter How to find whether a parameter exists in the request object? (Servlets) 1.boolean hasFoo = !(request.getParameter("foo") == null || request.getParameter("foo").equals("")); Answer 2. boolean hasParameter = request.getParameterMap().contains(theParameter); (which works in Servlet 2.3+) Question

How can I send user authentication information while makingURLConnection? (Servlets) You'll want to use HttpURLConnection.setRequestProperty and set all the Answer appropriate headers to HTTP authorization. Question

Question Can we use the constructor, instead of init(), to initialize servlet? (Servlets) Answer Yes , of course you can use the constructor instead of init(). There's nothing to stop you. But you shouldn't. The original reason for init() was that ancient versions of Java couldn't dynamically invoke constructors with arguments, so there was no way to give the constructur a ServletConfig. That no longer applies, but servlet containers still will only call your no-arg constructor. So you won't have access to a ServletConfig or

ServletContext. How can a servlet refresh automatically if some new data has entered the database? (Servlets) Answer You can use a client-side Refresh or Server Push Question

Question The code in a finally clause will never fail to execute, right? (Servlets) Answer Using System.exit(1); in try block will not allow finally code to execute. Question What is HttpTunneling? (Servlets) HTTP tunneling is used to encapsulate other protocols within the HTTP or HTTPS protocols. Normally the intra-network of an organization is blocked by a firewall and the network is exposed to the outer world only through a Answer specific web server port , that listens for only HTTP requests. To use any other protocol, that by passes the firewall, the protocol is embedded in HTTP and send as HttpRequest. What is Server Side Push and how is it implemented and when is it useful? (Servlets) Server Side push is useful when data needs to change regularly on the clients application or browser, without intervention from client. Standard examples might include apps like Stock's Tracker, Current News etc. As such server cannot connect to client's application automatically. The mechanism used is, when client first connects to Server, (Either through login etc..), then Server keeps the TCP/IP connection open. Answer It's not always possible or feasible to keep the connection to Server open. So another method used is, to use the standard HTTP protocols ways of refreshing the page, which is normally supported by all browsers. <META HTTP-EQUIV="Refresh" CONTENT="5;URL=/servlet/stockquotes/">

Question

This will refresh the page in the browser automatically and loads the new data every 5 seconds. Question What are the phases in JSP? (Servlets) a) Translation phase ? conversion of JSP to a Servlet source, and then Compilation of servlet source into a class file. The translation phase is Answer typically carried out by the JSP engine itself, when it receives an incoming request for the JSP page for the first time b) init(), service() and destroy() method as usual as Servlets. How many cookies can one set in the response object of the servlet? Also, are there any restrictions on the size of cookies? (Servlets) If the client is using Netscape, the browser can receive and store 300 total cookies Answer 4 kilobytes per cookie (including name) 20 cookies per server or domain Question

What?s the difference between sendRedirect( ) and forward( ) methods? (Servlets) A sendRedirect method creates a new request (it?s also reflected in browser?s URL ) where as forward method forwards the same request to the new target(hence the chnge is NOT reflected in browser?s URL). Answer The previous request scope objects are no longer available after a redirect because it results in a new request, but it?s available in forward. SendRedirectis slower compared to forward. Question

Is there some sort of event that happens when a session object gets bound or unbound to the session? (Servlets) HttpSessionBindingListener will hear the events When an object is added and/or remove from the session object, or when the session is invalidated, Answer in which case the objects are first removed from the session, whether the session is invalidated manually or automatically (timeout). Question

What do the differing levels of bean storage (page, session, app) mean? (Servlets) page life time - NO storage. This is the same as declaring the variable in a scriptlet and using it from there. session life time - request.getSession(true).putValue "myKey", myObj); Answer application level ? getServletConfig().getServletContext().setAttribute("myKey ",myObj ) request level - The storage exists for the lifetime of the request, which may be forwarded between jsp's and servlets Question

Is it true that servlet containers service each request by creating a new thread? If that is true, how does a container handle a sudden dramatic Question surge in incoming requests without significant performance degradation? (Servlets) The implementation depends on the Servlet engine. For each request generally, a new Thread is created. But to give performance boost, most containers, create and maintain a thread pool at the server startup time. To service a request, they simply borrow a thread from the pool and when Answer they are done, return it to the pool. For this thread pool, upper bound and lower bound is maintained. Upper bound prevents the resource exhaustion problem associated with unlimited thread allocation. The lower bound can instruct the pool not to keep too many idle threads, freeing them if needed. Question Can I just abort processing a JSP? (Servlets) Yes. Because your JSP is just a servlet method, you can just put Answer (whereever necessary) a < % return; % >

Question What is URL Encoding and URL Decoding ? (Servlets) Answer URL encoding is the method of replacing all the spaces and other extra characters into their Characters and Decoding is the reverse process converting all Hex Characters back their norm For Example consider this URL, /ServletsDirectory/Hello'servlet/

When Encoded using URLEncoder.encode("/ServletsDirectory/Hello'servlet/") the output is http%3A%2F%2Fwww.javacommerce.com%2FServlets+Directory%2FHello%27servlet%2FTh back using URLDecoder.decode("http%3A%2F%2Fwww.javacommerce.com%2FServlets+Directory%2FH Do objects stored in a HTTP Session need to be serializable? Or can it store any object? (Servlets) Yes, the objects need to be serializable, but only if your servlet container supports persistent sessions. Most lightweight servlet engines (like Answer Tomcat) do not support this. However, many EJB-enabled servlet engines do. Even if your engine does support persistent sessions, it is usually possible to disable this feature. Question

Question What is the difference between session and cookie? (Servlets) The difference between session and a cookie is two-fold. 1) session should work regardless of the settings on the client browser. even if users decide to forbid the cookie (through browser settings) session still works. there is no way to disable sessions from the client browser. 2) session and cookies differ in type and amount of information they are Answer capable of storing. Javax.servlet.http.Cookie class has a setValue() method that accepts Strings. javax.servlet.http.HttpSession has a setAttribute() method which takes a String to denote the name and java.lang.Object which means that HttpSession is capable of storing any java object. Cookie can only store String objects.

Question

What is the difference between ServletContext and ServletConfig? (Servlets)

Both are interfaces. The servlet engine implements the ServletConfig interface in order to pass configuration information to a servlet. The server passes an object that Answer implements the ServletConfig interface to the servlet's init() method. The ServletContext interface provides information to servlets regarding the environment in which they are running. It also provides standard way for servlets to write events to a log file. What are the differences between GET and POST service methods? (Servlets) A GET request is a request to get a resource from the server. Choosing GET as the "method" will append all of the data to the URL and it will show up in the URL bar of your browser. The amount of information you can send back using a GET is restricted as URLs can only be 1024 characters. A POST request is a request to post (to send) form data to a resource on the Answer server. A POST on the other hand will (typically) send the information through a socket back to the webserver and it won't show up in the URL bar. You can send much more information to the server this way - and it's not restricted to textual data either. It is possible to send files and even binary data such as serialized Java objects! Question

Question What is the difference between GenericServlet and HttpServlet? (Servlets) GenericServlet is for servlets that might not use HTTP, like for instance FTP service.As of only Http is implemented completely in HttpServlet. Answer The GenericServlet has a service() method that gets called when a client request is made. This means that it gets called by both incoming requests and the HTTP requests are given to the servlet as they are What is the Max amount of information that can be saved in a Session Object ? (Servlets) As such there is no limit on the amount of information that can be saved in a Session Object. Only the RAM available on the server machine is the limitation. The only limit is the Session ID length(Identifier) , which should not exceed more than 4K. If the data to be store is very huge, then it's Answer preferred to save it to a temporary file onto hard disk, rather than saving it in session. Internally if the amount of data being saved in Session exceeds the predefined limit, most of the servers write it to a temporary cache on Hard disk. Question

Question Explain the status codes of HttpResponse object. (Servlets) 100-199 ----> Informational 200-299 ----> Request was succesful Answer 300-399 ----> Request file has moved 400-499 ----> Client error 500-599 ----> Server error Question What is Servlet chaining? (Servlets) Answer response object from one servlet is passed as request to another Servlet. Try this example and you will come to know what servlet chaining is all about. public class ServletA extends HttpServlet { public void init(ServletConfig config) throws ServletException {} public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException { String aValue = request.getParameter("valueA"); System.out.println("ServletA read: "+aValue); request.setAttribute("ReadTheValue","Yes"); //do other servlet stuff...just don't open a write and output to the page.... request.getRequestDispatcher("/servletB").forward(req,resp); } } public class ServletB extends HttpServlet { public void init(ServletConfig config) throws ServletException {} public void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException { String aValue = request.getParameter("valueA"); System.out.println("ServletB also got the value: "+aValue); System.out.println("ServletB also found attribute: "+request.getAttribute("ReadTheValue")); //finish the task and write to the page. } }

Advantages: 1) Chaining reduces the demand on a single servlet 2) Modular design 3) Enables different complex processes to be maintained by more than one person. 4) Allows steps in a process to be modified (eg servlet 1 -> servlet 2 or servlet 1 -> servlet 3 -> servlet 2, etc) 5) Removes the complexity in a single servlet I am using servlets. I need to store an object NOT a string in a cookie. Is Question that possible? The helpfile says BASE64 encoding is suggested for use with binary values. How can I do that? (Servlets) You could serialize the object into a ByteArrayOutputStream and then Base64 encode the resulting byte[]. We must keep in mind the size limitations of a cookie and the overhead of transporting it back and forth between the browser and the server. Limitations are: Answer * at most 300 cookies * at most 4096 bytes per cookie (as measured by the characters that comprise the cookie non-terminal in the syntax description of the SetCookie2 header, and as received in the Set-Cookie2 header) * at most 20 cookies per unique host or domain name Suppose that Myservlet implements SingleThreadModel and there are Local variables, Instance variables, Question Class variables,Request attributes,Session attributes and Context attributes. Which among these variables would be thread safe? (Servlets) Answer Local,Instance and request variables are thread safe. I have the following deployed in my web.xml file. <session-config> <session-timeout>30 Question

then I perform setMaxInactiveInterval(2400) for my session object.The First one denotes 30 minutes and second one denoted 40 minutes. After how long would the session expire? (Servlets) 40 minutes. The timeout defined in your DD is used until you redefine it Answer with setMaxInactiveInterval Question The first time a Jsp page is requested which method is called? (Servlets) The first time a jsp page is requested jspInit method is called and the Answer request is handled by _jspService

Assume there is a servlet which is placed inside a .jar file and the .jar file is placed under WEB-INF/lib folder. Now you have the same servlet class under WEB-INF/classes folder. Which servlet Question class do you think will be called when there is a request to that particular servlet? i.e. the servlet in WEB-INF/lib or the servlet in WEB-INF/classes (Servlets) The servlet in WEB-INF/classes folder will be called. classes under WEBAnswer INF/classes will override the classes under WEB-INF/lib.

Question

How do I support both GET and POST protocol from the same Servlet? (Servlets) The easy way is, just support POST, then have your doGet() method call your doPost() method:

public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { Answer doPost(req, res); } Implementing the service() method is usually not what you want to do, since HttpServlet provides its own implementation of service() that turns around and calls doGet() and doPost() Question What is the difference between the doGet and doPost methods? (Servlets) doGet() is called in response to an HTTP GET request. This happens when users click on a link, or enter a URL into the browser's address bar. It also happens with some HTML FORMs (those with METHOD="GET" specified in the FORM tag). doPost() is called in response to an HTTP POST request. This happens with Answer some HTML FORMs (those with METHOD="POST" specified in the FORM tag). Both methods are called by the default (superclass) implementation of service in the HttpServlet base class. You should override one or both to perform your servlet's actions. You probably shouldn't override service(). How does one choose between overriding the doGet(), doPost(), and service() methods? (Servlets) Answer The differences between the doGet() and doPost() methods are that they are called in the HttpServlet that your servlet extends by its service() method when it recieves a GET or a POST request from a HTTP protocol request. Question

A GET request is a request to get a resource from the server. This is the case of a browser requesting a web page. It is also possible to specify parameters in the request, but the length of the parameters on the whole is limited. This

is the case of a form in a web page declared this way in html:

or . A POST request is a request to post (to send) form data to a resource on the server. This is the case of of a form in a web page declared this way in html: . In this case the size of the parameters can be much greater. The GenericServlet has a service() method that gets called when a client request is made. This means that it gets called by both incoming requests and the HTTP requests are given to the servlet as they are (you must do the parsing yourself). The HttpServlet instead has doGet() and doPost() methods that get called when a client request is GET or POST. This means that the parsing of the request is done by the servlet: you have the appropriate method called and have convenience methods to read the request parameters. NOTE: the doGet() and doPost() methods (as well as other HttpServlet methods) are called by the service() method. What is the diffence between client side validations and server side Question validations and under what scenerio we use these validations. Which one is better? (Servlets) When you say Clien side validation its Javascript validation. This validation doesn't require a request to the server. The validation is done within the browser itself. Server side validation, The validation is done at the server end. You will need to make a request to the server and pass all the form data to the server to do the validation. There is no rule that says use Client side validation for this and use Server Answer side valiedation for that. It all depends on your application. If there are any simple validations, e.g. check if First name is entered and entered email address is valid you can go for Client side validation. If there are business rules that needs to validate user input, then you will go for Server side validation. Server side validation has more control over your data validation and is secure (You dont display your validation logic to the users) Question What is the directory structure to create a web application? (Servlets) Answer Any web application needs to have the following directory structure Assume your web application name is javagalaxy then you would have javagalaxy

- (put all your jsp's, java script files, css files here. You may create different folders to segregate files) -WEB-INF (put your web.xml file here) -WEB-INFlib ( put all your library files here e.g. struts.jar, log4j.jar) -WEB-INFclasses (put all your class files here e.g. servlets, struts action class) Question How we invoke multi threading in servlet? (Servlets) Servlets are multi-threaded by default. If you need to make a servlet Answer single threaded, implement SingleThreadModel interface in the servlet Question Is Servlet a thread or a process? (Servlets) Answer Servlet is a thread. A process holds a collection of threads Question What happens if a Servlet is not loaded into memory? (Servlets) If a Servlet is not loaded in memory because it is shutting down or starting up, it will not be listening for requests and cannot accept connections. Any Answer clients that attempt to connect to the server will normally fail after a client-specific timeout period.

Question

Can you please tell me how many instances of servlet will be created for a very complex 500 users system? Question by : Sonal (Servlets) This actually depends on your configuration in web.xml 1. 200 2. 1

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In the first point you are asking the server to create 200 instances of a given servlet. There are Pros and Cons for this. This actually takes much of your memory. In the second point you are creating only one servlet instance at startup. This is a good practice. So when the server is unable to server requests by just this one instance, then the server would create another instance of the servlet to handle more requests. This would continue until the server can handle the requests. There is no specific number of instances that can be created by a server for a given servlet. This depends on your infrastructure.

In a single servlet program the three methods (i.e service(),doGet(),doPost()) are available by default? Question Which method is initalize first? Can there be any command to invoke the doGet() first? (Servlets) Answer First time when a servlet is invoked init() gets called. init() is called only once in a servlets life cycle.

service(), doGet() and doPost() gets called after init() is called. This is where your request gets processed. service(), doGet() and doPost() are available by default to your servlet, but it depends on what method you are overriding. You can have only service() or doGet() or both doGet() and doPost(). or all of them. It all depends on what you wanted to acheive in your servlet. By default service() is called. If you want doGet() to be called then avoid overriding service() Question What is the difference between 2 tier & 3 tier architecture? (Servlets) In 2 tier architecture client directly interacts with the database but in 3 tier architecture there is a middle tier that connects between client and database. Answer So in 3 tier when the user asks for some information from DB, middle tier interacts with DB and does necessary processing on the records and gives information to user. Question What are the Difference between Functions and Procedures? (SQL) 1. Functions are used for computations where as Procedures can be used for performing business logic 2. Functions MUST return a value, Procedures need not be. 3) you can have DML(insert, update, delete) statements in a Function. But, you cannot call such a function in a SQL query.. eg: suppose, if u have a function that is updating a table.. you can't call that function in any sql query. Answer

- select myFunction(field) from sometable; will throw error. 4) function parameters are always IN, no OUT is possible 5) function returns 1 value only. procedure can return multiple values(max. 1024) 6) we can select the fields from function.in the case of procdure we cannot select the fields. 7) Function do not return the images,text whereas stored procedures returns all

Question What is the difference between UNION and UNION ALL ? (SQL) UNION ALL selects all records between two queries including duplicates, whereas UNION takes the time to sort records in order to remove duplicates. In short UNION only selects distinct values, UNION ALL selects all values. Answer e.g. SELECT Date FROM table1 UNION SELECT Date FROM table2 SELECT Date FROM table1 UNION ALL SELECT Date FROM table2 Question What is a join and explain different types of joins. (SQL) Answer Joins are used in queries to explain how different tables are related. Joins also let you select data from a table depending upon data from another table. Types of joins INNER JOIN, OUTER JOIN, CROSS JOIN. OUTER JOIN is further classified as LEFT OUTER JOINS, RIGHT OUTER JOINS and FULL OUTER JOINS. Table details Student ID FirstName LastName BirthDate Attendance ID AttendanceDate AttendanceCode MinutesAbsent StudentSchedule ID Course Cross Join - A cross join merges two tables on every record in a geometric fashion, every record of one table is combined with every record from the other table. Two tables of 10 records each in a cross join will create a table of 100 (10 X 10) records. Inner Join - An Inner join is used to match two tables based on values of a common field. An inner join gets data from both tables where the specified data exists in both tables. For E.g. List of students in the database that were absent on December 4, 2003, you would use an inner join between

the two tables "Student" and "Attendance" SELECT Student.ID, Student.FirstName, Student.LastName, Attendance.AttendanceCode, Attendance.MinutesAbsent FROM Student INNER JOIN Attendance ON Student.ID=Attendance.ID WHERE Attendance.AttendanceDate='12/4/2003' Outer Join - An outer join gets data from the source table at all times, and returns data from the outer joined table ONLY if it matches the criteria. When using outer joins, fields will be set to NULL if data does not exist in the outer-joined table. For E.g. SELECT Student.ID, Student.FirstName, Student.LastName, Attendance.AttendanceCode, Attendance.MinutesAbsent FROM Student INNER JOIN StudentSchedule ON StudentSchedule.ID=Student.ID< br /> LEFT OUTER JOIN Attendance ON Student.ID=Attendance.ID AND Attendance.AttendanceDate='12/4/2003' WHERE StudentSchedule.Course='E NGLISH 9' a Left Outer join - All outer joins retrieve records from both tables, just as an inner join does. However, an outer join retrieves all of the records from one of the tables. A column in the result is NULL if the corresponding input table did not contain a matching record. b Right Outer join - The right outer join is similar to the left outer join in that it retrieves all the records from one side of the relationship, but this time it's the right table. Only records where the condition values match are retrieved from the left. c Full Outer Join - The full outer join retrieves all records from both the left and the right table. For 'n' number of SQL SELECT statements connected by UNION, how Question many times should we specify UNION to eliminate the duplicate rows? (SQL) Answer Only Once Question In the WHERE clause what is BETWEEN and IN? (SQL) Answer IN keyword helps to limit the selection criteria to one or more discrete values, the BETWEEN keyword allows for selecting a range. The syntax for the BETWEEN clause is as follows: SELECT "column_name" FROM "table_name" WHERE "column_name" BETWEEN 'value1' AND 'value2' This will select all rows whose column has a value between 'value1' and 'value2'. The syntax for using the IN keyword is as follows:

SELECT "column_name" FROM "table_name" WHERE "column_name" IN ('value1', 'value2', ...) The number of values in the parenthesis can be one or more, with each values separated by comma. Values can be numerical or characters. If there is only one value inside the parenthesis, this commend is equivalent to WHERE "column_name" = 'value1' Question Is BETWEEN inclusive of the range values specified? Answer Yes

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What is 'LIKE' used for in WHERE clause? What are the wildcard characters? (SQL) LIKE is another keyword that is used in the WHERE clause. Basically, LIKE allows you to do a search based on a pattern rather than specifying exactly what is desired (as in IN) or spell out a range (as in BETWEEN). The syntax for is as follows: SELECT "column_name" FROM "table_name" WHERE "column_name" LIKE {PATTERN} ‘%’ ( for a string of any character ) and ‘_’ (for any single character ) are the two wild card characters.

Question When do you use a LIKE statement? (SQL) To do partial search e.g. to search employee by name, you need not specify the complete name; using LIKE, you can search for partial string matches. Answer Example SQL : SELECT EMPNO FROM EMP WHERE EMPNAME LIKE 'RAMESH%' % is used to represent remaining all characters in the name. This query fetches all records contains RAMESH in six characters. Question What is a self join and give an example. (SQL) Joining two instances of a same table. Sample SQL : SELECT A.EMPNAME , B.EMPNAME Answer FROM EMP A, EMP B WHERE A.MGRID = B.EMPID Question What is a transaction? (SQL) Answer A transaction is a logicl unit of work where all steps must be commited or

rolled back. Question What is ACID? (SQL) ACID stands for Atomicity, Consistency, Isolation and Duralbility. These are the properties of a transaction. 1. Atomicity - states that database modifications must follow an "all or nothing" rule. Each transaction is said to be "atomic." If one part of the transaction fails, the entire transaction fails. It is critical that the database management system maintain the atomic nature of transactions in spite of any DBMS, operating system or hardware failure.

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2. Consistency - states that only valid data will be written to the database. If, for some reason, a transaction is executed that violates the database’s consistency rules, the entire transaction will be rolled back and the database will be restored to a state consistent with those rules. On the other hand, if a transaction successfully executes, it will take the database from one state that is consistent with the rules to another state that is also consistent with the rules. 3. Isolation - requires that multiple transactions occurring at the same time not impact each other's execution. For example, if Joe issues a transaction against a database at the same time that Mary issues a different transaction, both transactions should operate on the database in an isolated manner. The database should either perform Joe's entire transaction before executing Mary's or vice-versa. This prevents Joe's transaction from reading intermediate data produced as a side effect of part of Mary’s transaction that will not eventually be committed to the database. Note that the isolation property does not ensure which transaction will execute first, merely that they will not interfere with each other. 4. Durability - ensures that any transaction committed to the database will not be lost. Durability is ensured through the use of database backups and transaction logs that facilitate the restoration of committed transactions in spite of any subsequent software or hardware failures.

Question How do you perform a search based on part of a word? (SQL) SELECT * FROM emp WHERE dept LIKE '%war%'. The above query will return a recordset with Answer records consisting 'war' in the department name. E.g. software(war), hardware(war) Question What is GROUP BY?

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The GROUP BY keywords have been added to SQL because aggregate functions (like SUM) return the aggregate of all column values every time they are called. Without the GROUP BY functionality, finding the sum for each individual group of column values was not possible. The corresponding SQL syntax is, SELECT "column_name1", SUM("column_name2") FROM "table_name" GROUP BY "column_name1"

What is the difference between dropping, truncating and deleting from a table. (SQL) 1. Dropping - (Table structure + Data are deleted), Invalidates the dependent objects ,Drops the indexes Answer 2. Truncating - (Data alone deleted), Performs auto commit and is faster than delete 3. Delete - (Data alone deleted), Auto commit is not performed Question

What are cursors? Explain different types of cursors. What are the Question disadvantages of cursors? How can you avoid cursors? (SQL) Cursors allow row-by-row prcessing of the resultsets. Different Types of cursors - Static, Dynamic, Forward-only, Keyset-driven. Disadvantages of cursors - Each time you fetch a row from the cursor, it results in a network roundtrip, where as a normal SELECT query makes only one rowundtrip, however large the Answer resultset is. Cursors are also costly because they require more resources and temporary storage (results in more IO operations). Furthere, there are restrictions on the SELECT statements that can be used with some types of cursors. Most of the times, set based operations can be used instead of cursors. Question What are triggers? How to invoke a trigger on demand? (SQL) Triggers are special kind of stored procedures that get executed automatically when an INSERT, UPDATE or DELETE operation takes place on a table. Triggers can't be invoked on demand. They get triggered only when an associated action (INSERT, UPDATE, DELETE) happens Answer on the table on which they are defined. Triggers are generally used to implement business rules, auditing. Triggers can also be used to extend the referential integrity checks, but wherever possible, use constraints for this purpose, instead of triggers, as constraints are much faster.

Question What is a tuple? (SQL) In a table, a tuple is a row. Each row in the table represents a collection Answer of related data values.

What errors would arise when you try to execute a query and at the same time DB is down? (SQL) Answer An SQLException would arise Question

How do you select second maximum marks from a Student table with a single statement? (SQL) select max(marks) from Student where marks < (select max(marks) Answer from Student); Question

How to display the dept information from department table. (Oracle DB) (SQL) Answer select * from dept; Question

Question How to display the details of all employees. (Oracle DB) Answer select * from emp;

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Question How to display the name and job for all employees. (Oracle DB) Answer select ename, job from emp; Question How to display name and salary for all employees. (Oracle DB) Answer select ename, sal from emp;

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How to display employee number and total salary for each employee. (Oracle DB) (SQL) Answer select empno, sal+comm from emp; Question

How to display employee name and annual salary for all employees. (Oracle DB) (SQL) Answer select empno, empname, 12*sal+nvl(comm,0) annualsal from emp; Question

How to display the names of all employees who are working in department number 10. (Oracle DB) (SQL) Answer select ename from emp where deptno=10;

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How to display the names of all employees working as clerks and drawing a salary more than 3000. (Oracle DB) (SQL) Answer select ename from emp where job='CLERK' and sal>3000; Question

How to display employee number and names for employees who earn commission. (Oracle DB) (SQL) Answer select empno, ename from emp where comm is not null and comm>0; Question

How to display names of employees who do not earn any commission. (Oracle DB) (SQL) Answer Select empno, ename from emp where comm is null or comm=0; Question

How to display the names of employees who are working as clerk, Question salesman or analyst and drawing a salary more than 3000. (Oracle DB) (SQL) select ename from emp where (job='CLERK' or job='SALESMAN' or job='ANALYST') and sal>3000; Answer (or) select ename from emp where job in('CLERK','SALESMAN','ANALYST') and sal>3000; How to display the names of employees who are working in the company for the past 5 years. (Oracle DB) (SQL) Answer select ename from emp where sysdate-hiredate>5*365; Question

How to display the list of employees who have joined the company before 30th June 90 sor after 31st dec 90. (Oracle DB) (SQL) select * from emp where hiredate between '30-jun-1990' and '31Answer dec-1990'; Question

Question How to display current date. (Oracle DB) Answer select sysdate from dual;

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How to display the list of users in your database (using log table). (Oracle DB) (SQL) Answer select * from dba_users; Question

How to display the names of all tables from the current user. (Oracle DB) (SQL) Answer select * from tab; Question

Question How to display the name of the current user. (Oracle DB) Answer show user;

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How to display the names of employees working in department number Question 10 or 20 or 40 or employees working as clerks, salesman or analyst. (Oracle DB) (SQL) Answer select ename from emp where deptno in (10,20,40) or job in

('CLERK','SALESMAN','ANALYST'); How to display the names of employees whose name starts with alphabet S. (Oracle DB) (SQL) Answer select ename from emp where ename like 'S%'; Question

How to display employee names for employees whose name ends with alphabet. (Oracle DB) (SQL) Answer select ename from emp where ename like '%S'; Question

How to display the names of employees whose names have second alphabet A in their names. (Oracle DB) (SQL) Answer select ename from emp where ename like '_S%'; Question

How to display the names of employees whose name is exactly five characters in length. (Oracle DB) (SQL) select ename from emp where length(ename)=5; Answer (or) select ename from emp where ename like '_____'; Question

How to display the names of employees who are not working as managers. (Oracle DB) (SQL) select * from emp minus (select * from emp where empno in (select mgr from emp)); (or) select * from emp where empno not in (select mgr from emp where mgr is Answer not null); (or) select * from emp e where empno not in (select mgr from emp where e.empno=mgr) Question

How to display the names of employees who are not working as SALESMAN or CLERK or ANALYST. (Oracle DB) (SQL) select ename from emp where job not in Answer ('CLERK','SALESMAN','ANALYST'); Question

How to display all rows from EMP table. The system should wait after every screen full of information. (Oracle DB) (SQL) Answer set pause on; Question

How to display the total number of employees working in the company. (Oracle DB) (SQL) Answer select count(*) from emp; Question

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How to display the total salary being paid to all employees. (Oracle DB) (SQL)

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select sum(sal)+sum(nvl(comm,0)) from emp;

Question How to display the maximum salary from emp table. (Oracle DB) Answer select max(sal) from emp; Question How to display the minimum salary from emp table. (Oracle DB) Answer select min(sal) from emp; Question How to display the average salary from emp table. (Oracle DB) Answer select avg(sal) from emp;

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How to display the maximum salary being paid to CLERK. (Oracle DB) (SQL) Answer select max(sal) from emp where job='CLERK'; Question

How to display the maximum salary being paid in dept no 20. (Oracle DB) (SQL) Answer select max(sal) from emp where deptno=20; Question

How to display the min Sal being paid to any SALESMAN. (Oracle DB) (SQL) Answer select min(sal) from emp where job='SALESMAN'; Question

Question How to display the average salary drawn by managers. (Oracle DB) Answer select avg(sal) from emp where job='MANAGER';

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How to display the total salary drawn by analyst working in dept no 40. (Oracle DB) (SQL) Answer select sum(sal)+sum(nvl(comm,0)) from emp where deptno=40; Question

How to display the names of employees in order of salary i.e. the name of the employee earning lowest salary should appear first. (Oracle DB) (SQL) Answer select ename from emp order by sal; Question

How to display the names of employees in descending order of salary. (Oracle DB) (SQL) Answer select ename from emp order by sal desc; Question

How to display the details from emp table in order of emp name. (Oracle DB) (SQL) Answer select ename from emp order by ename; Question

Question How to display empno, ename, deptno, and sal. Sort the output first

based on name and within name by deptno and within deptno by Sal; (Oracle DB) (SQL) Answer select * from emp order by ename,deptno,sal; How to display the name of the employee along with their annual salary Question (Sal * 12). The name of the employee earning highest annual salary should appear first. (Oracle DB) (SQL) select ename, 12*(sal+nvl(comm,0)) Annual from emp order by Answer 12*(sal+nvl(comm,0)) desc; How to display name, Sal, hra, pf, da, total Sal for each employee. The output should be in the order of Question total Sal, hra 15% of Sal, da 10% of sal, pf 5% of sal total salary will be (sal*hra*da)-pf. (Oracle DB) (SQL) select ename,sal,sal*15/100 HRA, sal*5/100 PF, sal*10/100 Answer DA,sal+sal*15/100-sal*5/100+sal*10/100 TOTAL_SALARY from emp How to display dept numbers and total number of employees within each group. (Oracle DB) (SQL) Answer select deptno,count(*) from emp group by deptno; Question

How to display the various jobs and total number of employees with each job group. (Oracle DB) (SQL) Answer select job, count(*) from emp group by job; Question

How to display department numbers and total salary for each department. (Oracle DB) (SQL) Answer select deptno, sum(sal) from emp group by deptno; Question

How to display department numbers and maximum salary for each department. (Oracle DB) (SQL) Answer select deptno, max(sal),min(sal) from emp group by deptno; Question

How to display the various jobs and total salary for each job. (Oracle DB) (SQL) Answer select job, sum(sal) from emp group by job; Question

How to display each job along with minimum sal being paid in each job group. (Oracle DB) (SQL) Answer select job, min(sal) from emp group by job;

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How to display the department numbers with more than three employees in each dept. (Oracle DB) (SQL) Answer select deptno, count(*) from emp group by deptno having count(*)>3; Question

How to display the various jobs along with total sal for each of the jobs where total sal is greater than 40000. (Oracle DB) (SQL) Answer select job, sum(sal) from emp group by job having sum(sal)>40000; Question

How to display the various jobs along with total number of employees in Question each job. The output should contain only those jobs with more than three employees. (Oracle DB) (SQL) Answer select job, count(*) from emp group by job having count(*)>3; Question How to display the name of emp who earns highest sal. (Oracle DB) Answer select ename from emp where sal=(select max(sal) from emp);

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How to display the employee number and name of employee working as CLERK and earning highest salary among CLERKS. (Oracle DB) (SQL) select empno, ename from emp where job='CLERK' and sal=(select Answer max(sal) from emp where job='CLERK'); Question

How to display the names of the salesman who earns a salary more than the highest salary of any clerk. (Oracle DB) (SQL) select ename from emp where job='SALESMAN' and sal > Answer (select max(sal) from emp where job='CLERK'); Question

How to display the names of clerks who earn salary more than that of James of that of sal lesser than that of Scott. (Oracle DB) (SQL) select ename from emp where job='CLERK' and sal<(select sal from emp Answer where ename='SCOTT') and sal>(select sal from emp where ename='JAMES'); Question

How to display the names of employees who earn a Sal more than that of James or that of salary greater than that of Scott. (Oracle DB) (SQL) select ename from emp where sal < Answer (select sal from emp where ename='SCOTT') and sal > (select sal from emp where ename='JAMES'); Question

How to display the names of the employees who earn highest salary in their respective departments. (Oracle DB) (SQL) select * from emp e where sal = Answer (select max(sal) from emp where deptno=e.deptno) Question

How to display the names of employees who earn highest salaries in their respective job groups. (Oracle DB) (SQL) select * from emp e where sal in Answer (select max(sal) from emp group by job having e.job=job)

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Question How to display the employee names who are working in accountings

dept. (Oracle DB) (SQL) select ename from emp where deptno = (select deptno from dept where dname="ACCOUNTING"); Answer (or) select ename from emp where deptno in (select deptno from dept where dname="ACCOUNTING"); How to display the employee names who are working in Chicago. (Oracle DB) (SQL) select ename from emp where deptno = Answer (select deptno from dept where loc='CHICAGO'); Question

How to display the job groups having total salary greater then the maximum salary for managers. (Oracle DB) (SQL) select job, sum(sal) from emp group by job having sum(sal) > Answer (select max(sal) from emp where job='MANAGER'); Question

How to display the names of employees from department number 10 with Question salary greater than that of any employee working in other departments. (Oracle DB) (SQL) select ename,sal,deptno from emp e where deptno=10 and sal > Answer any(select sal from emp where e.deptno!=deptno); How to display the names of employee from department number 10 with Question salary greater then that of all employee working in other departments. (Oracle DB) (SQL) select ename, sal, deptno from emp e where deptno=10 and sal > Answer any(select sal from emp where e.deptno != deptno); Question How to display the names of employees in Upper case. (Oracle DB) Answer select upper(ename) from emp;

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Question How to display the names of employees in lower case. (Oracle DB) Answer select lower(ename) from emp;

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Question How to display the name of employees in proper case. (Oracle DB) Answer select initcap(ename) from emp;

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How to find out the length of your name using appropriate function. (Oracle DB) (SQL) Answer select length('India') from dual; Question

Question How to display the length of all employees' names. (Oracle DB) Answer select sum(length(ename)) from emp;

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How to display the name of the employee concatenate with EMP no. (Oracle DB) (SQL) select ename||empno from emp; Answer (or) select concat(ename,empno) from emp; Question

How to Use appropriate function and extract 3 characters starting from 2 Question characters from the following string 'Oracle' i.e. the output should be 'rac'. (Oracle DB) (SQL) Answer select substr('oracle',2,3) from dual; How to find the first occurrence of character a from the following string 'computer maintenance corporation'. (Oracle DB) (SQL) Answer select instr('computer maintenance corporation','a',1,1) from dual; Question

How to Replace every occurrence of alphabet A with B in the string Allen's (user translate function). (Oracle DB) (SQL) Answer select replace('Allens','A','b') from dual; Question

How to display the information from EMP table. Wherever job 'manager' Question is found it should be displayed as boss(replace function). (Oracle DB) (SQL) Answer select empno, ename, replace(job, 'MANAGER', 'Boss') JOB from emp; How to display empno, ename, deptno from EMP table. Instead of display Question department numbers display the related department name (use decode function). (Oracle DB) (SQL) select e.empno, e.ename, d.dname from emp e,dept d where e.deptno = Answer d.deptno; Question How to display your age in days. (Oracle DB) (SQL) Answer select round(sysdate-to_date('15-aug-1947')) from dual; Question How to display your age in months. (Oracle DB) (SQL) select floor(months_between(sysdate,'15-aug-1947')) "age in months" Answer from dual; How to display current date as 15th august Friday nineteen forty seven. (Oracle DB) (SQL) Answer select to_char(sysdate,'ddth month day year') from dual; Question

How to display the following output for each row from EMP table as 'scott Question has joined the company on Wednesday 13th august nineteen ninety'. (Oracle DB) (SQL) Answer select ename||' has joined the company on '||to_char(hiredate,'day ddth

month year') from emp; How to find the date of nearest Saturday after current day. (Oracle DB) (SQL) Answer select next_day(sysdate, 'SATURDAY') from dual; Question

Question How to display current time. (Oracle DB) (SQL) Answer select to_char(sysdate,'hh:mi:ss') Time from dual; How to display the date three months before the current date. (Oracle DB) (SQL) Answer select add_months(sysdate,-3) from dual; Question

How to display the common jobs from department number 10 and 20. (Oracle DB) (SQL) select job from emp where deptno=10 and job in(select job from emp where deptno=20); Answer (or) select job from emp where deptno=10 intersect select job from emp where deptno=20; Question

How to display the jobs found in department number 10 and 20 eliminate duplicate jobs. (Oracle DB) (SQL) select distinct(job) from emp where deptno=10 and job in(select job from emp where deptno=20); Answer (or) select job from emp where deptno=10 intersect select job from emp where deptno=20; Question

Question How to display the jobs which are unique to dept no 10. (Oracle DB) (SQL) select job from emp where deptno=10 minus select job from emp where deptno!=10; Answer (or) select job from emp where deptno = 10 and job not in (select job from emp where deptno<>10); How to display the details of those who do not have any person working under them. (Oracle DB) (SQL) select empno from emp where empno not in (select mgr from emp where Answer mgr is not null); Question

How to display the details of employees who are in sales dept and grade is 3. (Oracle DB) (SQL) Answer select * from emp where sal>=(select losal from salgrade where grade=3) and sal<=(select hisal from salgrade where grade=3) and deptno=(select deptno from dept Question

where dname='SALES'); How to display those who are not managers and who are managers any one. (Oracle DB) (SQL) select * from emp where empno in(select mgr from emp) union Answer select * from emp where empno not in(select mgr from emp where mgr is not null); Question

How to display those employees whose name contains not less than 4 chars. (Oracle DB) (SQL) Answer Select * from emp where length(ename)>4;

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How to display those departments whose name start with 'S' while location name end with 'O'. (Oracle DB) (SQL) Answer select * from dept where dname like 'S%' and loc like '%O'; Question

How to display those employees whose manager name is JONES. (Oracle DB) (SQL) select * from emp where mgr=(select empno from emp where Answer ename='JONES'); Question

How to display those employees whose salary is more than 3000 after giving 20% increment. (Oracle DB) (SQL) select * from emp where sal*120/100 > 3000; Answer (or) select * from emp where sal+sal*20/100 > 3000; Question

Question How to display all employees with there dept name. (Oracle DB) (SQL) Answer select ename, dname from emp e, dept d where e.deptno = d.deptno; Question How to display ename who are working in sales dept. (Oracle DB) (SQL) select empno, ename from emp where deptno=(select deptno from dept Answer where dname='SALES'); How to display employee name, deptname, salary and comm. for those Question Sal in between 2000 to 5000 while location is Chicago. (Oracle DB) (SQL) select empno,ename,deptno from emp where deptno=(select deptno Answer from dept where loc='CHICAGO') and sal between 2000 and 5000; How to display those employees whose salary greater than his manager salary. (Oracle DB) (SQL) select * from emp e where sal>(select sal from emp where Answer empno=e.mgr); Question

How to display those employees who are working in the same dept where his manager is working. (Oracle DB) (SQL) select * from emp e where deptno = (select deptno from emp where Answer empno=e.mgr); Question

How to display those employees who are not working under any manger. (Oracle DB) (SQL) Answer select * from emp where mgr is null or empno=mgr; Question

How to display grade and employees name for the dept no 10 or 30 but Question grade is not 4, while joined the company before 31-dec-82. (Oracle DB) (SQL) select empno,ename,sal,deptno,hiredate,grade from emp e,salgrade s where e.sal>=s.losal Answer and e.sal<=s.hisal and deptno in(10,30) and grade<>4 and hiredate<'01dec-1981'; How to update the salary of each employee by 10% increments that are not eligible for commission. (Oracle DB) (SQL) Answer update emp set sal=sal+(sal*10/100) where comm is null; Question

How to delete those employees who joined the company before 31Question dec-82 while there dept location is 'NEW YORK' or 'CHICAGO'. (Oracle DB) (SQL) delete from emp where hiredate<'31-dec-1982' and deptno in Answer (select deptno from dept where loc in('NEW YORK','CHICAGO')); How to display employee name, job, deptname, location for all who are working as managers. (Oracle DB) (SQL) select ename,job,dname,loc from emp e, dept d where Answer e.deptno=d.deptno and empno in (select mgr from emp); Question

How to display those employees whose manager names is Jones, and also display there manager name. (Oracle DB) (SQL) select e.empno, e.ename, m.ename MANAGER from emp e, emp m Answer where e.mgr=m.empno and m.ename='JONES'; Question

How to display name and salary of ford if his Sal is equal to high Sal of his grade. (Oracle DB) (SQL) select ename,sal from emp e where ename='FORD' and sal=(select hisal Answer from salgrade where grade=(select grade from salgrade where e.sal>=losal and e.sal<=hisal)); Question

Question How to display employee name, his job, his dept name, his manager name, his grade and make out of an

under department wise. (Oracle DB) (SQL) break on deptno; select d.deptno, e.ename, e.job, d.dname, m.ename, s.grade from Answer emp e, emp m, dept d, salgrade s where e.deptno=d.deptno and e.sal between s.losal and s.hisal and e.mgr=m.empno order by e.deptno; How to List out all the employees name, job, and salary grade and department name for every one in the Question company except 'CLERK'. Sort on salary display the highest salary. (Oracle DB) (SQL) select empno, ename, sal, dname, grade from emp e, dept d, salgrade s Answer where e.deptno=d.deptno and e.sal between s.losal and s.hisal and e.job<>'CLERK' order by sal; How to display employee name, his job and his manager. display also employees who are without manager. (Oracle DB) (SQL) select e.ename, e.job, m.ename Manager from emp e,emp m where Answer e.mgr=m.empno union select ename,job,'no manager' from emp where mgr is null; Question

Question How to find out the top 5 earner of company. (Oracle DB) (SQL) select * from emp e where 5>(select count(*) from emp where sal>e.sal) Answer order by sal desc; How to display the name of those employees who are getting highest salary. (Oracle DB) (SQL) Answer select empno,ename,sal from emp where sal=(select max(sal) from emp);

Question

How to display those employees whose salary is equal to average of maximum and minimum. (Oracle DB) (SQL) Answer select * from emp where sal=(select (max(sal)+min(sal))/2 from emp); Question

How to display count of employees in each department where count greater than 3. (Oracle DB) (SQL) Answer select deptno, count(*) from emp group by deptno having count(*)>3; Question

How to display dname where at least 3 are working and display only dname. (Oracle DB) (SQL) select dname from dept where deptno in Answer (select deptno from emp group by deptno having count(*)>3); Question

How to display name of those managers name whose salary is more than average salary of secompany. (Oracle DB) (SQL) select ename, sal from emp where empno in(select mgr from emp) and Answer sal > (select avg(sal) from emp); Question

How to display those managers name whose salary is more than an average salary of his employees. (Oracle DB) (SQL) select ename, sal from emp e where empno in(select mgr from emp) and Answer e.sal>(select avg(sal) from emp where mgr=e.empno); Question

How to display employee name, Sal, comm and net pay for those employees whose net pay are greater Question than or equal to any other employee salary of the company? (Oracle DB) (SQL) select ename, sal, comm, sal+nvl(comm,0) netPay from emp where Answer sal+nvl(comm.,0)>=any(select sal from emp); How to display those employees whose salary is less than his manager but more than salary of any other managers. (Oracle DB) (SQL) select * from emp e where sal<(select sal from emp where empno = Answer e.mgr) and sal>any(select sal from emp where empno!=e.mgr); Question

How to find out the last 5(least) earner of the company? (Oracle DB) (SQL) select * from emp e where 5>(select count(*) from emp where sal<e.sal) Answer order by sal; Question

How to find out the number of employees whose salary is greater than there manager salary? (Oracle DB) (SQL) select count(*) from emp e where sal>(select sal from emp where Answer empno=e.mgr); Question

How to display those manager who are not working under president but they are working under any other manager? (Oracle DB) (SQL) select * from emp e where mgr in(select empno from emp where Answer ename<>'KING');

Question

How to delete those department where no employee working? (Oracle DB) (SQL) delete from dept d where 0=(select count(*) from emp where Answer deptno=d.deptno); Question

How to delete those records from EMP table whose deptno not available in dept table? (Oracle DB) (SQL) Answer delete from emp where deptno not in(select deptno from dept); Question

How to display those earners whose salary is out of the grade available in Sal grade table? (Oracle DB) (SQL) select * from emp where sal<(select min(losal) from salgrade) or Answer sal>(select max(hisal) from salgrade); Question

How to display employee name, Sal, comm. and whose net pay is greater than any other in the company? (Oracle DB) (SQL) Select ename, sal, comm from emp where sal+sal*15/100-sal*5/100 Answer +sal*10/100 = (select max(sal+sal*15/100-sal*5/100+sal*10/100) from emp); Question

How to display name of those employees who are going to retire 31Question dec-99. If the maximum job is period is 18 years? (Oracle DB) (SQL) Answer select * from emp where (to_date('31-dec-1999')-hiredate)/365>18; How to display those employees whose salary is ODD value? (Oracle DB) (SQL) Answer select * from emp where mod(sal,2)=1; Question

How to display those employees whose salary contains at least 4 digits? (Oracle DB) (SQL) Answer select * from emp where length(sal)>=4; Question

How to display those employees who joined in the company in the month of DEC? (Oracle DB) (SQL) Answer select * from emp where upper(to_char(hiredate,'mon'))='DEC'; Question

How to display those employees whose name contains "A"? (Oracle DB) (SQL) Answer select * from emp where instr(ename,'A',1,1)>0; Question

How to display those employees whose deptno is available in salary? (Oracle DB) (SQL) Answer select * from emp where instr(sal,deptno,1,1)>0;

Question

How to display those employees whose first 2 characters from hire datelast 2 characters of salary? (Oracle DB) (SQL) select substr(hiredate,0,2)||substr(sal,length(sal)-1,2) from emp; Answer select concat( substr(hiredate,0,2), substr(sal,length(sal)-1,2) ) from emp; Question

How to display those employees whose 10% of salary is equal to the year of joining? (Oracle DB) (SQL) Answer select * from emp where to_char(hiredate,'yy')=sal*10/100; Question

How to display those employees who are working in sales or research? (Oracle DB) (SQL) select * from emp where deptno in(select deptno from dept where dname Answer in('SALES','RESEARCH')); Question

Question How to display the grade of Jones? (Oracle DB) (SQL) select grade from salgrade where losal<=(select(sal) from emp where Answer ename='JONES') and hisal>=(select(sal) from emp where ename='JONES'); How to display those employees who joined the company before 15th of the month? (Oracle DB) (SQL) select empno,ename from emp where hiredate<(to_date('15-'|| Answer to_char(hiredate,'mon')||'-'||to_char(hiredate,'yyyy'))); Question

How to delete those records where no of employee in a particular department is less than 3? (Oracle DB) (SQL) delete from emp where deptno in(select deptno from emp group by Answer deptno having count(*)>3); Question

How to delete those employees who joined the company 21 years back from today? (Oracle DB) (SQL) select * from emp where round((sysdate-hiredate)/365)>21; or Answer select * from emp where (to_char (sysdate, 'yyyy')-to_char (hiredate ,'yyyy') )>21; Question

How to display the department name the no of characters of which is Question equal to no of employees in any other department? (Oracle DB) (SQL) Select dname from dept where length(dname) in (select count(*) from Answer emp group by deptno); How to display those employees who are working as manager? (Oracle DB) (SQL) Answer select * from emp where empno in(select mgr from emp); Question

How to count the no of employees who are working as manager (use set operation)? (Oracle DB) (SQL) Answer select count(*) from emp where empno in(select mgr from emp);

Question

How to display the name of then dept those employees who joined the company on the same date? (Oracle DB) (SQL) select empno,ename,hiredate,deptno from emp e where hiredate in Answer (select hiredate from emp where empno<>e.empno); Question

How to display the manager who is having maximum number of employees working under him? (Oracle DB) (SQL) Select mgr from emp group by mgr having count(*)=(select Answer max(count(mgr)) from emp group by mgr); Question

How to list out employees name and salary increased by 15% and expressed as whole number of dollars? (Oracle DB) (SQL) select empno,ename,lpad(concat('$',round(sal*115/100)),7) salary from Answer emp; Question

How to produce the output of the EMP table "EMPLOYEE_AND_JOB" for ename and job? (Oracle DB) (SQL) Answer select * from EMPLOYEE_AND_JOB; Question

How to list all employees with hire date in the format 'June 4 1988'? (Oracle DB) (SQL) Answer select to_char(hiredate,'month dd yyyy') from emp; Question

How to print a list of employees displaying 'Less Salary' if less than 1500 if exactly 1500 display Question as 'Exact Salary' and if greater than 1500 display 'More Salary'? (Oracle DB) (SQL) select empno,ename,'Less Salary '||sal from emp where sal<1500 union Answer select empno,ename,'More Salary '||sal from emp where sal>1500 union select empno,ename,'Exact Salary '||sal from emp where sal=1500 How to write query to calculate the length of employee has been with the company? (Oracle DB) (SQL) Answer Select round(sysdate-hiredate) from emp; Question

How to display those mangers who are getting less than his employees Sal. (Oracle DB) (SQL) Select empno from emp e where sal
How to print the details of all the employees who are sub ordinate to Blake. (Oracle DB) (SQL) Select * from emp where mgr=(select empno from emp where Answer ename='BLAKE'); Question

How to display those who working as manager using co related sub query. (Oracle DB) (SQL) Answer Select * from emp where empno in(select mgr from emp);

Question

How to display those employees whose manger name is Jones and also with his manager name. (Oracle DB) (SQL) Select * from emp where mgr=(select empno from emp where Answer ename='JONES') union select * from emp where empno=(select mgr from emp where ename='JONES'); Question

How to define variable representing the expressions used to calculate on employee's total annual renumaration. (Oracle DB) (SQL) Answer define emp_ann_sal=(sal+nvl(comm,0))*12; Question

How to use the variable in a statement which finds all employees who can earn 30,000 a year or more. (Oracle DB) (SQL) Answer select * from emp where &emp_ann_sal rel="nofollow">30000; Question

How to find out how many mangers are there with out listing them. (Oracle DB) (SQL) Answer Select count (*) from EMP where empno in (select mgr from EMP); Question

How to find out the avg sal and avg total remuneration for each job type remember salesman earn commission. (Oracle DB) (SQL) select job,avg(sal+nvl(comm,0)),sum(sal+nvl(comm,0)) from emp group Answer by job; Question

How to check whether all employees number are indeed unique. (Oracle DB) (SQL) select count(empno),count(distinct(empno)) from emp having Answer count(empno)=(count(distinct(empno))); Question

How to list out the lowest paid employees working for each manager, Question exclude any groups where min sal is less than 1000 sort the output by sal. (Oracle DB) (SQL) select e.ename,e.mgr,e.sal from emp e where sal in(select min(sal) from Answer emp where mgr=e.mgr) and e.sal>1000 order by sal; How to list ename, job, annual sal, deptno, dname and grade who earn 30000 per year and who are not clerks. (Oracle DB) (SQL) Select e.ename, e.job, (e.sal+nvl(e.comm,0))*12, e.deptno, d.dname, s.grade from emp e, salgrade s , dept d Answer where e.sal between s.losal and s.hisal and e.deptno=d.deptno and (e.sal+nvl(comm,0))*12> 30000 and e.job <> 'CLERK'; Question

How to find out the all employees who joined the company before their manager. (Oracle DB) (SQL) Select * from emp e where hiredate<(select hiredate from emp where Answer empno=e.mgr); Question

How to list out the all employees by name and number along with their Question manager's name and number also display 'No Manager' who has no manager. (Oracle DB) (SQL) Answer select e.empno,e.ename,m.empno Manager,m.ename ManagerName from emp e,emp m where e.mgr=m.empno

union select empno,ename,mgr,'No Manager' from emp where mgr is null; How to find out the employees who earned the highest Sal in each job typed sort in descending Sal order. (Oracle DB) (SQL) select * from emp e where sal =(select max(sal) from emp where Answer job=e.job); Question

How to find out the employees who earned the min Sal for their job in ascending order. (Oracle DB) (SQL) select * from emp e where sal =(select min(sal) from emp where Answer job=e.job) order by sal; Question

How to find out the most recently hired employees in each dept order by hire date (Oracle DB) (SQL) Answer select * from emp order by deptno,hiredate desc; Question

How to display ename, sal and deptno for each employee who earn a Sal Question greater than the avg of their department order by deptno (Oracle DB) (SQL) select ename,sal,deptno from emp e where sal>(select avg(sal) from Answer emp where deptno=e.deptno) order by deptno; How to display the department where there are no employees (Oracle DB) (SQL) select deptno,dname from dept where deptno not in(select Answer distinct(deptno) from emp); Question

How to display the dept no with highest annual remuneration bill as compensation. (Oracle DB) (SQL) select deptno,sum(sal) from emp group by deptno having sum(sal) = Answer (select max(sum(sal)) from emp group by deptno); Question

How to find in which year did most people join the company. display the year and number of employees (Oracle DB) (SQL) select count(*),to_char(hiredate,'yyyy') from emp group by Answer to_char(hiredate,'yyyy'); Question

Question How to display avg sal figure for the dept (Oracle DB) Answer select deptno,avg(sal) from emp group by deptno;

(SQL)

How to write a query of display against the row of the most recently hired Question employee. display ename hire date and column max date showing. (Oracle DB) (SQL) Answer select empno,hiredate from emp where hiredate=(select max (hiredate)

from emp); How to display employees who can earn more than lowest Sal in dept no 30 (Oracle DB) (SQL) select * from emp where sal>(select min(sal) from emp where Answer deptno=30);

Question

How to find employees who can earn more than every employees in dept no 30 (Oracle DB) (SQL) select * from emp where sal>(select max(sal) from emp where Answer deptno=30); select * from emp where sal>all(select sal from emp where deptno=30); Question

Question How to select dept name dept no and sum of Sal (Oracle DB) (SQL) break on deptno on dname; Answer select e.deptno,d.dname,sal from emp e, dept d where e.deptno=d.deptno order by e.deptno; How to find all dept's which have more than 3 employees (Oracle DB) (SQL) Answer select deptno from emp group by deptno having count(*)>3; Question

How to display the half of the enames in upper case and remaining lower case (Oracle DB) (SQL) select concat ( upper ( substr ( ename, 0 , length (ename)/ 2) ), Answer lower (substr (ename, length(ename) / 2+1, length(ename) )) ) from emp; Question

Question How to create a copy of emp table. (Oracle DB) Answer Create table emp1 as select * from emp;

(SQL)

Question How to select ename if ename exists more than once. (Oracle DB) (SQL) select distinct(ename) from emp e where ename in(select ename from Answer emp where e.empno<>empno); Question How to display all enames in reverse order. (Oracle DB) Answer select ename from emp order by ename desc;

(SQL)

How to display those employee whose joining of month and grade is equal. (Oracle DB) (SQL) select empno,ename from emp e, salgrade s where e.sal between s.losal Answer and s.hisal and to_char(hiredate,'mm')=grade; Question

Question

How to display those employee whose joining date is available in dept no (Oracle DB) (SQL)

Answer

select * from emp where to_char(hiredate,'dd')=deptno;

How to display those employees name as follows A ALLEN, B BLAKE (Oracle DB) (SQL) Answer select substr(ename,1,1)||' '||ename from emp;

Question

Question How to list out the employees ename, sal, PF from emp (Oracle DB) Answer Select ename,sal,sal*15/100 PF from emp; Question How to create table emp with only one column empno (Oracle DB) Answer Create table emp (empno number(5));

(SQL)

(SQL)

How to add this column to emp table ename Varchar(20). (Oracle DB) (SQL) Answer alter table emp add ename varchar2(20) not null; Question

How to add primary key constraint after the table has been created? (Oracle DB) (SQL) Answer alter table emp add constraint emp_empno primary key (empno); Question

How to increase the length of ename column to 30 characters. (Oracle DB) (SQL) Answer alter table emp modify ename varchar2(30); Question

Question How to add salary column to emp table. (Oracle DB) Answer alter table emp add sal number(7,2);

(SQL)

I want to give a validation saying that sal cannot be greater 10,000(note give a name to this column). How to do this? (Oracle DB) (SQL) Answer alter table emp add constraint emp_sal_check check (sal<10000); Question

How to add column called as mgr to your emp table. This column should Question be related to empno. Give a command to add this constraint (Oracle DB) (SQL) Alter table emp add mgr number (5); Answer Alter table emp add constraint emp_mgr foreign key (empno); How to add dept no column to your emp table. This dept no column Question should be related to deptno column of dept table (Oracle DB) (SQL) Alter table emp add deptno number (3); Answer Alter table emp1 add constraint emp1_deptno foreign key(deptno) references dept(deptno);

How to create table called as new emp. Using single command create this Question table as well as to get data into this table (use create table as) (Oracle DB) (SQL) Answer create table newemp as select *from emp; How to create table called as newemp. This table should contain only empno,ename, dname (Oracle DB) (SQL) create table newemp as select empno,ename,dname from emp e , dept d Answer where e.deptno=d.deptno; Question

How to delete the rows of employees who are working in the company for more than 2 years. (Oracle DB) (SQL) Answer delete from emp where floor(sysdate-hiredate)>2*365; Question

How to provide a commission to employees who are not earning any commission. (Oracle DB) (SQL) Answer update emp set comm=300 where comm is null; Question

Write a query, If any employee has commission his commission should be incremented by 10% of his salary. (Oracle DB) (SQL) Answer update emp set comm=comm*10/100 where comm is not null; Question

How to display employee name and department name for each employee. (Oracle DB) (SQL) Answer select ename,dname from emp e, dept d where e.deptno=d.deptno; Question

How to display employee number, name and location of the department in which he is working. (Oracle DB) (SQL) Answer Select empno, ename, loc from emp e, dept d where e.deptno=d.deptno; Question

How to display ename, dname even if there no employees working in a particular department(use outer join). (Oracle DB) (SQL) Select ename, dname from emp e, dept d where e.deptno (+)= Answer d.deptno; Question

Question How to display employee name and his manager name. (Oracle DB) (SQL) Answer Select e.ename, m.ename from emp e, emp m where e.mgr=m.empno; How to display the department name along with total salary in each department. (Oracle DB) (SQL) Answer Select deptno, sum(sal) from emp group by deptno; Question

Question

How to display the department name and total number of employees in each department. (Oracle DB) (SQL)

Answer

select deptno,count(*) from emp group by deptno;

Question How to display the current date and time (Oracle DB) (SQL) Answer select to_char(sysdate,'month mon dd yy yyyy hh:mi:ss') from dual; Question What is the advantage of DispatchAction class? (Struts) Using DispatchAction class you can overcome writing one Action class for one business entity. Answer Instead you can write multiple business entities in one Action class and call the business method that is required by the application. Can you write your own method in Action class other than execute() and Question call that user method directly? (Struts) Answer Yes, We can create any number of methods in Action class and instruct the action tag in struts-config.xml file to call that user method. This is possible by using DispatchAction class. A sample code would look like this public class StudentAction extends DispatchAction { public ActionForward create(ActionMapping mapping, ActionForm form, HttpServletRequest req, HttpServletResponse res) throws Exception { return something; } public ActionForward read(ActionMapping mapping, ActionForm form, HttpServletRequest req, HttpServletResponse res) throws Exception { return something; } public ActionForward update(ActionMapping mapping, ActionForm form, HttpServletRequest req, HttpServletResponse res) throws Exception { return something; } }

In the above Action class if the user wants to call any method, he would do something like this in my struts-config.xml file parameter="methodToCall" scope="request" validate="true" input="/student.jsp"> In this configuration file the value of the parameter methodToCall determines the method in the StudentAction class to be invoked. For example, the JSP from which you are calling this action, can set the methodToCall parameter to any of the methods it wants to. How you will enable front-end validation based on the xml in validation.xml? (Struts) The tag to allow front-end validation based on the xml in validation.xml. For example the code: generates the client side java script for the form "logonForm" as defined in the validation.xml file. The when added in the jsp file generates the client site validation script. Question

What happens when the user has not provided required information that is needed by the struts Question validator framework? Explain how struts handles to display the validation messages? (Struts) When the user has not provided any required information, the validation messages are displayed to the user and this block of code does this job. Answer

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Question Brief on how struts validation is working (Struts) Struts uses 2 XML files to validate the user form. validator-rules.xml and validation.xml files. The validator-rules.xml defines a set of standard validations and these validations Answer are used in validation.xml file. Generally the rules defined in validator-rules.xml file are typical java class files that does the validation in the backend. Question Explain about ActionForm class (Struts) ActionForm class defines get and set methods for information provided by the client in the Answer HTML form. It has reset (for reseting the defined variables to its initital state) and validate method for validating the user input. Question Explain about Action Class (Struts) The Action Class is part of the Model and is a wrapper around the business logic. The purpose of Action Class is to translate the HttpServletRequest to the business logic. To use the Action, we need to Subclass and overwrite the execute() method. In the Action Class all the database/business processing are done. It is advisable to Answer perform all the database related stuffs in the Action Class. The ActionServlet (command) passes the parameterize class to Action Form using the execute() method. The return type of the execute method is ActionForward which is used by the Struts Framework to forward the request to the file as per the value of the returned ActionForward object. Do you write an ActionForm class for every form that you need to process? (Struts) Answer No, I define DynaValidatorForm as this class takes care of defining get/set methods and doing the validation for me. And I need to define this class in strutsconfig.xml file. An e.g. would be Question



Question How would struts handle "messages" required for the application? (Struts) Messages are defined in a .properties file as name value pairs. To make these messages available to the application, you need to place the .properties file in WEBINF/classes folder. And define the following tag in struts-config.xml file Answer <message-resources parameter="ApplicationResources" /> And in order to display a message in a jsp you would use What is the name of the XML file which handles requests and gives Question responses? And acts as a controller? (Struts) Answer struts-config.xml file Question Explain about ActionServlet in Apache Struts (Struts) ActionServlet acts as a controller in Struts. There can be only one ActionServlet defined Answer for a context. ActionServlet is defined in web.xml file of a context e.g. WEB-INF/web.xml Question Can I have more than one struts-config.xml file? (Struts) Answer Yes. A sample configuration in web.xml file would look like this <servlet> <servlet-name>action <servlet-class>org.apache.struts.action.ActionServlet <param-name>config <param-value>/WEB-INF/struts-config.xml <param-name>config/exercise <param-value>/WEB-INF/exercise/struts-config.xml <param-name>config/upload <param-value>/WEB-INF/upload/struts-config.xml

Question What is Struts Validator Framework? (Struts) Struts Framework provides the functionality to validate the form data. It can be use to validate the data on the users browser as well as on the server side. Struts Framework emits Answer the java scripts and it can be used to validate the form data on the client browser. Server side validation of form can be accomplished by sub classing your From Bean with DynaValidatorForm class. Question What is the role of ActioMapping object in Struts Action class? (Struts) An ActionMapping knows about the mapping of a particular request to an Answer instance of a particular Action class. What happens internally when actionMappings.findForward("target") method is called in Action class. (Struts) actionMappings.findForward("target") method returns ActionForward object. An ActionForward represents a destination to which the controller, RequestProcessor, might be directed to perform a Answer RequestDispatcher.forward or HttpServletResponse.sendRedirect to, as a result of processing activities of an Action class. Question

Expalin JSP,Servlet Combination VS Struts Which is better and how? (Struts) Struts is a framework based on Model-View-Controller (MVC) design paradigm. The Model represents the business or database code, the View represents the page design code, and the Controller represents the navigational code. The Struts framework is designed to help developers create web applications Answer that utilize a MVC architecture. Struts framework internally uses Servlet Technology for processing the request and response. Question

Sun Microsystems has provided the JSP / Servlet API specifications, and its the web server or application server vendor who implements the logic for the API. Question Explain ActionForward against JSP:forward (Struts) jsp:forward is part of JSTL and will be used in JSP pages. It internally uses RequestDispatcher to forward the request to new jsp or servlet page Answer ActionForward is part of Struts framework and is used in Action class. It internally uses RequestDispatcher to forward the request to ActionServlet Question How to handle multiple forms using Struts?

(Struts)

What I understand from your question is, You are having multiple forms in different pages and needs to process this information to server at the last form. e.g. Form 1, 2, 3 and 4 in 4 different web pages and process the data to server at the final web page. It is easy to have a form that spans multiple jsp's with struts. Make sure that your form bean is part Answer of the session and not the request object. Also be sure to include some type of form validation routine that checks to make sure all of the required information has been submitted before you send the whole form off to the server for processing. You cannot have single web page with multiple forms and submit all of it at once. When you click on submit button in a form only the form that has submit button will be submitted to server.

Question What are the tokens available in Struts? (Struts) Answer Any one would like like to answer this question? Question What is the difference between Action messages & Action errors? Answer Any one would like like to answer this question?

(Struts)

Why does it take so much time to access an Applet having Swing Components the first time? (Swing) Because behind every swing component are many Java objects and resources. This takes time to create them in memory. JDK 1.3 from Sun Answer has some improvements which may lead to faster execution of Swing applications. Question

Why does JComponent have add() and remove() methods but Component does not? (Swing) because JComponent is a subclass of Container, and can contain other Answer components and jcomponents Question

Question How would you create a button with rounded edges? (Swing) There are 2 ways. The first thing is to know that a JButton?s edges are drawn by a Border. so you can override the Button?s paintComponent(Graphics) method and draw a circle or rounded rectangle Answer (whatever), and turn off the border. Or you can create a custom border that draws a circle or rounded rectangle around any component and set the button?s border to it. Question How would you detect a keypress in a JComboBox?

(Swing)

Answer

Add a KeyListener to the JComboBox?s editor component instead of adding a KeyListener to the JComboBox itself

Why should the implementation of any Swing callback (like a listener) execute quickly? (Swing) Because callbacks are invoked by the event dispatch thread which will be Answer blocked processing other events for as long as your method takes to execute. Question

Why would you use SwingUtilities.invokeAndWait or SwingUtilities.invokeLater? (Swing) I want to update a Swing component but I?m not in a callback. If I want the update to happen immediately (perhaps for a progress bar component) Answer then I?d use invokeAndWait. If I don?t care when the update occurs, I?d use invokeLater. Question

If your UI seems to freeze periodically, what might be a likely reason? (Swing) A callback implementation like ActionListener.actionPerformed or Answer MouseListener.mouseClicked is taking a long time to execute thereby blocking the event dispatch thread from processing other UI events. Question

Question Which Swing methods are thread-safe? (Swing) Answer The only thread-safe methods are repaint(), revalidate(), and invalidate() Why won’t the JVM terminate when I close all the application windows? (Swing) The AWT event dispatcher thread is not a daemon thread. You must Answer explicitly call System.exit to terminate the JVM.

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JFrame is a heavy weight component. Since it extends an awt Frame, is it Thread Safe? (Swing) JFrame itself is, since it is just a java.awt.Frame in essence, but the root Answer pane/content pane is not, so it effectively follows the same rules for Swing containers and is not considered thread safe. Question

What is the difference between invokeAndWait() and invokeLater()? (Swing) invokeAndWait() blocks until the Runnable task is complete; it's synchronous. Answer invokeLater() posts an action event to the event queue and returns immediately; it's asynchronous. Question

Question What is the difference between a Scrollbar and a ScrollPane? (Swing) Answer A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own

scrolling Question What do heavy weight components mean? (Swing) Heavy weight components like Abstract Window Toolkit (AWT), depend on the local windowing toolkit. For example, java.awt.Button is a heavy weight component, when it is running on the Java platform for Unix platform, it maps to a real Motif button. In this relationship, the Motif button is called the peer to the java.awt.Button. If you create two Buttons, Answer two peers and hence two Motif Buttons are also created. The Java platform communicates with the Motif Buttons using the Java Native Interface. For each and every component added to the application, there is an additional overhead tied to the local windowing system, which is why these components are called heavy weight. Question Difference between paint() and paintComponent() (Swing) The key point is that the paint() method invokes three methods in the following order: a) paintComponent() b) paintBorder() c) paintChildren() Answer As a general rule, in Swing, you should be overriding the paintComponent method unless you know what you are doing. paintComponent() paints only component (panel) but paint() paints component and all its children. What is the difference between paint(), repaint() and update() methods within an applet which contains images? (Swing) paint : is only called when the applet is displayed for the first time, or when part of the applet window has to be redisplayed after it was hidden. repaint : is used to display the next image in a continuous loop by calling the update method. Answer update : you should be aware that, if you do not implement it yourself, there is a standard update method that does the following : · it will reset the applet window to the current background color (i.e. it will erase the current image) it will call paint to construct the new image Question

Question What are peerless components? (Swing) Answer The peerless components are called light weight components Question When should the method invokeLater()be used? (Swing) This method is used to ensure that Swing components are updated Answer through the event-dispatching thread. Question What is the difference between yielding and sleeping? (Threads) When a task invokes its yield() method, it returns to the ready state. Answer When a task invokes its sleep() method, it returns to the waiting state.

Question When a thread blocks on I/O, what state does it enter? Answer A thread enters the waiting state when it blocks on I/O.

(Threads)

Question When a thread is created and started, what is its initial state? (Threads) Answer A thread is in the ready state after it has been created and started. Question What invokes a thread's run() method? (Threads) After a thread is started, via its start() method or that of the Thread Answer class, the JVM invokes the thread's run() method when the thread is initially executed. What method is invoked to cause an object to begin executing as a separate thread? (Threads) The start() method of the Thread class is invoked to cause an object to Answer begin executing as a separate thread. Question

What is the purpose of the wait(), notify(), and notifyAll() methods? (Threads) The wait(),notify(), and notifyAll() methods are used to provide an efficient way for threads to wait for a shared resource. When a thread Answer executes an object's wait() method, it enters the waiting state. It only enters the ready state after another thread invokes the object's notify() or notifyAll() methods. Question

Question What are the high-level thread states? (Threads) Answer The high-level thread states are ready, running, waiting, and dead Question What happens when a thread cannot acquire a lock on an object? (Threads) If a thread attempts to execute a synchronized method or synchronized Answer statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available. How does multithreading take place on a computer with a single CPU? (Threads) The operating system's task scheduler allocates execution time to Answer multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially. Question

What happens when you invoke a thread's interrupt method while it is sleeping or waiting? (Threads) When a task's interrupt() method is executed, the task enters the ready Answer state. The next time the task enters the running state, an InterruptedException is thrown. Question

Question What state is a thread in when it is executing? Answer An executing thread is in the running state

(Threads)

What are three ways in which a thread can enter the waiting state? (Threads) A thread can enter the waiting state by invoking its sleep() method, by blocking on I/O, by unsuccessfully attempting to acquire an object's lock, or Answer by invoking an object's wait() method. It can also enter the waiting state by invoking its (deprecated) suspend() method.

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Question What method must be implemented by all threads? (Threads) All tasks must implement the run() method, whether they are a subclass Answer of Thread or implement the Runnable interface. What are the two basic ways in which classes that can be run as threads may be defined? (Threads) A thread class may be declared as a subclass of Thread, or it may Answer implement the Runnable interface. Question

If there are 20 threads waiting in the waiting pool with same priority, how can you invoke 15th thread from the waiting pool? (Threads) Its not possible do call a particular thread. The methods notify() will calls thread from waiting pool, but there is no guaranty which thread is invoked. Answer The method notifyAll() method puts all the waiting threads from the waiting pool in ready state. Question

Question Explain about Deadlock state of a thread (Threads) Answer Synchronizing run() is a good example of a simple deadlock scenario, where a thread is blocked forever, waiting for something to happen that can't. Let's look at a few examples that are more realistic than this. The most common deadlock scenario occurs when two threads are both waiting for each other to do something. The following (admittedly contrived) code snippet makes what's going on painfully obvious: see code at: http://www.javagalaxy.com:8080/Threads/View.jsp? slno=3&tbl=0 Now, imagine a scenario whereby one thread (call it Wilma) calls fred(), passes through the synchronization of lock_1, and is then preempted, allowing another thread (call it Betty) to execute. Betty calls barney(), acquires lock_2, and tries to acquire lock_1, but can't because Wilma has it. Betty is now blocked, waiting for lock_1 to become available, so Wilma wakes up and tries to acquire lock_2 but can't because Betty has it. Wilma and Betty are now deadlocked. Neither one can ever execute. (Note that lock_1 and lock_2 have to be one-element arrays rather than simple ints, because only objects have monitors in Java; the argument to

synchronized must be an object. An array is a first-class object in Java; a primitive-type such as int is not. Consequently, you can synchronize on it. Moreover, a one-element array is efficient to bring into existence compared to a more elaborate object (like an Integer) since it's both small and does not require a constructor call. Also, note that I can keep the reference to the lock as a simple Object reference, since I'll never access the array elements. Question What is a daemon thread? (Threads) Daemon threads are service providers for other threads running in the same process as the daemon thread. When the only remaining threads in a process are daemon threads, the interpreter exits. This is because when only daemon threads remain, there is no other thread for which a daemon Answer thread can provide a service. An example would be AWT-Event threads. Another example would be a mail daemon. When a mail message is recieved by a mail server it generally needs to be forwarded on to another machine. One way to do this is to have a daemon check for mail every so often and cause the mail to be forwarded.

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Can you explain the difference between green threads and native threads? (Threads) Green threads is thread mechanism implemented in JVM itself. It is blind and can run on any OS, so actually all threads are run in one native thread and scheduling is up to JVM. This is disadvantageously for SMP systems, since only one processor can serve Java application. Native threads is a mechanism based on OS threading mechanism. This allows to use features of hardware and OS. For example,there is IBM's JDK for AIX that supports native threads. The perfomance of applications can be highly imploved by this.

I have created a program with a main method that instantiates and starts Question three threads, the first two of which are daemons. Why daemons does die when normal thread die? (Threads) Because of nature of daemon threads. They are alive if exists at least one Answer "normal user's" thread. Otherwise they die immediately Question When will a Thread Object be garbage collected? (Threads) Since Thread is also an Object, it will only garbage collected when the reference count is zero. You may think it is quite non-sense. the thread is useless when it enter "dead" state. why not garbage collect it? That's Answer because the thread object itself may contain some other useful information even the thread dead , e.g. the result of the execution of the thread. Thus, it is not sensible to do garbage collect when the reference count is not zero. Question What happens when you call yield() on a thread? (Threads) Causes the currently executing thread object to temporarily pause and Answer allow other threads to execute

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What is the difference between "green" threads and "native" threads on Linux OS? (Threads) Native threads can switch between threads pre-emptively, switching control from a running thread to a non-running thread at any time. Green threads only switch when control is explicitly given up by a thread (Thread.yield(), Object.wait(), etc.) or a thread performs a blocking operation (read(), etc.). On multi-CPU machines, native threads can run more than one thread simultaneously by assigning different threads to different CPUs. Green threads run on only one CPU. Native threads create the appearance that many Java processes are running: each thread takes up its own entry in the process table. One clue that these are all threads of the same process is that the memory size is identical for all the threads - they are all using the same memory.

Question What is the difference between Multitasking and Multithreading? (Threads) Multitasking is the ability of an operating system to execute more than one program simultaneously. Though we say so but in reality no two programs on a single processor machine can be executed at the same time. The CPU switches from one program to the next so quickly that appears as if all of the programs are executing at the same time. Multithreading is the ability of an operating system to execute the different parts of the program, called threads, simultaneously. The program has to be designed well so that the different Answer threads do not interfere with each other. This concept helps to create scalable applications because you can add threads as and when needed. Individual programs are all isolated from each other in terms of their memory and data, but individual threads are not as they all share the same memory and data variables. Hence, implementing multitasking is relatively easier in an operating system than implementing multithreading. Which UML diagrams would give a chronological perspective of a scenario? (UML) Answer Sequence Diagram Question

Which UML diagram would you use to view and understand large complex systems, on a high level? (UML) Answer Package Diagrams Question

Give 2 examples of non-functional requirements considered during the Design Phase? (UML) Answer Performance and Security Question

What diagrams would you use in OOAD, to capture Hardware Topology, using Nodes and Links? (UML) Answer Deployment Diagrams Question

Question What is the term used to describe the dependency between classes? Answer Coupling

(UML)

Question Why is XML such an important development? (XML) It removes two constraints which were holding back Web developments:
1. § dependence on a single, inflexible document type (HTML) which was being much abused for tasks it was never designed for;
2. the complexity of full SGML, whose syntax allows many Answer powerful but hard-to-program options.
§ XML allows the flexible development of user-defined document types. It provides a robust, nonproprietary, persistent, and verifiable file format for the storage and transmission of text and data both on and off the Web; and it removes the more complex options of SGML, making it easier to program for.

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Is it possible to write the contents of org.w3c.dom.Document object into an .xml file? (XML) Yes its possible. One to achieve this is by using Xerces. Xerces is an XML parser. You would use the following code

org.apache.xml.serialize.OutputFormat format = new Answer org.apache.xml.serialize.OutputFormat(myDocument); org.apache.xml.serialize.XMLSerializer output = new org.apache.xml.serialize.XMLSerializer(new FileOutputStream(new File("test.xml")), format); output.serialize(myDocument); What is the difference between DOM and SAX? What would you use if an option is given? (XML) Answer DOM parses an XML document and returns an instance of org.w3c.dom.Document. This document object's tree must then be "walked" in order to process the different elements. DOM parses the ENTIRE Document into memory, and then makes it available to you. The size of the Document you can parse is limited to the memory available. Question

SAX uses an event callback mechanism requiring you to code methods to handle events thrown by the parser as it encounters different entities within the XML document. SAX throws events as the Document is being parsed. Only the current element is actually in memory, so there is no limit to the size of a

Document when using SAX. The specific parser technology that will be used will be determined by the requirements of your application. If you need the entire document represented, you will most likely use DOM builder implementation. If you only care about parts of the XML document and/or you only need to parse the document once, you might be better served using SAX implementation. Question What is SOAP? (XML) The Simple Object Access Protocol (SOAP) uses XML to define a protocol for the exchange of information in distributed computing environments. Answer SOAP consists of three components: an envelope, a set of encoding rules, and a convention for representing remote procedure calls. Question What is DOM? (XML) The Document Object Model (DOM) is an interface specification maintained by the W3C DOM Workgroup that defines an application Answer independent mechanism to access, parse, or update XML data. In simple terms it is a hierarchical model that allows developers to manipulate XML documents easily. Question Can you walk us through the steps necessary to parse XML file? DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); Answer factory.setValidating(true); DocumentBuilder domBuilder = factory.newDocumentBuilder(); Document doc = domBuilder.parse(XMLFile);

(XML)

Question Is it necessary to validate XML file against a DTD? (XML) Although XML does not require data to be validated against a DTD, many Answer of the benefits of using the technology are derived from being able to validate XML documents against business or technical architecture rules. Question What is XPath? (XML) XPath stands for XML Path Language XPath is a syntax for defining parts of an XML document XPath is used to navigate through elements and attributes in an XML document Answer XPath contains a library of standard functions XPath is a major element in XSLT XPath is designed to be used by both XSLT and XPointer XPath is a W3C Standard Question What is XSL? (XML) Answer XSLT - a language for transforming XML documents

XSLT is used to transform an XML document into another XML document, or another type of document that is recognized by a browser, like HTML and XHTML. Normally XSLT does this by transforming each XML element into an (X)HTML element. Question What is a DTD and a Schema? (XML) The XML Document Type Declaration contains or points to markup declarations that provide a grammar for a class of documents. This grammar is known as a document type definition or DTD. The DTD can point to an external subset containing markup declarations, or can contain the markup declarations directly in an internal subset, or can even do both. Answer A Schema is: XML Schemas express shared vocabularies and allow machines to carry out rules made by people. They provide a means for defining the structure, content and semantics of XML documents. Schemas are a richer and more powerful of describing information than what is possible with DTDs. Question What is the difference between DTD and Schema? (XML) Answer The first, and probably most significant, difference between XML Schemas and XML DTDs is that XML Schemas use XML document syntax. While transforming the syntax to XML doesn’t automatically improve the quality of the description, it does make those descriptions far more extensible than they were in the original DTD syntax. Declarations can have richer and more complex internal structures than declarations in DTDs, and schema designers can take advantage of XML’s containment hierarchies to add extra information where appropriate - even sophisticated information like documentation. There are a few other benefits from this approach. XML Schemas can be stored along with other XML documents in XMLoriented data stores, referenced, and even styled, using tools like XLink, XPointer, and XSL. The largest addition XML Schemas provide to the functionality of the descriptions is a vastly improved data typing system. XML Schemas provide data-oriented data types in addition to the more documentoriented data types XML 1.0 DTDs support, making XML more suitable for data interchange applications. Built-in datatypes include strings, booleans, and time values, and the XML Schemas draft provides a mechanism for generating additional data types. Using that system, the draft provides support for all of the XML 1.0 data types (NMTOKENS, IDREFS, etc.) as well as data-specific types like decimal,integer, date, and time. Using XML Schemas, developers can build their own libraries of easily interchanged data types and use them inside

schemas or across multiple schemas. The current draft of XML Schemas also uses a very different style for declaring elements and attributes to DTDs. In addition to declaring elements and attributes individually, developers can create models archetypes - that can be applied to multiple elements and refined if necessary. This provides a lot of the functionality SOX had developed to support object-oriented concepts like inheritance. Archetype development and refinement will probably become the mark of the high-end schema developer, much as the effective use of parameter entities was the mark of the high-end DTD developer. Archetypes should be easier to model and use consistently, however. XML Schemas also support namespaces, a key feature of the W3C’s vision for the future of XML. While it probably wouldn’t be impossible to integrate DTDs and namespaces, the W3C has decided to move on, supporting namespaces in its newer developments and not retrofitting XML 1.0. In many cases, provided that namespace-prefixes don’t change or simply aren’t used, DTD’s can work just fine with namespaces, and should be able to interoperate with namespaces and schema processing that relies on namespaces. There will be a few cases, however, where namespaces may force developers to use the newer schemas rather than the older DTDs.