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EJB Interview Questions Q:What are the different kinds of enterprise beans? A: Different kind of enterrise beans are Stateless session bean, Stateful session bean, Entity bean, Message-driven bean........... Q:What is Session Bean? A: A session bean is a non-persistent object that implements some business logic running on the server. One way to think of a session object........... Q:What is Entity Bean? A: The entity bean is used to represent data in the database. It provides an objectoriented interface to ........... Q:What are the methods of Entity Bean? A: An entity bean consists of 4 groups of methods, create methods........... Q:What is the difference between Container-Managed Persistent (CMP) bean and Bean-Managed Persistent(BMP) ? A: Container-managed persistence (CMP) and bean-managed persistence (BMP). With CMP, the container manages the persistence of the entity bean............ Q:What are the callback methods in Entity beans? A: Callback methods allows the container to notify the bean of events in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface............ Q:What is software architecture of EJB? A: Session and Entity EJBs consist of 4 and 5 parts respectively, a remote interface........... Q:Can Entity Beans have no create() methods? A: Yes. In some cases the data is inserted NOT using Java application,........... Q:What is bean managed transaction? A: If a developer doesn't want a Container to manage transactions, it's possible to implement all database operations manually........... Q:What are transaction attributes? A: The transaction attribute specifies how the Container must manage transactions for a method when a client invokes the method via the enterprise bean’s home or...........

Q:What are transaction isolation levels in EJB? A: Transaction_read_uncommitted , Transaction_read_committed Transaction_repeatable_read...........

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EJB Interview Questions Question:How EJB Invocation happens? Question:Is it possible to share an HttpSession between a JSP and EJB? What happens when I change a value in the HttpSession from inside an EJB? Question:The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes? Question:Can the primary key in the entity bean be a Java primitive type such as int? Question:Can you control when passivation occurs? Question:What is the advantage of using Entity bean for database operations, over directly using JDBC API to do database operations? When would I use one over the other? Question:What is EJB QL? Question:Brief description about local interfaces? Question:What are the special design care that must be taken when you work with local interfaces? Question:What happens if remove( ) is never invoked on a session bean? Question:What is the difference between Message Driven Beans and Stateless Session beans? Question:How can I call one EJB from inside of another EJB? Question:What is an EJB Context? Question:The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes?

Q:How EJB Invocation happens? A: Step 1: Retrieve Home Object reference from Naming Service via JNDI. step 2: Return Home Object reference to the client. step 3: Create me a new EJB Object through Home Object interface. step 4: Create EJB Object from the Ejb Object step 5: Return EJB Object reference to the client. step 6: Invoke business method using EJB Object reference. step 7: Delegate request to Bean (Enterprise Bean). [ Received from Ramana Bhavanasi] TOP Q:Is it possible to share an HttpSession between a JSP and EJB? What happens when I change a value in the HttpSession from inside an EJB? A: 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-by-value", 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 ? in terms of object oriented design. This is because you 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. TOP [ Received from Nishit Kamdar] Q:The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes? A: 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. 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 application server. The way it is implemented is again up to the implementer. TOP [ Received from Vishal Khasgiwala ] Q:Can the primary key in the entity bean be a Java primitive type such as int? A: The primary key can't be a primitive type--use the primitive wrapper 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) TOP [ Received from Prasanna Inamanamelluri ] Q:Can you control when passivation occurs? A: 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. 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). [ Received from Prasanna Inamanamelluri ]

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Q:What is the advantage of using Entity bean for database operations, over directly using JDBC API to do database operations? When would I use one over the other? A: 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. TOP [ Received from Prasanna Inamanamelluri ] Q:What is EJB QL? A: 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 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. TOP [ Received from Prasanna Inamanamelluri ] Q:Brief description about local interfaces?

A: EEJB 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. TOP [ Received from Prasanna Inamanamelluri ] Q:What are the special design care that must be taken when you work with local interfaces? A: EIt 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-byreference. This means that in order to use local interfaces safely, application developers need to carefully consider potential deployment scenarios up 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 longterm costs of these choices, especially when changing requirements and component reuse are taken into account, need to be factored into the design decision. TOP [ Received from Prasanna Inamanamelluri ] Q:What happens if remove( ) is never invoked on a session bean? A: 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. 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. TOP [ Received from Prasanna Inamanamelluri ] Q:What is the difference between Message Driven Beans and Stateless Session beans?

A: 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 cannot be directly accessed by internal or external clients. Clients interact with messagedriven 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. TOP [ Received from Prasanna Inamanamelluri ] Q:How can I call one EJB from inside of another EJB? A: EJBs can be clients of other EJBs. It just works. Use JNDI to locate the Home Interface of the other bean, then acquire an instance reference, and so forth. [ Received from Prasanna Inamanamelluri ]

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Q:What is an EJB Context? A: 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 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. TOP [ Received from Prasanna Inamanamelluri ] Q:The EJB container implements the EJBHome and EJBObject classes. For every request from a unique client, does the container create a separate instance of the generated EJBHome and EJBObject classes? A: 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. 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 application server. The way it is implemented is again up to the implementer. [ Received from Vishal Khasgiwala ]

Struts Interview Questions

Question:What is Struts? Question:What is Jakarta Struts Framework? Question:What is ActionServlet? Question:How you will make available any Message Resources Definitions file to the Struts Framework Environment? Question:What is Action Class? Question:What is ActionForm? Question:What is Struts Validator Framework? Question:Give the Details of XML files used in Validator Framework? Question:How you will display validation fail errors on jsp page? Question:How you will enable front-end validation based on the xml in validation.xml? Question:How to get data from the velocity page in a action class? Q:What is Struts? A: The core of the Struts framework is a flexible control layer based on standard technologies like Java Servlets, JavaBeans, ResourceBundles, and XML, as well as various Jakarta Commons packages. Struts encourages application architectures based on the Model 2 approach, a variation of the classic Model-View-Controller (MVC) design paradigm. Struts provides its own Controller component and integrates with other technologies to provide the Model and the View. For the Model, Struts can interact with standard data access technologies, like JDBC and EJB, as well as most any third-party packages, like Hibernate, iBATIS, or Object Relational Bridge. For the View, Struts works well with JavaServer Pages, including JSTL and JSF, as well as Velocity Templates, XSLT, and other presentation systems. The Struts framework provides the invisible underpinnings every professional web application needs to survive. Struts helps you create an extensible development environment for your application, based on published standards and proven design patterns. [ Received from Ramakrishna Potluri ] TOP Q:What is Jakarta Struts Framework? A: Jakarta Struts is open source implementation of MVC (Model-View-Controller) pattern for the development of web based applications. Jakarta Struts is robust architecture and can be used for the development of application of any size. Struts framework makes it much easier to design scalable, reliable Web applications with Java. TOP [ Received from Dhiraj Sharma] Q:What is ActionServlet? A: The class org.apache.struts.action.ActionServlet is the called the ActionServlet. In the the Jakarta Struts Framework this class plays the role of controller. All the requests to the server goes through the controller. Controller is responsible for handling all the requests. TOP [ Received from Dhiraj Sharma]

Q:How you will make available any Message Resources Definitions file to the Struts Framework Environment? A: T Message Resources Definitions file are simple .properties files and these files contains the messages that can be used in the struts project. Message Resources Definitions files can be added to the struts-config.xml file through <messageresources /> tag. Example: <message-resources parameter=\"MessageResources\" />. [ Received from Dhiraj Sharma]

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Q:What is Action Class? A: 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 perform all the database related stuffs in the Action Class. The ActionServlet (commad) passes the parameterized 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. TOP [ Received from Dhiraj Sharma] Q:What is ActionForm? A: An ActionForm is a JavaBean that extends org.apache.struts.action.ActionForm. ActionForm maintains the session state for web application and the ActionForm object is automatically populated on the server side with data entered from a form on the client side. TOP [ Received from Dhiraj Sharma] Q:What is Struts Validator Framework? A: 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 the java scripts and it can be used 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. The Validator framework was developed by David Winterfeldt as third-party addon to Struts. Now the Validator framework is a part of Jakarta Commons project and it can be used with or without Struts. The Validator framework comes integrated with the Struts Framework and can be used without doing any extra settings. [ Received from Dhiraj Sharma]

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Q:Give the Details of XML files used in Validator Framework? A: The Validator Framework uses two XML configuration files validator-rules.xml and validation.xml. The validator-rules.xml defines the standard validation routines, these are reusable and used in validation.xml. to define the form specific validations. The validation.xml defines the validations applied to a form bean.

[ Received from Dhiraj Sharma]

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Q:How you will display validation fail errors on jsp page? A: Following tag displays all the errors: [ Received from Dhiraj Sharma]

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Q:How you will enable front-end validation based on the xml in validation.xml? A: 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. TOP [ Received from Dhiraj Sharma] Q:How to get data from the velocity page in a action class? A: We can get the values in the action classes by using data.getParameter(\"variable name defined in the velocity page\"); [ Received from Shri Prakash Kunwar]

Q: What is Jakarta Struts Framework? A: Jakarta Struts is open source implementation of MVC (Model-View-Controller) pattern for the development of web based applications. Jakarta Struts is robust architecture and can be used for the development of application of any size. Struts framework makes it much easier to design scalable, reliable Web applications with Java. Q: What is ActionServlet? A: The class org.apache.struts.action.ActionServlet is the called the ActionServlet. In the the Jakarta Struts Framework this class plays the role of controller. All the requests to the server goes through the controller. Controller is responsible for handling all the requests. Q: How you will make available any Message Resources Definitions file to the Struts Framework Environment? A: Message Resources Definitions file are simple .properties files and these files contains the messages that can be used in the struts project. Message Resources Definitions files can be added to the struts-config.xml file through <message-resources /> tag. Example: <message-resources parameter="MessageResources" />

Q: What is Action Class? A: The Action is part of the controller. 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. The ActionServlet (commad) passes the parameterized class to Action Form using the execute() method. There should be no database interactions in the action. The action should receive the request, call business objects (which then handle database, or interface with J2EE, etc) and then determine where to go next. Even better, the business objects could be handed to the action at runtime (IoC style) thus removing any dependencies on the model. 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. Q: Write code of any Action Class? A: Here is the code of Action Class that returns the ActionForward object. TestAction.java package roseindia.net; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import import import import

org.apache.struts.action.Action; org.apache.struts.action.ActionForm; org.apache.struts.action.ActionForward; org.apache.struts.action.ActionMapping;

public class TestAction extends Action { public ActionForward execute( ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception{ return mapping.findForward("testAction"); } }

Q: What is ActionForm? A: An ActionForm is a JavaBean that extends org.apache.struts.action.ActionForm. ActionForm maintains the session state for web application and the ActionForm object is automatically populated on the server side with data entered from a form on the client side. Q: What is Struts Validator Framework? A: 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 the java scripts and it can be used 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. The Validator framework was developed by David Winterfeldt as third-party add-on to Struts. Now the Validator framework is a part of Jakarta Commons project and it can be used with or without Struts. The Validator framework comes integrated with the Struts Framework and can be used without doing any extra settings. Q. Give the Details of XML files used in Validator Framework? A: The Validator Framework uses two XML configuration files validator-rules.xml and validation.xml. The validator-rules.xml defines the standard validation routines, these are reusable and used in validation.xml. to define the form specific validations. The validation.xml defines the validations applied to a form bean. Q. How you will display validation fail errors on jsp page? A: Following tag displays all the errors: Q. How you will enable front-end validation based on the xml in validation.xml? A: 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 is RequestProcessor and RequestDispatcher? Answer: The controller is responsible for intercepting and translating user input into actions to be performed by the model. The controller is responsible for selecting the next view based on user input and the outcome of model operations. The Controller receives the request from the browser, invoke a business operation and coordinating the view to return to the client. The controller is implemented by a java servlet, this servlet is centralized point of control for the web application. In struts framework the controller responsibilities are implemented by several different components like The ActionServlet Class The RequestProcessor Class The Action Class The ActionServlet extends the javax.servlet.http.httpServlet class. The ActionServlet class is not abstract and therefore can be used as a concrete controller by your application. The controller is implemented by the ActionServlet class. All incoming requests are

mapped to the central controller in the deployment descriptor as follows. <servlet> <servlet-name>action <servlet-class>org.apache.struts.action.ActionServlet All request URIs with the pattern *.do are mapped to this servlet in the deployment descriptor as follows. <servlet-mapping> <servlet-name>action *.do *.do A request URI that matches this pattern will have the following form. http://www.my_site_name.com/mycontext/actionName.do The preceding mapping is called extension mapping, however, you can also specify path mapping where a pattern ends with /* as shown below. <servlet-mapping> <servlet-name>action /do/* *.do A request URI that matches this pattern will have the following form. http://www.my_site_name.com/mycontext/do/action_Name The class org.apache.struts.action.requestProcessor process the request from the controller. You can sublass the RequestProcessor with your own version and modify how the request is processed. Once the controller receives a client request, it delegates the handling of the request to a helper class. This helper knows how to execute the business operation associated with the requested action. In the Struts framework this helper class is descended of org.apache.struts.action.Action class. It acts as a bridge between a client-side user action and business operation. The Action class decouples the client request from the business model. This decoupling allows for more than one-to-one mapping between the user request and an action. The Action class also can perform other functions such as authorization, logging before invoking business operation. the Struts Action class contains several methods, but most important method is the execute() method. public ActionForward execute(ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws Exception; The execute() method is called by the controller when a request is received from a client. The controller creates an instance of the Action class if one doesn’t already exist. The strut framework will create only a single instance of each Action class in your application.

Action are mapped in the struts configuration file and this configuration is loaded into memory at startup and made available to the framework at runtime. Each Action element is represented in memory by an instance of the org.apache.struts.action.ActionMapping class . The ActionMapping object contains a path attribute that is matched against a portion of the URI of the incoming request. path= "/somerequest" type="com.somepackage.someAction" scope="request" name="someForm" validate="true" input="somejsp.jsp" Once this is done the controller should determine which view to return to the client. The execute method signature in Action class has a return type org.apache.struts.action.ActionForward class. The ActionForward class represents a destination to which the controller may send control once an action has completed. Instead of specifying an actual JSP page in the code, you can declaratively associate as action forward through out the application. The action forward are specified in the configuration file. path= "/somerequest" type="com.somepackage.someAction" scope="request" name="someForm" validate="true" input="somejsp.jsp" The action forward mappings also can be specified in a global section, independent of any specific action mapping. public interface RequestDispatcher Defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server. The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name. This interface is intended to wrap servlets, but a servlet container can create

RequestDispatcher objects to wrap any type of resource. getRequestDispatcher public RequestDispatcher getRequestDispatcher(java.lang.String path) Returns a RequestDispatcher object that acts as a wrapper for the resource located at the given path. A RequestDispatcher object can be used to forward a request to the resource or to include the resource in a response. The resource can be dynamic or static. The pathname must begin with a "/" and is interpreted as relative to the current context root. Use getContext to obtain a RequestDispatcher for resources in foreign contexts. This method returns null if the ServletContext cannot return a RequestDispatcher. Parameters: path - a String specifying the pathname to the resource Returns: a RequestDispatcher object that acts as a wrapper for the resource at the specified path See Also: RequestDispatcher, getContext(java.lang.String) getNamedDispatcher public RequestDispatcher getNamedDispatcher(java.lang.String name) Returns a RequestDispatcher object that acts as a wrapper for the named servlet. Servlets (and JSP pages also) may be given names via server administration or via a web application deployment descriptor. A servlet instance can determine its name using ServletConfig.getServletName(). This method returns null if the ServletContext cannot return a RequestDispatcher for any reason. Parameters: name - a String specifying the name of a servlet to wrap Returns: a RequestDispatcher object that acts as a wrapper for the named servlet See Also: RequestDispatcher, getContext(java.lang.String), ServletConfig.getServletName() Question: Why cant we overide create method in StatelessSessionBean? Answer: From the EJB Spec : - A Session bean's home interface defines one or morecreate(...) methods. Each create method must be named create and must match one of the ejbCreate methods defined in the enterprise Bean class. The return type of a create method must be the enterprise Bean's remote interface type. The home interface of a

stateless session bean must have one create method that takes no arguments. Question: Is struts threadsafe?Give an example? Answer: Struts is not only thread-safe but thread-dependant. The response to a request is handled by a light-weight Action object, rather than an individual servlet. Struts instantiates each Action class once, and allows other requests to be threaded through the original object. This core strategy conserves resources and provides the best possible throughput. A properly-designed application will exploit this further by routing related operations through a single Action. Question: Can we Serialize static variable? Answer: Serialization is the process of converting a set of object instances that contain references to each other into a linear stream of bytes, which can then be sent through a socket, stored to a file, or simply manipulated as a stream of data. Serialization is the mechanism used by RMI to pass objects between JVMs, either as arguments in a method invocation from a client to a server or as return values from a method invocation. In the first section of this book, There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of any particular object's state. 2. Base class fields are only handled if the base class itself is serializable. 3. Transient fields. There are four basic things you must do when you are making a class serializable. They are: 1. 2. 3. 4.

Implement the Serializable interface. Make sure that instance-level, locally defined state is serialized properly. Make sure that superclass state is serialized properly. Override equals( )and hashCode( ). it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process .... (Source: http://www.oreilly.com/catalog/javarmi/chapter/ch10.html)

Question: What are the uses of tiles-def.xml file, resourcebundle.properties file, validation.xml file? Answer: tiles-def.xml is is an xml file used to configure tiles with the struts application. You can define the layout / header / footer / body content for your View. See more at http://www.roseindia.net/struts/using-tiles-defs-xml.shtml. The resourcebundle.properties file is used to configure the message (error/ other messages) for the struts applications.

The file validation.xml is used to declare sets of validations that should be applied to Form Beans. Fpr more information please visit http://www.roseindia.net/struts/address_struts_validator.shtml. Question: What is the difference between perform() and execute() methods? Answer: Perform method is the method which was deprecated in the Struts Version 1.1. In Struts 1.x, Action.perform() is the method called by the ActionServlet. This is typically where your business logic resides, or at least the flow control to your JavaBeans and EJBs that handle your business logic. As we already mentioned, to support declarative exception handling, the method signature changed in perform. Now execute just throws Exception. Action.perform() is now deprecated; however, the Struts v1.1 ActionServlet is smart enough to know whether or not it should call perform or execute in the Action, depending on which one is available. Question: What are the various Struts tag libraries? Answer: Struts is very rich framework and it provides very good and user friendly way to develop web application forms. Struts provide many tag libraries to ease the development of web applications. These tag libraries are: * Bean tag library - Tags for accessing JavaBeans and their properties. * HTML tag library - Tags to output standard HTML, including forms, text boxes, checkboxes, radio buttons etc.. * Logic tag library - Tags for generating conditional output, iteration capabilities and flow management * Tiles or Template tag library - For the application using tiles * Nested tag library - For using the nested beans in the application Question: What do you understand by DispatchAction? Answer: DispatchAction is an action that comes with Struts 1.1 or later, that lets you combine Struts actions into one class, each with their own method. The org.apache.struts.action.DispatchAction class allows multiple operation to mapped to the different functions in the same Action class. For example: A package might include separate RegCreate, RegSave, and RegDelete Actions, which just perform different operations on the same RegBean object. Since all of these operations are usually handled by the same JSP page, it would be handy to also have them handled by the same Struts Action. A very simple way to do this is to have the submit button modify a field in the form which indicates which operation to perform. <SCRIPT>function set(target) {document.forms[0].dispatch.value=target;} SAVE

SAVE AS NEW DELETE

Then, in the Action you can setup different methods to handle the different operations, and branch to one or the other depending on which value is passed in the dispatch field. String dispatch = myForm.getDispatch(); if ("create".equals(dispatch)) { ... if ("save".equals(dispatch)) { ...

The Struts Dispatch Action [org.apache.struts.actions] is designed to do exactly the same thing, but without messy branching logic. The base perform method will check a dispatch field for you, and invoke the indicated method. The only catch is that the dispatch methods must use the same signature as perform. This is a very modest requirement, since in practice you usually end up doing that anyway. To convert an Action that was switching on a dispatch field to a DispatchAction, you simply need to create methods like this public ActionForward create( ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { ... public ActionForward save( ActionMapping mapping, ActionForm form, HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { ...

Cool. But do you have to use a property named dispatch? No, you don't. The only other step is to specify the name of of the dispatch property as the "parameter" property of the action-mapping. So a mapping for our example might look like this:

If you wanted to use the property "o" instead, as in o=create, you would change the mapping to
name="regForm" scope="request" validate="true" parameter="o"/ rel="nofollow">

Again, very cool. But why use a JavaScript button in the first place? Why not use several buttons named "dispatch" and use a different value for each? You can, but the value of the button is also its label. This means if the page designers want to label the button something different, they have to coordinate the Action programmer. Localization becomes virtually impossible. (Source: http://husted.com/struts/tips/002.html). Question: How Struts relates to J2EE? Answer: Struts framework is built on J2EE technologies (JSP, Servlet, Taglibs), but it is itself not part of the J2EE standard. Question: What is Struts actions and action mappings? Answer: A Struts action is an instance of a subclass of an Action class, which implements a portion of a Web application and whose perform or execute method returns a forward. An action can perform tasks such as validating a user name and password. An action mapping is a configuration file entry that, in general, associates an action name with an action. An action mapping can contain a reference to a form bean that the action can use, and can additionally define a list of local forwards that is visible only to this action. An action servlet is a servlet that is started by the servlet container of a Web server to process a request that invokes an action. The servlet receives a forward from the action and asks the servlet container to pass the request to the forward's URL. An action servlet must be an instance of an org.apache.struts.action.ActionServlet class or of a subclass of that class. An action servlet is the primary component of the controller. Question: Can I setup Apache Struts to use multiple configuration files? Answer: Yes Struts can use multiple configuration files. Here is the configuration example: <servlet> <servlet-name>banking <servlet-class>org.apache.struts.action.ActionServlet <param-name>config

<param-value>/WEB-INF/struts-config.xml, /WEB-INF/struts-authentication.xml, /WEB-INF/struts-help.xml 1 Question: What are the disadvantages of Struts? Answer: Struts is very robust framework and is being used extensively in the industry. But there are some disadvantages of the Struts: a) High Learning Curve Struts requires lot of efforts to learn and master it. For any small project less experience developers could spend more time on learning the Struts. b) Harder to learn Struts are harder to learn, benchmark and optimize. Question: What is Struts Flow? Answer: Struts Flow is a port of Cocoon's Control Flow to Struts to allow complex workflow, like multi-form wizards, to be easily implemented using continuations-capable JavaScript. It provides the ability to describe the order of Web pages that have to be sent to the client, at any given point in time in an application. The code is based on a proof-ofconcept Dave Johnson put together to show how the Control Flow could be extracted from Cocoon. (Ref: http://struts.sourceforge.net/struts-flow/index.html ) Question: What are the difference between and ? Answer: : This tag is used to output locale-specific text (from the properties files) from a MessageResources bundle. : This tag is used to output property values from a bean. is a commonly used tag which enables the programmers to easily present the data. Question: What is LookupDispatchAction? Answer: An abstract Action that dispatches to the subclass mapped execute method. This is useful in cases where an HTML form has multiple submit buttons with the same name. The button name is specified by the parameter property of the corresponding ActionMapping. (Ref. http://struts.apache.org/1.2.7/api/org/apache/struts/actions/LookupDispatchAction.html). Question: What are the components of Struts? Answer: Struts is based on the MVC design pattern. Struts components can be categories into Model, View and Controller. Model: Components like business logic / business processes and data are the part of Model. View: JSP, HTML etc. are part of View

Controller: Action Servlet of Struts is part of Controller components which works as front controller to handle all the requests. Question: What are Tag Libraries provided with Struts? Answer: Struts provides a number of tag libraries that helps to create view components easily. These tag libraries are: a) Bean Tags: Bean Tags are used to access the beans and their properties. b) HTML Tags: HTML Tags provides tags for creating the view components like forms, buttons, etc.. c) Logic Tags: Logic Tags provides presentation logics that eliminate the need for scriptlets. d) Nested Tags: Nested Tags helps to work with the nested context. Question: What are the core classes of the Struts Framework? Answer: Core classes of Struts Framework are ActionForm, Action, ActionMapping, ActionForward, ActionServlet etc. Question: What are difference between ActionErrors and ActionMessage? Answer: ActionMessage: A class that encapsulates messages. Messages can be either global or they are specific to a particular bean property. Each individual message is described by an ActionMessage object, which contains a message key (to be looked up in an appropriate message resources database), and up to four placeholder arguments used for parametric substitution in the resulting message. ActionErrors: A class that encapsulates the error messages being reported by the validate() method of an ActionForm. Validation errors are either global to the entire ActionForm bean they are associated with, or they are specific to a particular bean property (and, therefore, a particular input field on the corresponding form). Question: How you will handle exceptions in Struts? Answer: In Struts you can handle the exceptions in two ways: a) Declarative Exception Handling: You can either define global exception handling tags in your struts-config.xml or define the exception handling tags within .. tag. Example: <exception key="database.error.duplicate" path="/UserExists.jsp" type="mybank.account.DuplicateUserException"/> b) Programmatic Exception Handling: Here you can use try{}catch{} block to handle the exception.

Servlet Interview Questions Question:Explain the life cycle methods of a Servlet. Question:What if the static modifier is removed from the signature of the main method? Question:Explain the directory structure of a web application. Question:What are the common mechanisms used for session tracking? Question:Explain ServletContext. Question:What is preinitialization of a servlet? Question:What is the difference between Difference between doGet() and doPost()? Question:What is the difference between HttpServlet and GenericServlet? Question:What is the difference between ServletContext and ServletConfig?

Q:Explain the life cycle methods of a Servlet. A: The javax.servlet.Servlet interface defines the three methods known as life-cycle method. public void init(ServletConfig config) throws ServletException public void service( ServletRequest req, ServletResponse res) throws ServletException, IOException public void destroy() First the servlet is constructed, then initialized wih the init() method. Any request from client are handled initially by the service() method before delegating to the doXxx() methods in the case of HttpServlet. The servlet is removed from service, destroyed with the destroy() methid, then garbaged collected and finalized. TOP

Q:What is the difference between the getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface and javax.servlet.ServletContext interface? A: The getRequestDispatcher(String path) method of javax.servlet.ServletRequest interface accepts parameter the path to the resource to be included or forwarded to, which can be relative to the request of the calling servlet. If the path begins with a "/" it is interpreted as relative to the current context root. The getRequestDispatcher(String path) method of javax.servlet.ServletContext interface cannot accepts relative paths. All path must sart with a "/" and are interpreted as relative to curent context root. TOP

Q:Explain the directory structure of a web application. A: The directory structure of a web application consists

of

two

parts.

A A

public

private resource

WEB-INF 1. 2. 3. lib directory

directory

directory which

folder

contains consists

called public

resource

WEB-INF folder.

of

classes

web.xml directory TOP

Q:What are the common mechanisms used for session tracking? A: Cookies SSL URL- rewriting

sessions TOP

Q:Explain ServletContext. A: ServletContext interface is a window for a servlet to view it's environment. A servlet can use this interface to get information such as initialization parameters for the web applicationor servlet container's version. Every web application has one and only one ServletContext and is accessible to all active resource of that application. TOP

Q:What is preinitialization of a servlet? A: A container doesnot initialize the servlets ass soon as it starts up, it initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or preinitializing a servlet. TOP [ Received from Amit Bhoir ] Q:What is the difference between Difference between doGet() and doPost()? A: A doGet() method is limited with 2k of data to be sent, and doPost() method doesn't have this limitation. A request string for doGet() looks like the following: http://www.allapplabs.com/svt1?p1=v1&p2=v2&...&pN=vN doPost() method call doesn't need a long text tail after a servlet name in a request. All parameters are stored in a request itself, not in a request string, and it's impossible to guess the data transmitted to a servlet only looking at a request string. TOP [ Received from Amit Bhoir ] Q:What is the difference between HttpServlet and GenericServlet? A: A GenericServlet has a service() method aimed to handle requests. HttpServlet extends GenericServlet and adds support for doGet(), doPost(), doHead() methods (HTTP 1.0) plus doPut(), doOptions(), doDelete(), doTrace() methods (HTTP 1.1). Both these classes are abstract. TOP [ Received from Amit Bhoir ]

Q:What is the difference between ServletContext and ServletConfig? A: ServletContext: Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a log file.The ServletContext object is contained within the ServletConfig object, which the Web server provides the servlet when the servlet is initialized ServletConfig: The object created after a servlet is instantiated and its default constructor is read. It is created to pass initialization information to the servlet. [ Received from Sivagopal Balivada ]

JSP Interview Questions Question:What is a output comment? Question:What is a Hidden comment? Question:What is an Expression? Question:What is a Declaration ? Question:What is a Scriptlet? Question:What are implicit objects? List them? Question:Difference between forward and sendRedirect? Question:What are the different scope values for the <jsp:useBean>? Question:Explain the life-cycle methods in JSP? Q:What is a output comment? A: A comment that is sent to the client in the viewable page source.The JSP engine handles an output comment as uninterpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser. JSP Syntax ] --> Example

is (new

a

commnet sent to client java.util.Date()).toLocaleString()

Displays in the page

1 on %> source: TOP

Q:What is a Hidden Comment? A: A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags. A hidden comment is not sent to the client, either in the displayed JSP page or the HTML page source. The hidden comment is useful when you want to hide or "comment out" part of your JSP page. You can use any characters in the body of the comment except the closing --%> combination. If you need to use --%> in your comment, you can escape it by

typing JSP <%-- comment --%>

--%\>.

Syntax

Examples <%@ page language="java" %> <%-- This comment will not be visible to the colent in the page source --%> TOP

Q:What is a Expression? A: An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. Because the value of an expression is converted to a String, you can use an expression within text in a JSP file. Like <%= someexpression %> <%= (new java.util.Date()).toLocaleString() %> You cannot use a semicolon to end an expression TOP

Q:What is a Declaration? A: A declaration declares one or more variables or methods for use later in the JSP source file. A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as they are separated by semicolons. The declaration must be valid in the scripting language used in the JSP file. <%! <%! int <%! int a, b, c; %>

somedeclarations i =

0;

%> %> TOP

Q:What is a Scriptlet? A: A scriptlet can contain any number of language statements, variable or method declarations, or expressions that are valid in the page scripting language.Within scriptlet tags, you can 1.Declare variables or methods to use later in the file (see also Declaration). 2.Write expressions valid in the page scripting language (see also Expression). 3.Use any of the JSP implicit objects or any object declared with a <jsp:useBean> tag. You must write plain text, HTML-encoded text, or other JSP tags outside the

scriptlet. Scriptlets are executed at request time, when the JSP engine processes the client request. If the scriptlet produces output, the output is stored in the out object, from which you can display it. TOP

Q:What are implicit objects? List them? A: Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects re listed below • request • response • pageContext • session • application • out • config • page •

exception TOP

Q:Difference between forward and sendRedirect? A: When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completly with in the web container. When a sendRedirtect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completly new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward. TOP

Q:What are the different scope valiues for the <jsp:useBean>? A: The different scope values for <jsp:useBean> are 1. 2. 3.session 4.application

page request

TOP

Q:Explain the life-cycle mehtods in JSP? A: THe generated servlet class for a JSP page implements the HttpJspPage interface of the javax.servlet.jsp package. Hte HttpJspPage interface extends the JspPage interface which inturn extends the Servlet interface of the javax.servlet package. the generated servlet class thus implements all the methods of the these three

interfaces. The JspPage interface declares only two mehtods - jspInit() and jspDestroy() that must be implemented by all JSP pages regardless of the clientserver protocol. However the JSP specification has provided the HttpJspPage interfaec specifically for the JSp pages serving HTTP requests. This interface declares one method _jspService(). The jspInit()- The container calls the jspInit() to initialize te servlet instance.It is called before any other method, and is called only once for a servlet instance. The _jspservice()- The container calls the _jspservice() for each request, passing it the request and the response objects. The jspDestroy()- The container calls this when it decides take the instance out of service. It is the last method called n the servlet instance.

JSP Interview Questions Question:How do I prevent the output of my JSP or Servlet pages from being cached by the browser? Question:How does JSP handle run-time exceptions? Question:How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it? Question:How do I use a scriptlet to initialize a newly instantiated bean? 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? Question:What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization? Question:How can I enable session tracking for JSP pages if the browser has disabled cookies? Q:How do I prevent the output of my JSP or Servlet pages from being cached by the browser? A: 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. <% 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 %> [ Received from Sumit Dhamija ]

TOP

Q:How does JSP handle run-time exceptions? A: You can use the errorPage attribute of the page directive to have uncaught runtime 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\" %> 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. [ Received from Sumit Dhamija ]

TOP

Q:How can I implement a thread-safe JSP page? What are the advantages and Disadvantages of using it? A: 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 the admin screen for your JSP engine. More importantly, avoid using the 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 . TOP [ Received from Sumit Dhamija ] Q:How do I use a scriptlet to initialize a newly instantiated bean? A: 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 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 --%> [ Received from Sumit Dhamija ]

TOP

Q: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? A: You could make a simple wrapper function, like <%!

String return } %>

(s

blanknull(String == null)

?

s) \"\"

{ s;

:

then use it inside your JSP form, like [ Received from Sumit Dhamija ]

TOP

Q:What's a better approach for enabling thread-safe servlets and JSPs? SingleThreadModel Interface or Synchronization? A: 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. 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. [ Received from Sumit Dhamija ]

TOP

Q:How can I enable session tracking for JSP pages if the browser has disabled cookies? A: 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. hello1.jsp <%@ page <% Integer num = session.putValue("num",num); String url %> \'>hello2.jsp

session=\"true\" new

%> Integer(100);

=response.encodeURL("hello2.jsp");

hello2.jsp <%@ page session="true" %> <% Integer i= (Integer )session.getValue("num"); out.println("Num value in session is " + i.intValue()); %> [ Received from Vishal Khasgiwala ]

JSP Interview Questions Question:What is the difference between variable declared inside a declaration part and variable declared in scriplet part? Question:Is there a way to execute a JSP from the comandline or from my own application?

Q:What is the difference b/w variable declared inside a declaration part and variable declared in scriplet part? A: Variable declared inside declaration part is treated as a global variable.that means after convertion jsp file into servlet that variable will be in outside of service method or it will be declared as instance variable.And the scope is available to complete jsp and to complete in the converted servlet class.where as if u declare a variable inside a scriplet that variable will be declared inside a service method and the scope is with in the service method. [ Received from Neelam Gangadhar] TOP Q:Is there a way to execute a JSP from the comandline or from my own application?

A: There is a little tool called JSPExecutor that allows you to do just that. The developers (Hendrik Schreiber & Peter Rossbach <[email protected]>) aim was not to write a full blown servlet engine, but to provide means to use JSP for generating source code or reports. Therefore most HTTPspecific features (headers, sessions, etc) are not implemented, i.e. no reponseline or header is generated. Nevertheless you can use it to precompile JSP for your website. TOP [ Received from Nitin Jawarkar ]

Question: What do you understand by JSP Actions? Answer: JSP actions are XML tags that direct the server to use existing components or control the behavior of the JSP engine. JSP Actions consist of a typical (XML-based) prefix of "jsp" followed by a colon, followed by the action name followed by one or more attribute parameters. There are six JSP Actions: <jsp:include/> <jsp:forward/> <jsp:plugin/> <jsp:usebean/> <jsp:setProperty/> <jsp:getProperty/>

Question: What is the difference between <jsp:include page = ... > and <%@ include file = ... >?. Answer: Both the tag includes the information from one page in another. The differences are as follows: <jsp:include page = ... >: This is like a function call from one jsp to another jsp. It is executed ( the included page is executed and the generated html content is included in the content of calling jsp) each time the client page is accessed by the client. This approach is useful to for modularizing the web application. If the included file changed then the new content will be included in the output. <%@ include file = ... >: In this case the content of the included file is textually embedded in the page that have <%@ include file=".."> directive. In this case in the included file changes, the changed content will not included in the output. This approach is used when the code from one jsp file required to include in multiple jsp files.

Question: What is the difference between <jsp:forward page = ... > and response.sendRedirect(url),?. Answer: The <jsp:forward> element forwards the request object containing the client

request information from one JSP file to another file. The target file can be an HTML file, another JSP file, or a servlet, as long as it is in the same application context as the forwarding JSP file. sendRedirect sends HTTP temporary redirect response to the browser, and browser creates a new request to go the redirected page. The response.sendRedirect kills the session variables. Question: Identify the advantages of JSP over Servlet. a) Embedding of Java code in HTML pages b) Platform independence c) Creation of database-driven Web applications d) Server-side programming capabilities Answer :- Embedding of Java code in HTML pages Write the following code for a JSP page: <%@ page language = "java" %> <TITLE>RESULT PAGE <% PrintWriter print = request.getWriter(); print.println("Welcome"); %> Suppose you access this JSP file, Find out your answer. a) A blank page will be displayed. b) A page with the text Welcome is displayed c) An exception will be thrown because the implicit out object is not used d) An exception will be thrown because PrintWriter can be used in servlets only Answer :- A page with the text Welcome is displayed Question: What are implicit Objects available to the JSP Page? Answer: Implicit objects are the objects available to the JSP page. These objects are created by Web container and contain information related to a particular request, page, or application. The JSP implicit objects are:

Variable

Class

Description The context for the JSP page's servlet and any Web application javax.servlet.ServletContext components contained in the same application. config javax.servlet.ServletConfig Initialization information for the JSP page's servlet. exception java.lang.Throwable Accessible only from an error page. out javax.servlet.jsp.JspWriter The output stream. The instance of the JSP page's servlet processing the page java.lang.Object current request. Not typically used by JSP page authors. The context for the JSP page. Provides a single API to pageContext javax.servlet.jsp.PageContext manage the various scoped attributes. Subtype of request The request triggering the execution of the JSP page. javax.servlet.ServletRequest Subtype of The response to be returned to the client. Not typically response javax.servlet.ServletResponse used by JSP page authors. session javax.servlet.http.HttpSession The session object for the client.

Question: What are all the different scope values for the <jsp:useBean> tag? Answer:<jsp:useBean> tag is used to use any java object in the jsp page. Here are the scope values for <jsp:useBean> tag: a) page b) request c) session and d) application Question: What is JSP Output Comments? Answer: JSP Output Comments are the comments that can be viewed in the HTML source file. Example: and Question: What is expression in JSP? Answer: Expression tag is used to insert Java values directly into the output. Syntax for the Expression tag is: <%= expression %> An expression tag contains a scripting language expression that is evaluated, converted to a String, and inserted where the expression appears in the JSP file. The following

expression tag displays time on the output: <%=new java.util.Date()%> Question: What types of comments are available in the JSP? Answer: There are two types of comments are allowed in the JSP. These are hidden and output comments. A hidden comments does not appear in the generated output in the html, while output comments appear in the generated output. Example of hidden comment: <%-- This is hidden comment --%> Example of output comment: Question: What is JSP declaration? Answer: JSP Decleratives are the JSP tag used to declare variables. Declaratives are enclosed in the <%! %> tag and ends in semi-colon. You declare variables and functions in the declaration tag and can use anywhere in the JSP. Here is the example of declaratives: <%@page contentType="text/html" %> <%! int cnt=0; private int getCount(){ //increment cnt and return the value cnt++; return cnt; } %>

Values of Cnt are:

<%=getCount()%>

Question: What is JSP Scriptlet? Answer: JSP Scriptlet is jsp tag which is used to enclose java code in the JSP pages. Scriptlets begins with <% tag and ends with %> tag. Java code written inside scriptlet executes every time the JSP is invoked. Example: <%

//java codes String userName=null; userName=request.getParameter("userName"); %> Question: What are the life-cycle methods of JSP? Answer: Life-cycle methods of the JSP are: a) jspInit(): The container calls the jspInit() to initialize the servlet instance. It is called before any other method, and is called only once for a servlet instance. b)_jspService(): The container calls the _jspservice() for each request and it passes the request and the response objects. _jspService() method cann't be overridden. c) jspDestroy(): The container calls this when its instance is about to destroyed. The jspInit() and jspDestroy() methods can be overridden within a JSP page.

J2EE INTERVIEW QUESTIONS Question: What is J2EE? Answer: J2EE Stands for Java 2 Enterprise Edition. J2EE is an environment for developing and deploying enterprise applications. J2EE specification is defined by Sun Microsystems Inc. The J2EE platform is one of the best platform for the development and deployment of enterprise applications. The J2EE platform is consists of a set of services, application programming interfaces (APIs), and protocols, which provides the functionality necessary for developing multi-tiered, web-based applications. You can download the J2EE SDK and development tools from http://java.sun.com/. Question: What do you understand by a J2EE module? Answer: A J2EE module is a software unit that consists of one or more J2EE components of the same container type along with one deployment descriptor of that type. J2EE specification defines four types of modules: a) EJB b) Web c) application client and d) resource adapter In the J2EE applications modules can be deployed as stand-alone units. Modules can also be assembled into J2EE applications. Question: Tell me something about J2EE component? Answer: J2EE component is a self-contained functional software unit supported by a container and configurable at deployment time. The J2EE specification defines the following J2EE components:

• • •

Application clients and applets are components that run on the client. Java servlet and JavaServer Pages (JSP) technology components are Web components that run on the server. Enterprise JavaBeans (EJB) components (enterprise beans) are business components that run on the server. J2EE components are written in the Java programming language and are compiled in the same way as any program in the language. The difference between J2EE components and "standard" Java classes is that J2EE components are assembled into a J2EE application, verified to be well formed and in compliance with the J2EE specification, and deployed to production, where they are run and managed by the J2EE server or client container. Source: J2EE v1.4 Glossary

Question: What are the contents of web module? Answer: A web module may contain: a) JSP files b) Java classes c) gif and html files and d) web component deployment descriptors Question: Differentiate between .ear, .jar and .war files. Answer: These files are simply zipped file using java jar tool. These files are created for different purposes. Here is the description of these files: .jar files: These files are with the .jar extenstion. The .jar files contains the libraries, resources and accessories files like property files. .war files: These files are with the .war extension. The war file contains the web application that can be deployed on the any servlet/jsp container. The .war file contains jsp, html, javascript and other files for necessary for the development of web applications. .ear files: The .ear file contains the EJB modules of the application. Question: What is the difference between Session Bean and Entity Bean? Answer: Session Bean: Session is one of the EJBs and it represents a single client inside the Application Server. Stateless session is easy to develop and its efficient. As compare to entity beans session beans require few server resources. A session bean is similar to an interactive session and is not shared; it can have only one client, in the same way that an interactive session can have only one user. A session bean is not persistent and it is destroyed once the session terminates. Entity Bean: An entity bean represents persistent global data from the database. Entity

beans data are stored into database. Question: Why J2EE is suitable for the development distributed multi-tiered enterprise applications? Answer: The J2EE platform consists of multi-tiered distributed application model. J2EE applications allows the developers to design and implement the business logic into components according to business requirement. J2EE architecture allows the development of multi-tired applications and the developed applications can be installed on different machines depending on the tier in the multi-tiered J2EE environment . The J2EE application parts are: a) Client-tier components run on the client machine. b) Web-tier components run on the J2EE server. c) Business-tier components run on the J2EE server and the d) Enterprise information system (EIS)-tier software runs on the EIS servers Question: Why do understand by a container? Answer: Normally, thin-client multi-tiered applications are hard to write because they involve many lines of intricate code to handle transaction and state management, multithreading, resource pooling, and other complex low-level details. The componentbased and platform-independent J2EE architecture makes J2EE applications easy to write because business logic is organized into reusable components. In addition, the J2EE server provides underlying services in the form of a container for every component type. Because you do not have to develop these services yourself, you are free to concentrate on solving the business problem at hand (Source: http://java.sun.com/j2ee/1.3/docs/tutorial/doc/Overview4.html ). In short containers are the interface between a component and the low-level platform specific functionality that supports the component. The application like Web, enterprise bean, or application client component must be assembled and deployed on the J2EE container before executing. Question: What are the services provided by a container? Answer: The services provided by container are as follows: a) Transaction management for the bean b) Security for the bean c) Persistence of the bean d) Remote access to the bean e) Lifecycle management of the bean f) Database-connection pooling g) Instance pooling for the bean

Question: What are types of J2EE clients? Answer: J2EE clients are the software that access the services components installed on the J2EE container. Following are the J2EE clients: a) Applets b) Java-Web Start clients c) Wireless clients d) Web applications Question: What is Deployment Descriptor? Answer: A deployment descriptor is simply an XML(Extensible Markup Language) file with the extension of .xml. Deployment descriptor describes the component deployment settings. Application servers reads the deployment descriptor to deploy the components contained in the deployment unit. For example ejb-jar.xml file is used to describe the setting of the EJBs. Question: What do you understand by JTA and JTS? Answer: JTA stands for Java Transaction API and JTS stands for Java Transaction Service. JTA provides a standard interface which allows the developers to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK uses the JTA transaction manager to implement the transaction. The code developed by developers does not calls the JTS methods directly, but only invokes the JTA methods. Then JTA internally invokes the JTS routines. JTA is a high level transaction interface used by the application code to control the transaction. Question: What is JAXP? Answer: The Java API for XML Processing (JAXP) enables applications to parse and transform XML documents independent of a particular XML processing implementation. JAXP or Java API for XML Parsing is an optional API provided by Javasoft. It provides basic functionality for reading, manipulating, and generating XML documents through pure Java APIs. It is a thin and lightweight API that provides a standard way to seamlessly integrate any XML-compliant parser with a Java application. More at http://java.sun.com/xml/ Question: What is J2EE Connector architecture? Answer: J2EE Connector Architecture (JCA) is a Java-based technology solution for connecting application servers and enterprise information systems (EIS) as part of enterprise application integration (EAI) solutions. While JDBC is specifically used to connect Java EE applications to databases, JCA is a more generic architecture for connection to legacy systems (including databases). JCA was developed under the Java Community Process as JSR 16 (JCA 1.0) and JSR 112 (JCA 1.5). As of 2006, the current version of JCA is version 1.5. The J2EE Connector API is used by J2EE tools developers and system integrators to create resource adapters. Home page for J2EE Connector

architecture http://java.sun.com/j2ee/connector/. Question: What is difference between Java Bean and Enterprise Java Bean? Answer: Java Bean as is a plain java class with member variables and getter setter methods. Java Beans are defined under JavaBeans specification as Java-Based software component model which includes the features like introspection, customization, events, properties and persistence. Enterprise JavaBeans or EJBs for short are Java-based software components that comply with Java's EJB specification. EJBs are delpoyed on the EJB container and executes in the EJB container. EJB is not that simple, it is used for building distributed applications. Examples of EJB are Session Bean, Entity Bean and Message Driven Bean. EJB is used for server side programming whereas java bean is a client side. Bean is only development but the EJB is developed and then deploy on EJB Container. Question: What is the difference between JTS and JTA? Answer: In any J2EE application transaction management is one of the most crucial requirements of the application. Given the complexity of today's business requirements, transaction processing occupies one of the most complex segments of enterprise level distributed applications to build, deploy and maintain. JTS specifies the implementation of a Java transaction manager. JTS specifies the implementation of a Transaction Manager which supports the Java Transaction API (JTA) 1.0 This transaction manager supports the JTA, using which application servers can be built to support transactional Java applications. Internally the JTS implements the Java mapping of the OMG OTS 1.1 specifications. The Java mapping is specified in two packages: org.omg.CosTransactions and org.omg.CosTSPortability. The JTS thus provides a new architecture for transactional application servers and applications, while complying to the OMG OTS 1.1 interfaces internally. This allows the JTA compliant applications to interoperate with other OTS 1.1 complaint applications through the standard IIOP. Java-based applications and Java-based application servers access transaction management functionality via the JTA interfaces. The JTA interacts with a transaction management implementation via JTS. Similarly, the JTS can access resources via the JTA XA interfaces or can access OTS-enabled non-XA resources. JTS implementations can interoperate via CORBA OTS interfaces. The JTA specifies an architecture for building transactional application servers and defines a set of interfaces for various components of this architecture. The components are: the application, resource managers, and the application server. The JTA specifies standard interfaces for Java-based applications and application servers to interact with transactions, transaction managers, and resource managers JTA transaction management provides a set of interfaces utilized by an application server to manage the beginning and completion of transactions. Transaction synchronization and propagation services are also provided under the domain of transaction management. In the Java transaction model, the Java application components can conduct transactional operations on JTA compliant resources via the JTS. The JTS acts as a layer over the OTS.

The applications can therefore initiate global transactions to include other OTS transaction managers, or participate in global transactions initiated by other OTS compliant transaction managers. Question: Can Entity Beans have no create() methods? Answer: Entity Beans can have no create() methods. Entity Beans have no create() method, when entity bean is not used to store the data in the database. In this case entity bean is used to retrieve the data from database. Question: What are the call back methods in Session bean? Answer: Callback methods are called by the container to notify the important events to the beans in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface.The callback methods example are ejbCreate(), ejbPassivate(), and ejbActivate(). Question: What is bean managed transaction? Answer: In EJB transactions can be maintained by the container or developer can write own code to maintain the transaction. If a developer doesn’t want a Container to manage transactions, developer can write own code to maintain the database transaction. Question: What are transaction isolation levels in EJB? Answer: Thre are four levels of transaction isolation are: * Uncommitted Read * Committed Read * Repeatable Read * Serializable The four transaction isolation levels and the corresponding behaviors are described below: Isolation Level Read Uncommitted Read Committed Repeatable Read Serializable

Dirty Read Possible Not possible Not possible Not possible

Non-Repeatable Read Possible Possible Not possible Not possible

Java Collection Interview Questions

Phantom Read Possible Possible Possible Not possible

Question: Question: Question: Question: Question: Question: Question: Question: Question: Question: Question: Question: Question: Question:

What is the Collections API? What is the List interface? What is the Vector class?

What is an Iterator interface? Which java.util classes and interfaces support event handling? What is the GregorianCalendar class? What is the Locale class? What is the SimpleTimeZone class? What is the Map interface? What is the highest-level event class of the event-delegation model? What is the Collection interface? What is the Set interface? What is the typical use of Hashtable? I am trying to store an object using a key in a Hashtable. And some other object already exists in that location, then what will happen? The existing object will be overwritten? Or the new object will be stored elsewhere? Question: What is the difference between the size and capacity of a Vector? Question: Can a vector contain heterogenous objects? Q:What is the Collections API? A: The Collections API is a set of classes and interfaces that support operations on collections of objects. [ Received from Prasanna Inamanamelluri] TOP Q:What is the List interface? A: The List interface provides support for ordered collections of objects. [ Received from SPrasanna Inamanamelluri]

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Q:What is the Vector class? A: The Vector class provides the capability to implement a growable array of objects. [ Received from Prasanna Inamanamelluri] TOP Q:What is an Iterator interface? A: The Iterator interface is used to step through the elements of a Collection . [ Received from Prasanna Inamanamelluri]

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Q:Which java.util classes and interfaces support event handling? A: The EventObject class and the EventListener interface support event processing. TOP [ Received from Prasanna Inamanamelluri] Q:What is the GregorianCalendar class? A: The GregorianCalendar provides support for traditional Western calendars [ Received from Prasanna Inamanamelluri]

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Q:What is the Locale class? A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region .

[ Received from Prasanna Inamanamelluri]

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Q:What is the SimpleTimeZone class? A: The SimpleTimeZone class provides support for a Gregorian calendar . [ Received from Prasanna Inamanamelluri]

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Q:What is the Map interface? A: The Map interface replaces the JDK 1.1 Dictionary class and is used associate keys with values. [ Received from Prasanna Inamanamelluri] TOP Q:What is the highest-level event class of the event-delegation model? A: The java.util.EventObject class is the highest-level class in the event-delegation class hierarchy. [ Received from Prasanna Inamanamelluri] TOP Q:What is the Collection interface? A: The Collection interface provides support for the implementation of a mathematical bag - an unordered collection of objects that may contain duplicates. [ Received from Prasanna Inamanamelluri] TOP Q:What is the Set interface? A: The Set interface provides methods for accessing the elements of a finite mathematical set. Sets do not allow duplicate elements. [ Received from Prasanna Inamanamelluri]

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Q:What is the typical use of Hashtable? A: Whenever a program wants to store a key value pair, one can use Hashtable. [ Received from Sandesh Sadhale]

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Q:I am trying to store an object using a key in a Hashtable. And some other object already exists in that location, then what will happen? The existing object will be overwritten? Or the new object will be stored elsewhere? A: The existing object will be overwritten and thus it will be lost. [ Received from Sandesh Sadhale]

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Q:What is the difference between the size and capacity of a Vector? A: The size is the number of elements actually stored in the vector, while capacity is the maximum number of elements it can store at a given instance of time. [ Received from Sandesh Sadhale] TOP Q:Can a vector contain heterogenous objects? A: Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object. [ Received from Sandesh Sadhale]

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Q:Can a ArrayList contain heterogenous objects? A: Yes a ArrayList can contain heterogenous objects. Because a ArrayList stores everything in terms of Object. [ Received from Sandesh Sadhale]

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Q:What is an enumeration? A: An enumeration is an interface containing methods for accessing the underlying data structure from which the enumeration is obtained. It is a construct which collection classes return when you request a collection of all the objects stored in the collection. It allows sequential access to all the elements stored in the collection. [ Received from Sandesh Sadhale] TOP Q:Considering the basic properties of Vector and ArrayList, where will you use Vector and where will you use ArrayList? A: The basic difference between a Vector and an ArrayList is that, vector is synchronized while ArrayList is not. Thus whenever there is a possibility of multiple threads accessing the same instance, one should use Vector. While if not multiple threads are going to access the same instance then use ArrayList. Non synchronized data structure will give better performance than the synchronized one. [ Received from Sandesh Sadhale] TOP Q:Can a vector contain heterogenous objects? A: Yes a Vector can contain heterogenous objects. Because a Vector stores everything in terms of Object.

Java Interview Questions Question: What is the difference between an Interface and an Abstract class? What is the purpose of garbage collection in Java, and when is it used? Question: Question: Describe synchronization in respect to multithreading. Question: Explain different way of using thread? Question: What are pass by reference and passby value? Question: What is HashMap and Map? Question: Difference between HashMap and HashTable? Question: Difference between Vector and ArrayList? Question: Difference between Swing and Awt? Question: What is the difference between a constructor and a method? Question: What is an Iterator? Question: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers. Question: What is an abstract class? Question: What is static in java?

Question: What is final? Q:What is the difference between an Interface and an Abstract class? A: An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract. An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods. . TOP

Q:What is the purpose of garbage collection in Java, and when is it used? A: The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used. TOP

Q:Describe synchronization in respect to multithreading. A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors. TOP

Q:Explain different way of using thread? A: The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help. TOP

Q:What are pass by reference and passby value? A: Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed. TOP

Q:What is HashMap and Map? A: Map is Interface and Hashmap is class that implements that. TOP

Q:Difference between HashMap and HashTable? A: The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized. TOP

Q:Difference between Vector and ArrayList? A: Vector is synchronized whereas arraylist is not. TOP

Q:Difference between Swing and Awt? A: AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT. TOP

Q:What is the difference between a constructor and a method? A: A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator. A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator. TOP

Q:What is an Iterator? A: Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator. TOP

Q:State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers. A: public : Public class is visible in other packages, field is visible everywhere (class must be public too) private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature. protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature. default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package. TOP

Q:What is an abstract class? A: Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

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Q:What is static in java? A: Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass. TOP

Q:What is final? A: A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class is inherited. You can't change value of a final variable (is a constant).

Question: What if the main method is declared as private? Question: What if the static modifier is removed from the signature of the main method? Question: What if I write static public void instead of public static void? Question: What if I do not provide the String array as the argument to the method? Question: What is the first argument of the String array in main method? Question: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null? Question: How can one prove that the array is not null but empty using one line of code? Question: What environment variables do I need to set on my machine in order to be able to run Java programs? Question: Can an application have multiple classes having main method? Question: Can I have multiple main methods in the same class? Question: Do I need to import java.lang package any time? Why ? Question: Can I import same package/class twice? Will the JVM load the package twice at runtime? Question: What are Checked and UnChecked Exception? Question: What is Overriding? Question: What are different types of inner classes? Q:What if the main method is declared as private? A: The program compiles properly but at runtime it will give "Main method not public." message. TOP [ Received from Sandesh Sadhale] Q:What if the static modifier is removed from the signature of the main method? A: Program compiles. But at runtime throws an error "NoSuchMethodError". TOP [ Received from Sandesh Sadhale]

Q:What if I write static public void instead of public static void? A: Program compiles and runs properly. [ Received from Sandesh Sadhale]

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Q:What if I do not provide the String array as the argument to the method? A: Program compiles but throws a runtime error "NoSuchMethodError". TOP [ Received from Sandesh Sadhale] Q:What is the first argument of the String array in main method? A: The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name. TOP [ Received from Sandesh Sadhale] Q:If I do not provide any arguments on the command line, then the String array of Main method will be empty or null? A: It is empty. But not null. TOP [ Received from Sandesh Sadhale] Q:How can one prove that the array is not null but empty using one line of code? A: Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length. TOP [ Received from Sandesh Sadhale] Q:What environment variables do I need to set on my machine in order to be able to run Java programs? A: CLASSPATH and PATH are the two variables. TOP [ Received from Sandesh Sadhale] Q:Can an application have multiple classes having main method? A: Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method. TOP [ Received from Sandesh Sadhale] Q:Can I have multiple main methods in the same class? A: No the program fails to compile. The compiler says that the main method is already defined in the class. TOP [ Received from Sandesh Sadhale] Q:Do I need to import java.lang package any time? Why ? A: No. It is by default loaded internally by the JVM. [ Received from Sandesh Sadhale]

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Q:Can I import same package/class twice? Will the JVM load the package twice at runtime?

A: One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class. TOP [ Received from Sandesh Sadhale] Q:What are Checked and UnChecked Exception? A: A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method· Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be. TOP

Q:What is Overriding? A: When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass. When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private. TOP

Q:What are different types of inner classes? A: Nested top-level classes, Member classes, Local classes, Anonymous classes Nested top-level classes- If you declare a class within a class and specify the static modifier, the compiler treats the class just like any other top-level class. Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety. Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested toplevel class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class. Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement a more publicly available interface.Because local classes are not members, the

modifiers

public,

protected,

private,

and

static

are

not

usable.

Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Question:Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? Question:Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? Question:What is the difference between declaring a variable and defining a variable? Question:What is the default value of an object reference declared as an instance variable? Question:Can a top level class be private or protected? Question:What type of parameter passing does Java support? Question:Primitive data types are passed by reference or pass by value? Question:Objects are passed by value or by reference? Question:What is serialization? Question:How do I serialize an object to a file? Question:Which methods of Serializable interface should I implement? Question:How can I customize the seralization process? i.e. how can one have a control over the serialization process? Question:What is the common usage of serialization? Question:What is Externalizable interface? Question:When you serialize an object, what happens to the object references included in the object? Question:What one should take care of while serializing the object? Question:What happens to the static fields of a class during serialization?

Q:Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile? A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol symbol : class ABCD location: package io import java.io.ABCD; TOP [ Received from Sandesh Sadhale] Q:Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*? A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it's subpackage. TOP [ Received from Sandesh Sadhale] Q:What is the difference between declaring a variable and defining a

variable? A: In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization. e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions. TOP [ Received from Sandesh Sadhale] Q:What is the default value of an object reference declared as an instance variable? A: null unless we define it explicitly. TOP [ Received from Sandesh Sadhale] Q:Can a top level class be private or protected? A: No. A top level class can not be private or protected. It can have either "public" or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the "modifier private is not allowed here". This means that a top level class can not be private. Same is the case with protected. TOP [ Received from Sandesh Sadhale] Q:What type of parameter passing does Java support? A: In Java the arguments are always passed by value . [ Update from Eki and Jyothish Venu]

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Q:Primitive data types are passed by reference or pass by value? A: Primitive data types are passed by value. [ Received from Sandesh Sadhale]

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Q:Objects are passed by value or by reference? A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object . TOP [ Update from Eki and Jyothish Venu] Q:What is serialization? A: Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream. TOP [ Received from Sandesh Sadhale] Q:How do I serialize an object to a file? A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file. TOP [ Received from Sandesh Sadhale] Q:Which methods of Serializable interface should I implement? A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods. TOP [ Received from Sandesh Sadhale]

Q:How can I customize the seralization process? i.e. how can one have a control over the serialization process? A: Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal. You should implement these methods and write the logic for customizing the serialization process. TOP [ Received from Sandesh Sadhale] Q:What is the common usage of serialization? A: Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed. TOP [ Received from Sandesh Sadhale] Q:What is Externalizable interface? A: Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism. Thus if your class implements this interface, you can customize the serialization process by implementing these methods. TOP [ Received from Sandesh Sadhale] Q:When you serialize an object, what happens to the object references included in the object? A: The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process. Thus when an object is serialized, all the included objects are also serialized alongwith the original obect. TOP [ Received from Sandesh Sadhale] Q:What one should take care of while serializing the object? A: One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException. TOP [ Received from Sandesh Sadhale] Q:What happens to the static fields of a class during serialization? A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are 1. Serialization ignores static fields, because they are not part of ay particular state state. 2. Base class fields are only hendled if the base class itself is serializable. 3. Transient fields. TOP [ Received from Sandesh Sadhale Modified after P.John David comments.]

Question:Does Java provide any construct to find out the size of an object? Question:Give a simplest way to find out the time a method takes for execution without using any profiling tool? Question:What are wrapper classes?

Question:Why do we need wrapper classes? Question:What are checked exceptions? Question:What are runtime exceptions? Question:What is the difference between error and an exception?? Question:How to create custom exceptions? Question:If I want an object of my class to be thrown as an exception object, what should I do? Question:If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object? Question:How does an exception permeate through the code? Question:What are the different ways to handle exceptions? Question:What is the basic difference between the 2 approaches to exception handling...1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach? Question:Is it necessary that each try block must be followed by a catch block? Question:If I write return at the end of the try block, will the finally block still execute? Question:If I write System.exit (0); at the end of the try block, will the finally block still execute?

Q:Does Java provide any construct to find out the size of an object? A: No there is not sizeof operator in Java. So there is not direct way to determine the size of an object directly in Java. TOP [ Received from Sandesh Sadhale] Q:Give a simplest way to find out the time a method takes for execution without using any profiling tool? A: Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.

To put it in code... long start = method long end = System.currentTimeMillis ();

System.currentTimeMillis

(); ();

System.out.println ("Time taken for execution is " + (end - start)); Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amout of processing. TOP [ Received from Sandesh Sadhale] Q:What are wrapper classes? A: Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

[ Received from Sandesh Sadhale]

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Q:Why do we need wrapper classes? A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object. TOP [ Received from Sandesh Sadhale] Q:What are checked exceptions? A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions. TOP [ Received from Sandesh Sadhale] Q:What are runtime exceptions? A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time. TOP [ Received from Sandesh Sadhale] Q:What is the difference between error and an exception? A: An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.). TOP [ Received from Sandesh Sadhale] Q:How to create custom exceptions? A: Your class should extend class Exception, or some more specific type thereof. TOP [ Received from Sandesh Sadhale] Q:If I want an object of my class to be thrown as an exception object, what should I do? A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also. TOP [ Received from Sandesh Sadhale] Q:If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object? A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well. TOP [ Received from Sandesh Sadhale] Q:How does an exception permeate through the code? A: An unhandled exception moves up the method stack in search of a matching When

an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates. TOP [ Received from Sandesh Sadhale] Q:What are the different ways to handle exceptions? A: There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. and 2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions. TOP [ Received from Sandesh Sadhale] Q:What is the basic difference between the 2 approaches to exception handling. 1> try catch block and 2> specifying the candidate exceptions in the throws clause? When should you use which approach? A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use. TOP [ Received from Sandesh Sadhale] Q:Is it necessary that each try block must be followed by a catch block? A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method. TOP [ Received from Sandesh Sadhale] Q:If I write return at the end of the try block, will the finally block still execute? A: Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return. TOP [ Received from Sandesh Sadhale] Q:If I write System.exit (0); at the end of the try block, will the finally block still execute? A: No in this case the finally block will not execute because when you say System.exit (0); the control immediately goes out of the program, and thus finally

never executes. [ Received from Sandesh Sadhale]

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Question:How are Observer and Observable used? Question:What is synchronization and why is it important? Question:How does Java handle integer overflows and underflows? Question:Does garbage collection guarantee that a program will not run out of memory? Question:What is the difference between preemptive scheduling and time slicing? Question:When a thread is created and started, what is its initial state? Question:What is the purpose of finalization? Question:What is the Locale class? Question:What is the difference between a while statement and a do statement? Question:What is the difference between static and non-static variables? Question:How are this() and super() used with constructors? Question:What are synchronized methods and synchronized statements? Question:What is daemon thread and which method is used to create the daemon thread? Question:Can applets communicate with each other? Question:What are the steps in the JDBC connection? Question:How does a try statement determine which catch clause should be used to handle an exception?

Q:How are Observer and Observable used? A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects. TOP [Received from Venkateswara Manam] Q:What is synchronization and why is it important? A: With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without 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. TOP [ Received from Venkateswara Manam] Q:How does Java handle integer overflows and underflows? A: It uses those low order bytes of the result that can fit into the size of the type allowed by the operation. TOP [ Received from Venkateswara Manam] Q:Does garbage collection guarantee that a program will not run out of memory? A: 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 than they are garbage collected. It is also possible for programs to create objects that are not subject to garbage collection . TOP [ Received from Venkateswara Manam] Q:What is the difference between preemptive scheduling and time slicing? A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into 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. TOP [ Received from Venkateswara Manam] Q:When a thread is created and started, what is its initial state? A: A thread is in the ready state after it has been created and started. [ Received from Venkateswara Manam]

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Q:What is the purpose of finalization? A: The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. TOP [ Received from Venkateswara Manam] Q:What is the Locale class? A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region. TOP [ Received from Venkateswara Manam] Q:What is the difference between a while statement and a do statement? A: 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 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. TOP [ Received from Venkateswara Manam] Q:What is the difference between static and non-static variables? A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. TOP [ Received from Venkateswara Manam] Q:How are this() and super() used with constructors? A: This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor. TOP [ Received from Venkateswara Manam] Q:What are synchronized methods and synchronized statements? A: 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 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. TOP [ Received from Venkateswara Manam] Q:What is daemon thread and which method is used to create the daemon thread? A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. setDaemon method is used to create a daemon thread. TOP [ Received from Shipra Kamra] Q:Can applets communicate with each other? A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable. An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members. It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial. TOP [ Received from Krishna Kumar ] Q:What are the steps in the JDBC connection? A: While making a JDBC connection we go through the following steps : Step 1 : Register the database driver by using : Class.forName(\" driver classs for that specific database\" ); Step 2 : Now create a database connection using : Connection con = DriverManager.getConnection(url,username,password); Step 3: Now Create a query using : Statement stmt = Connection.Statement(\"select * from TABLE NAME\"); Step 4 : Exceute the query : stmt.exceuteUpdate(); [ Received from Shri Prakash Kunwar]

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Q:How does a try statement determine which catch clause should be used to

handle an exception? A: 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 appear. The first catch clause that is capable of handling the exceptionis executed. The remaining catch clauses are ignored. [ Received from P Rajesh

Question:Can an unreachable object become reachable again? Question:What method must be implemented by all threads? Question:What are synchronized methods and synchronized statements? Question:What is Externalizable? Question:What modifiers are allowed for methods in an Interface? Question:What are some alternatives to inheritance? Question:What does it mean that a method or field is "static"? ? Question:What is the difference between preemptive scheduling and time slicing? Question:What is the catch or declare rule for method declarations?

Q:Can an unreachable object become reachable again? A: An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects. TOP [Received from P Rajesh] Q:What method must be implemented by all threads? A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface. TOP [ Received from P Rajesh] Q:What are synchronized methods and synchronized statements? A: 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 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. TOP [ Received from P Rajesh] Q:What is Externalizable? A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in) TOP [ Received from Venkateswara Manam] Q:What modifiers are allowed for methods in an Interface? A: Only public and abstract modifiers are allowed for methods in interfaces.

[ Received from P Rajesh]

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Q:What are some alternatives to inheritance? A: 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 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). TOP [ Received from P Rajesh] Q:What does it mean that a method or field is "static"? A: 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. [ Received from P Rajesh]

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Q:What is the difference between preemptive scheduling and time slicing? A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into 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. TOP [ Received from P Rajesh] Q:What is the catch or declare rule for method declarations? A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause. TOP [ Received from P Rajesh]

Question:Is Empty .java file a valid source file? Question:Can a .java file contain more than one java classes? Question:Is String a primitive data type in Java? Question:Is main a keyword in Java? Question:Is next a keyword in Java? Question:Is delete a keyword in Java? Question:Is exit a keyword in Java? Question:What happens if you dont initialize an instance variable of any of the primitive types in Java? Question:What will be the initial value of an object reference which is defined as an

instance variable? Question:What are the different scopes for Java variables? Question:What is the default value of the local variables? Question:How many objects are created in the following piece of code? MyClass c1, c2, c3; c1 = new MyClass (); c3 = new MyClass (); Question:Can a public class MyClass be defined in a source file named YourClass.java? Question:Can main method be declared final? Question:What will be the output of the following statement? System.out.println ("1" + 3); Question:What will be the default values of all the elements of an array defined as an instance variable?

Q:Is Empty .java file a valid source file? A: Yes, an empty .java file is a perfectly valid source file. [Received from Sandesh Sadhale]

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Q:Can a .java file contain more than one java classes? A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class. TOP [ Received from Sandesh Sadhale] Q:Is String a primitive data type in Java? A: No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package. TOP [ Received from Sandesh Sadhale] Q:Is main a keyword in Java? A: No, main is not a keyword in Java. [ Received from Sandesh Sadhale]

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Q:Is next a keyword in Java? A: No, next is not a keyword. [ Received from Sandesh Sadhale]

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Q:Is delete a keyword in Java? A: No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does. TOP [ Received from Sandesh Sadhale] Q:Is exit a keyword in Java? A: No. To exit a program explicitly you use exit method in System object. [ Received from Sandesh Sadhale]

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Q:What happens if you dont initialize an instance variable of any of the primitive types in Java? A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false. TOP [ Received from Sandesh Sadhale] Q:What will be the initial value of an object reference which is defined as an instance variable? A: The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references. TOP [ Received from Sandesh Sadhale] Q:What are the different scopes for Java variables? A: The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time. 1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible. 2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope. 3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance. TOP [ Received from Sandesh Sadhale] Q:What is the default value of the local variables? A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized.. TOP [ Received from Sandesh Sadhale] Q:How many objects are created in the following piece of code? MyClass c1, c2, c3; c1 = new MyClass (); c3 = new MyClass (); A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized. TOP [ Received from Sandesh Sadhale] Q:Can a public class MyClass be defined in a source file named YourClass.java? A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension. TOP [ Received from Sandesh Sadhale]

Q:Can main method be declared final? A: Yes, the main method can be declared final, in addition to being public static. TOP [ Received fromSandesh Sadhale] Q:What will be the output System.out.println ("1" + 3); A: It will print 13. [ Received from Sandesh Sadhale]

of

the

following

statement?

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Q:What will be the default values of all the elements of an array defined as an instance variable? A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null. TOP [ Received from Sandesh Sadhale]

Question: What is transient variable? Answer: Transient variable can't be serialize. For example if a variable is declared as transient in a Serializable class and the class is written to an ObjectStream, the value of the variable can't be written to the stream instead when the class is retrieved from the ObjectStream the value of the variable becomes null. Question: Name the containers which uses Border Layout as their default layout? Answer: Containers which uses Border Layout as their default are: window, Frame and Dialog classes. Question: What do you understand by Synchronization? Answer: Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object while another thread is in the process of using or updating the object's value. Synchronization prevents such type of data corruption. E.g. Synchronizing a function: public synchronized void Method1 () { // Appropriate method-related code. } E.g. Synchronizing a block of code inside a function: public myFunction (){ synchronized (this) { // Synchronized code here. } }

Question: What is Collection API? Answer: The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables if effectively replaces. Example of classes: HashSet, HashMap, ArrayList, LinkedList, TreeSet and TreeMap. Example of interfaces: Collection, Set, List and Map. Question: Is Iterator a Class or Interface? What is its use? Answer: Iterator is an interface which is used to step through the elements of a Collection. Question: What is similarities/difference between an Abstract class and Interface? Answer: Differences are as follows: • •

• •

Interfaces provide a form of multiple inheritance. A class can extend only one other class. Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc. A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class. Interfaces are slow as it requires extra indirection to to find corresponding method in in the actual class. Abstract classes are fast.

Similarities: •

Neither Abstract classes or Interface can be instantiated.

Question: How to define an Abstract class? Answer: A class containing abstract method is called Abstract class. An Abstract class can't be instantiated. Example of Abstract class: abstract class testAbstractClass { protected String myString; public String getMyString() { return myString; } public abstract string anyAbstractFunction(); }

Question: How to define an Interface? Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface. Emaple of Interface: public interface sampleInterface { public void functionOne(); public long CONSTANT_ONE = 1000; } Question: Explain the user defined Exceptions? Answer: User defined Exceptions are the separate Exception classes defined by the user for specific purposed. An user defined can created by simply sub-classing it to the Exception class. This allows custom exceptions to be generated (using throw) and caught in the same way as normal exceptions. Example: class myCustomException extends Exception { // The class simply has to exist to be an exception } Question: Explain the new Features of JDBC 2.0 Core API? Answer: The JDBC 2.0 API includes the complete JDBC API, which includes both core and Optional Package API, and provides inductrial-strength database computing capabilities. New Features in JDBC 2.0 Core API: •

• • • • •

Scrollable result sets- using new methods in the ResultSet interface allows programmatically move the to particular row or to a position relative to its current position JDBC 2.0 Core API provides the Batch Updates functionality to the java applications. Java applications can now use the ResultSet.updateXXX methods. New data types - interfaces mapping the SQL3 data types Custom mapping of user-defined types (UTDs) Miscellaneous features, including performance hints, the use of character streams, full precision for java.math.BigDecimal values, additional security, and support for time zones in date, time, and timestamp values.

Question: Explain garbage collection? Answer: Garbage collection is one of the most important feature of Java. Garbage

collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program cann't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected. Question: How you can force the garbage collection? Answer: Garbage collection automatic process and can't be forced. Question: What is OOPS? Answer: OOP is the common abbreviation for Object-Oriented Programming. Question: Describe the principles of OOPS. Answer: There are three main principals of oops which are called Polymorphism, Inheritance and Encapsulation. Question: Explain the Encapsulation principle. Answer: Encapsulation is a process of binding or wrapping the data and the codes that operates on the data into a single entity. This keeps the data safe from outside interface and misuse. One way to think about encapsulation is as a protective wrapper that prevents code and data from being arbitrarily accessed by other code defined outside the wrapper. Question: Explain the Inheritance principle. Answer: Inheritance is the process by which one object acquires the properties of another object. Question: Explain the Polymorphism principle. Answer: The meaning of Polymorphism is something like one name many forms. Polymorphism enables one entity to be used as as general category for different types of actions. The specific action is determined by the exact nature of the situation. The concept of polymorphism can be explained as "one interface, multiple methods". Question: Explain the different forms of Polymorphism. Answer: From a practical programming viewpoint, polymorphism exists in three distinct forms in Java:

Method overloading Method overriding through inheritance Method overriding through the Java interface

• • •

Question: What are Access Specifiers available in Java? Answer: Access specifiers are keywords that determines the type of access to the member of a class. These are: Public Protected Private Defaults

• • • •

Question: Describe the wrapper classes in Java. Answer: Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type. Following table lists the primitive types and the corresponding wrapper classes: Primitive boolean

Wrapper java.lang.Boolean

byte

java.lang.Byte

char

java.lang.Character

double float int

java.lang.Double java.lang.Float java.lang.Integer

long

java.lang.Long

short

java.lang.Short

void

java.lang.Void

Question: Read the following program: public class test { public static void main(String [] args) {

int x = 3; int y = 1; if (x = y) System.out.println("Not equal"); else System.out.println("Equal"); } } What is the result? A. The output is “Equal” B. The output in “Not Equal” C. An error at " if (x = y)" causes compilation to fall. D. The program executes but no output is show on console. Answer: C