Brief History Of Java

Brief History of Java In 1990, Sun Microsystems began an internal project known as the Green Project to work on a new technology. In 1992, the Green Project was spun off and its interest directed toward building highly interactive devices for the cable TV industry. This failed to materialize. In 1994, the focus of the original team was re-targeted, this time to the use of Internet technology. A small web browser called HotJava was written. Oak was renamed to Java after learning that Oak had already been trademarked. In 1995, Java was first publicly released. In 1996, Java Development Kit (JDK) 1.0 was released. In 2002, JDK 1.4 (codename Merlin) was released, the most widely used version. In 2004, JDK 5.0 (codename Tiger) was released, the latest version.

James Gosling

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James Gosling is generally credited as the inventor of the Java programming language He was the first designer of Java and implemented its original compiler and virtual machine He is also known as the Father of Java He is currently the Chief Technical Officer of Sun Microsystems

What is Java? A multi-platform, network-centric, object-oriented programming language  Multi-platform  It can run on almost any computer platform  Network-centric  Designed with network in mind – “the network is the computer”  Designed for building applications for the Internet  Object-oriented  It incorporates object-oriented programming model

Java Platform Editions A Java Platform is the set of APIs, class libraries, and other programs used in developing Java programs for specific applications There are 3 Java Platform Editions 1. Java 2 Platform, Standard Edition (J2SE) • Core Java Platform targeting applications running on workstations 2. Java 2 Platform, Enterprise Edition (J2EE) • Component-based approach to developing distributed, multi-tier enterprise applications 3. Java 2 Platform, Micro Edition (J2ME) • Targeted at small, stand-alone or connectable consumer and embedded devices

Java Development Kit (JDK) Java Development Kit (JDK) • Is a set of Java tools for developing Java programs • Consists of Java API, Java Compiler, and JVM Java Application Programming Interface (API) • Is prewritten code, organized into packages of similar topics Java Virtual Machine (JVM) •Is an execution engine that runs compiled Java byte code

JDK JRE MyProgram.java Java API Java Virtual Machine Hardware - Based Platform

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A Java source file can include package, import and class declarations in that order The main() method is the start of execution of a Java application Each Java statement is terminated by a semicolon “;” Identifiers are case-sensitive Java keywords cannot be used as identifiers Each variable must be declared with a data type There are 8 primitive data types: boolean, char, byte, short, int, long, float and double There are 3 reference data types: class, array and interface

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Use unary, arithmetic operators for basic mathematical operations Use string operator to concatenate strings Use relational operators to compare objects Use conditional operator as alternative to ifelse() statement Use logical operators to compare boolean values Use assignment operators to assign values to variables Get familiar with object, shift and bitwise operators Java evaluates operators in order of precedence

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if() and switch() are used for branching statements

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while(), do-while() and for() are used for iterating statements

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break, continue and label are used to branch inside loops

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An array is a collection of values (either primitive or objects) all of which have the same data type

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There are 3 steps in creating an array:   

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Declaration Creation Initialization

Array elements are accessed using an index that starts at 0

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A method refers to what an object or class can do A method must have a return type, a name, and optional parameters The method signature refers to a method name and its parameters Return statement returns a value to its caller or returns control to its caller A method that does not return a value must specify void as a return type Calling a method should match its method signature When calling a method in the same class,

What is an Exception? 

An event during program execution that prevents the program from continuing normally

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An error condition that changes the normal flow of control in a program

Types of Exceptions 

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All exceptions in Java are objects of Throwable class Unchecked Exceptions  are exceptions derived from Error and RuntimeException classes  are usually irrecoverable and not handled explicitly  are not checked by the compiler Checked Exceptions 

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are exceptions derived from Exception class excluding the RuntimeException class must be handled explicitly

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Exceptional conditions sometimes occur in properly written code and must be handled Exception handling in Java is build around the “throw-and-catch” paradigm If a method can’t handle the exception, it can propagate the exception to the call stack All exceptions are objects of Throwable class

Defining a Class 

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A Class acts as the template from which an instance of an object is created. The class defines the properties of the object and the methods used to control the object's behavior. A Class specifies the structure of data as well as the methods which manipulate that data. Such data and methods are contained in each instance of the class.

Defining an Object

An object is a self-contained entity information an object must know: and with attributes behaviors behavior an object must do:  identity – uniqueness  attributes – structure  state – current condition

 methods – what it can do  events – what it responds to

Defining Abstraction

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Abstraction is the process of extracting common features from specific examples Abstraction is a process of defining the essential concepts while ignoring the inessential details

Different Types of Abstraction 

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Data Abstraction Programming languages define constructs to simplify the way information is presented to the programmer. Functional Abstraction Programming languages have constructs that ‘gift wrap’ very complex and low level instructions into instructions that are much more readable. Object Abstraction OOP languages take the concept even further and abstract programming constructs as objects.

Defining a Java Class    

A Java Class denotes a category of objects, and acts as a blueprint for creating such objects. It defines its members referred to as fields and methods. The fields (also known as variables or attributes) refer to the properties of the class. The methods (also known as operations) refer to behaviors that the class exhibits.

Class Members  

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A class member refers to one of the fields or methods of a class. Static members are variables and methods belonging to a class where only a single copy of variables and methods are shared by each object. Instance members are variables and methods belonging to objects where a copy of each variable and method is created for each object instantiated.

Class Modifiers Class modifiers change the way a class can be used.  Access modifiers describe how a class can be accessed.  Non-access modifiers describe how a class can be Modifier manipulated.Description 

(no modifier)

class is accessible within its package only

public

class is accessible by any class in any package class cannot be instantiated (a class cannot be abstract and final at the same time) class cannot be extended

abstract

final strictfp

class implements strict floating-point arithmetic

Access Modifiers private Private features of the Sample class can only be accessed from within the class itself.

default Only classes that are in the package may access default features of classes that are in the package

Clas s Packag e Sampl e Clas s Clas s

Clas s

protected

public

Classes that are in the package and all its subclasses may access protected features of the Sample class.

All classes may access public features of the Sample class.

Clas s

Clas s

* Default is not a modifier; it is just the name of the access level if no access modifier is specified.

Accessibility Scope 

Accessibility scope defines the boundary of access to a class and its members Scope static

Access

static code can access static members but not instance members nonnon-static code can access both static static members and instance members package a class and its members can be accessed within the package they are declared class class members can be accessed within the class block local variables can be accessed only within a block

•Encapsulation is the process of hiding an object’s implementation from another object, while presenting only the interfaces that should be visible. •Encapsulating a Class Members of a class must always be declared with the minimum level of visibility. Provide setters and getters (also known as accessors/mutators) to allow controlled access to private data. Provide other public methods (known as interfaces ) that other objects must adhere to in order to interact with the object.

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Abstraction is the process of formulating general concepts by extracting common properties of instances. A class is an abstraction of its instances. A Java Class denotes a category of objects. Class members refer to its fields and methods. Static members are variables and methods belonging to a class. Instance members are variables and methods belonging to objects. Instantiating a class means creating objects of its own type. Class modifiers include: (no modifier), public, abstract, final and strictfp. Member modifiers include: (no modifier), public, protected, private, static, final, abstract, strictfp, synchronized, native, transient and

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Encapsulation hides implementation details of a class. Encapsulating a class means declaring members with minimum level of visibility. Setters are methods whose only function is to alter the state of an object in a controlled manner. Getters are methods which only function is to return information about the state of an object. Constructors are methods which set the initial state of an object upon creation of the object.

Defining Inheritance 

Inheritance is the ability to derive new classes from existing ones. A derived class ("subclass") inherits the instance variables and methods of the base class ("superclass"), and may add new instance variables and methods.

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Inheritance defines a hierarchical relationship among classes wherein one class shares the attributes and methods defined in one or more classes.

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Inheritance is a relationship among classes in which one class shares the structure and behavior of another. A subclass inherits from a superclass.

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Relationships of Inheritance “is-a” relationship  a subclass can be used wherever a superclass can be used  implemented in Java by extending a class “has-a” relationship  a whole-class relationship between a class and its parts  also known as composition or aggregation  implemented in Java by instantiating an object inside a class “has-a” relationship “is-a” relationship Car

Vehicle Engine Car

Bus

Wheel

this and super

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this is a reference to the object of the current class  It can be used to distinguish instance variables from local variables  It can be assigned to other references, or passed as a parameter, or cast to other types  It cannot be used in a static context super is a reference to the object of a superclass  Used to access hidden members of the superclass  It cannot be assigned to other references, nor passed as a parameter, nor cast to other types 

Casting 

Casting is converting from one data type to another  Implicit casting is an implied casting operation  Explicit casting is a required casting operation  Primitive casting is converting a primitive data type to another data type  Widening conversion is casting a narrower data type to a broader data type  Narrowing conversion is casting a broader data type to a narrower data type  Reference casting is converting a reference data type to another  Upcasting is conversion up the inheritance hierarchy  Downcasting is conversion down the inheritance hierarchy  Casting between primitive and reference type is not allowed

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Inheritance is deriving a new class (subclass) from an existing class (superclass) Inheritance can exhibit an “is-a” or “has-a” relationship In Java, a class can only inherit from a single class All classes inherit from Object class – the highest in the inheritance hierarchy In Java, inheritance is implemented by the extends keyword this is a reference to the object of the current class super is a reference to the object of a superclass this() is used to call constructors within the same class super() is used to invoke constructors in the immediate superclass Casting is converting from one data type to another

Defining Polymorphism 

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Polymorphism is the ability of different objects to respond to the same message in different ways. This means that different objects can have very different method implementations for the same message. Polymorphism is the ability of a new object to implement the base functionality of a parent object in a new way. Polymorphism is an object's ability to behave differently depending on its type. Polymorphism is the ability of objects belonging to different types to respond to methods of the same name, each one according to the

Method Overloading 

Method Overloading is the process of declaring methods with the same name but different parameter types.

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A method can be overloaded in the same class or in a subclass.

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Which overloaded method to call is based on reference type and decided at compile time.

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Which overridden method to call is based on object type and decided during runtime.

Rules of Method Overloading •

Overloaded methods must change the argument list.

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Overloaded methods can change the return type.

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Overloaded methods can change the access modifier.

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Overloaded methods can declare new or broader checked exceptions.

Method Overriding 

Method Overriding allows a subclass to redefine methods of the same name from the superclass.

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The key benefit of overriding is the ability to define/defer behavior specific to subclasses.

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Which overridden method to call is based on object type and decided at runtime.

Rules of Method Overriding •

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An overridden method must have • the same name • the same number of parameters and types • the same return type as the overridden method. Overriding a method cannot narrow the method access level defined in the overridden method. Overriding a method cannot widen checked exceptions defined in the overridden method. Methods declared as private, static, or final cannot be overridden. A static method cannot override an instance method.

Overloading vs Overriding Criteria

Overloaded Method

Argumen t list Return type Exception s Access

Different

Overridden Method Same

Can change

Same

Can change

Cannot be wider

Can change

Cannot be narrower Based on object type and decided at runtime

level Invocatio Based on reference n type and decided at compile time

Defining Abstract Class 

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An Abstract Class is a class that provides common behavior across a set of subclasses, but is not itself designed to have instances of its own A class that dictates certain behavior but allows its subclasses to provide implementation A class designed only as a parent from which subclasses may be derived, but which is not itself suitable for instantiation A class often used to "abstract out" incomplete sets of features which may then be shared by a group of sibling sub-classes which add different variations of the missing pieces

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An Abstract Class cannot be instantiated An abstract class can SHOULD be extended An abstract class can have any number of abstract methods or none at all A class with at least one abstract method must be declared an abstract class A subclass can provide partial or full implementations of the inherited abstract methods A class that has at least one abstract method, whether declared or inherited from an abstract class, must be declared abstract.

Defining Interface 

An Interface defines a contract by specifying a set of method prototypes for which each class that implements it must adhere

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An interface is 100% abstract class

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An interface provides only a form for a class but no implementation An interface defines what a class can do but not how the class will do it

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Implementing Interface 

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Implementing an interface means providing implementations for its methods Interfaces are implemented using the implements keyword Rules on implementing the interface methods 1. Must have the same method signature and return type 2. Cannot narrow the method accessibility 3. Cannot specify broader checked exceptions Interface variables are implicitly public final static Interface methods are implicitly public

Abstract Class vs Interface Considerations in using either an interface or abstract class  An interface is useful because any class can implement it. But an interface, compared to an abstract class, is a pure API specification and contains no implementation.  If another method is added in an interface, all classes that implemented that interface will be broken  A good implementation of both is to create an interface and let the abstract class implement it. So when there is a need for adding methods, it can be safely added to the abstract class itself rather than the interface.

Defining Final Class 

A Final Class is considered complete, it cannot be improved or specialized

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A final class ensures that its state and behavior cannot be changed for safety and security

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To re-use a final class, you must utilize composition instead of inheritance

Rules on Final Class 

A final class cannot be extended or subclassed

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All methods of a final class have implementations

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All methods of a final class are implicitly final

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Members of the final class cannot be inherited or hidden

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Methods of the final class cannot be overridden

Abstract Class vs Final Class 

Abstract class provides a template of behaviors for subclasses to follow without actually dictating to the subclasses the exact implementation of those behaviors.

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Final class, since it cannot be extended, encourages composition, while abstract class encourages inheritance.

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Inheritance, through abstract classes, introduces tight coupling between classes and can weaken encapsulation by deferring actual implementation to subclasses.

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Final classes are used to prevent malicious code

What is a Collection? A Collection (also known as container) is an object that contains a group of objects treated as a single unit. Any type of objects can be stored, retrieved and manipulated as elements of collections. Collections Framework is a unified architecture for managing collections Main Parts of Collections Framework Interfaces Core interfaces defining common functionality exhibited by collections Implementations Concrete classes of the core interfaces providing data structures Operations Methods that perform various operations on

Collections Hierarchy

Set and List Collection implements

Set implements

HashSet

extends LinkedHashSet

List extends

implements

SortedSet implements TreeSet

LinkedList

Vector

ArrayList

List value

“Paul”

“Mark ”

“John”

“Paul”

“Luke ”

index

0

1

2

3

4

A List cares about the index. ArrayList

Vector

LinkedList

Set “Paul” “Peter”

“John” “Mark”

“Luke” “Fred”

A Set cares about uniqueness, it doesn’t allow duplicates. HashSet

LinkedHashSet

TreeSet

Map key

“Pl”

“Ma”

“Jn”

“ul”

“Le”

value

“Paul”

“Mark”

“John”

“Paul”

“Luke”

A Map cares about unique identifiers. HashMap

Hashtable

LinkedHash Map

TreeMap