Lecture Notes Java Software devalopment, Lecture notes of Java Programming

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Interfaces

 Using the keyword interface, you can fully abstract a class’ interface from its implementation.

 That is, using interface, you can specify what a class must do, but not how it does it.

 Interfaces are syntactically similar to classes, but they lack instance variables, and their methods are declared without any body.

 To implement an interface, a class must create the complete set of methods defined by the interface.

 However, each class is free to determine the details of its own implementation.

Difference

 You cannot instantiate an interface.

 An interface does not contain any constructors.

 All of the methods in an interface are abstract.

 An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.

 An interface is not extended by a class; it is implemented by a class.

 An interface can extend multiple interfaces.

Defining an Interface

 access interface name {

 return-type method-name1(parameter-list);

 return-type method-name2(parameter-list);

 type final-varname1 = value;

 type final-varname2 = value;

 return-type method-nameN(parameter-list);

 type final-varnameN = value;

 interface Callback {

 void callback(int param); }

 Once an interface has been defined, one or more classes can implement that interface.

 class classname [extends superclass] [implements interface [,interface...]] {

 // class-body}

 If a class implements more than one interface, the interfaces are separated with a comma.

 The methods that implement an interface must be declared public.

 class Client implements Callback {  // Implement Callback's interface  public void callback(int p) {  System.out.println("callback called with " + p);  }  }  Notice that callback( ) is declared using the public access specifier.

Interface Inheritence

 One interface may inherit another interface.  The inheritance syntax is the same for classes and interfaces.  interface MyInterface1 {  void myMethod1(…) ;  }  interface MyInterface2 extends MyInterface1 {  void myMethod2(…) ;  }  When a class implements an interface that inherits another interface, it must  provide implementations for all methods defined within the interface  inheritance chain.

 When a class implements an interface that inherits another interface, it must

 provide implementations for all inherited methods:

 class MyClass implements MyInterface2 {

 void myMethod1(…) { … }

 void myMethod2(…) { … }

 …

 }

 MyClass must implement all of A and B methods:  class MyClass implements B {  public void meth1() {  System.out.println("Implement meth1().");  }  public void meth2() {  System.out.println("Implement meth2().");  }  public void meth3() {  System.out.println("Implement meth3().");  }  }

 Create a new MyClass object, then invoke all interface methods on it:

 class IFExtend {

 public static void main(String arg[]) {

 MyClass ob = new MyClass();

 ob.meth1();

 ob.meth2();

 ob.meth3();

 }

 }

Why Java is Important

• Two reasons :

• Trouble with C/C++ language is that they are not

portable and are not platform independent

languages.

• Emergence of World Wide Web, which demanded

portable programs

• Portability and security necessitated the

invention of Java

History

• James Gosling - Sun Microsystems
• Co founder – Vinod Khosla
• Oak - Java, May 20, 1995, Sun World
• JDK Evolutions
  • JDK 1.0 (January 23, 1996)
  • JDK 1.1 (February 19, 1997)
  • J2SE 1.2 (December 8, 1998)
  • J2SE 1.3 (May 8, 2000)
  • J2SE 1.4 (February 6, 2002)
  • J2SE 5.0 (September 30, 2004)
  • Java SE 6 (December 11, 2006)
  • Java SE 7 (July 28, 2011)

What is java?

• A general-purpose object-oriented language.
• Write Once Run Anywhere (WORA).
• Designed for easy Web/Internet applications.
• Widespread acceptance.

How is Java different from C…

• C Language:
  • Major difference is that C is a structure oriented language and Java is an object oriented language and has mechanism to define classes and objects.
  • Java does not support an explicit pointer type
  • Java does not have preprocessor, so we cant use #define, #include and #ifdef statements.
  • Java does not include structures, unions and enum data types.
  • Java does not include keywords like goto, sizeof and typedef.
  • Java adds labeled break and continue statements.
  • Java adds many features required for object oriented programming.