Interfaces

Interfaces

• Interface is user defined data type just like class except that it have only abstract methods and final data members.
• Interfaces are used to define standard which must be followed by the class which are implementing those interfaces.

interface name
{
    Final Data members
    -------
    Abstract methods
    ------
}

1. We cannot create object of interface. i.e. you cannot instantiate an interface.
2. An interface does not contain any constructors.
3. All of the methods in an interface are abstract.
4. An interface cannot contain instance fields. The only fields that can appear in an interface must be declared both static and final.
5. You cannot have private members in interface.
6. By using interfaces we can achieve multiple inheritance.
• A interface can be extended from multiple interfaces by using keyword extends.
for ex:
interface Name extends IA , IB , IC
• A class can be implemented from multiple interfaces by using keyword implements.
for ex:
class Name implements IA , IB, IC
7. If a class is implementing a interface then it must override all abstract methods of that interface otherwise it will become an abstract class.

  interface Solid
  {
    public void printvolume();
  }
  interface Surface
  {
    public void printarea();
  }
  class Rectangle implements Surface
  {
    private int L,B;
    public Rectangle(int m,int n)
    {
      L=m;
      B=n;
    }
    public void printarea()
    {
      int a=L*B;
      System.out.println("Area is "+a);
    }
  }
  class Circle implements Surface
  {
    private int r;
    public Circle(int n)
    {
      r=n;
    }
    public void printarea()
    { 
      double a=3.14*r*r;
      System.out.println("Area is "+a);
    }
  }
  class Box implements Solid, Surface
  {
    private int L,B,H;
    public Box(int x,int y,int z)
    {
      L=x;
      B=y;
      H=z;
    }
    public void printvolume()
    {
      int v=L*B*H;
      System.out.println("Volume is "+v);
    }
    public void printarea( )
    {
      int a=2*L*H+2*B*H+2*L+B;
      System.out.println("Area is "+a);
    }
  }
  class demo
  {
    public static void main(String args[])
    {
      Circle a=new Circle(5);
      Rectangle b=new Rectangle(10,20);
      Box c=new Box(2,3,4);
      a.printarea();
      b.printarea();
      c.printarea();
      c.printvolumne();
    }
  }

Area is 78.5
Area is 200 
Area is 47 
Volume is 24

Tag or Marker interface

• An empty interface is known as tag or marker interface.
• For example Serializable, EventListener, Remote(java.rmi.Remote) are tag interfaces.
• These interfaces do not have any field and methods in it.
• They are used to provide some essential information to the JVM so that JVM may perform some useful operation.