Interface

All interface classes are by default Public and Abstract, and do not contain any completed methods.

Interface should implement (complete the body of a method) when all the abstract methods are inherited from the superclass (abstract class).

The actual use of interface will be structuring (setting or mandating to set off rules).

An example of a Java program with an interface

The following is an example of an interface in a Java program:

package MyFirstPackage;

interface sample_abs2 {
  void abstr_method ();
  void abstr_method1();
}

abstract  class sample_abs3 implements sample_abs2 {
  abstract void abstr_method2 ();

  void abstr_method2A(){
    System.out.println("Abstact class abstr_method2A");
  }
}

class Interface extends sample_abs3 {//implements is a keyword used to complete the
//Interface Class
  public void abstr_method () {
    System.out.println("Interface abstr_method implementation");
  }

  public void abstr_method1 () {
    System.out.println("Interface abstr_method1 implementation");
  }

  public void abstr_method2 () {
    System.out.println("Abstact class abstr_method2 implementation");
  }

  public static void main(String[] args) {
    System.out.println("interface class demo");
    Interface objI = new Interface();
    objI.abstr_method();
    objI.abstr_method1();
    objI.abstr_method2();
    objI.abstr_method2A();
  }
}

The output will be as follows:

interface class demo
Interface abstr_method implementation
Interface abstr_method1 implementation
Abstact class abstr_method2 implementation
Abstact class abstr_method2A

Abstract classes compared with interfaces

For more details on abstract classes versus interfaces, visit http://docs.oracle.com/javase/tutorial/java/IandI/abstract.html.

The this keyword

The this is a Java keyword that is used as an alternate for creating an instance for a nonstatic variable or method within a same class.

An example for the this keyword is as follows:

package MyFirstPackage;

class ThisKeyword{
  int id;  
  String name;

  //Constructor method
  ThisKeyword(int id, String name){
    //constructor variable and instance variable names are same
    this.id = id;  
    this.name = name;
  }

  void display(){
    System.out.println(id + " " + name);
  }

  public static void main(String args[]){
    ThisKeyword objThis1 = new ThisKeyword(1234,"John");
    ThisKeyword objThis2 = new ThisKeyword(2456,"Smith");
    objThis1.display();
    objThis2.display();
  }
}

The output for the preceding code will be as follows:

1234 John
2456 Smith