type of object while implementing interface

Question

Below is my code...

Its not that much complex.

Here I want to understand that In class D, b is an interface type variable and in that variable we are storing reference to new object of class(C) which implements that interface(B). How we are able to assign object of type C to interface B type variable b..? Class and Interface both are of different types then what makes it special when we implements a interface on class and we are able to do it which is my question

public interface A {

}

public interface B {

    public A methodABCD();
}

public class C implements B {

    final A typeA;

    public A methodABCD() {
        return typeA;
    }
}

public class D {
  static private B b;

  static public A methodABCD() {
    if (b == null) {
        b = new C();-------------->How..?
    }
    return b.methodABCD();
  }
}

Answer 1

Ok lets take an Example and illustrate it.

interface Animal {  
   public void eat();  
 }  
 class Human implements Animal{  
   public void eat(){  
     // Eat cakes and Buns   
   }  
 }  
 class Dog implements Animal{  
   public void eat(){  
     // Eat bones   
   }  
 }  

Now let see how you can use them

Animal h = new Human();     
Animal d = new Dog();

Now at some point you might want to change your Human to behave like a Dog.

   h =d;
   h.eat();// Eat bones 

Your thought became a possibility because, they belongs to the same type. Imagine that there is not Animal interface and see how difficult it is convert a Human to Dog.

You see the flexibility and the advantages in type varying. You are allowed to do that because of they both are Animal nothing but an Interface.