How to implement Interface in Java

Interface:

The interface is a kind of class, it contains methods and variable. It defines only abstract methods and final fields. It has the following syntax:

interface Interface-Name
{
 variable declaration;
 methods declaration;
}

Example:

 interface Area
{
 final static float pi=3.14;
 float calculate(float a, float b);
 void display();
}

Implement Interface:

Interfaces are used as Superclasses whose properties are inherited by classes. It is necessary to create a class that inherits the given interface:

class class-name implements interface-name
{
 body of class-name
}

Program:

interface Area
{
 final static float pi=3.14;
 float calculate(float p, float q)
}
class Rectangle implements Area
{
 public float calculate(float p, float q)
{
 return(x*y);
}
}
class Circle implements Area
{
 return (pi*p*p);
}
}
class Test
{
 public static void main(String args[])
{
 Rectangle rect=new Rectangle();
 Circle c1=new Circle();
 Area a1;            //Interface object
 a1=rect;
 System.out.println("Area of Rectangle :"+a1.calculate(25, 30));
 a1=c1;
 System.out.println("Area of Circle :"+a1.calculate(20,0);
}
}