Interface is third way to achieve polymorphism.Because when
you call a method on any interface variable then at run time
it is decided which object method to call.
Ex : 
Let there is a Interface A: 
public interface A
{
 void display();
}

And two classes B and C implement A
class B implements A
{
 display()
 {
    System.out.println("Class B");
  }
}

class C implements A
{
 display()
 {
    System.out.println("Class C");
  }
}

Then while using interface A..see what will happen
class mainClass
{
 public static void main(String args[])
 {
    A obj = new C();
    obj.display();
    obj = new B();
    obj.display();
 }
}

Output : Class C
         Class B
So it is clear that while run/execution time it is decided
which display() method to call..i.e class B or class C display()