Question

SITUATION: Say there is a class A and an interface B.

REQUIREMENT: If any class, say C, wants to create objects of A and use them, then that class will also have to implement interface B.Is there any way to enforce this condition?

WHY: Now a question may arise as to why I want to do such a thing. The reason is that when a class C creates objects of A and uses them, then those objects call certain methods of C. I want to declare those methods in interface B, so that C will invariably implement those methods.

Answer 1

Since you say that objects of class A will call methods on C, they will have to keep reference to C somehow. Make this reference of type B and you are done.

That is

public class A {
  public A(B arg) {
     ....
  }
}

Then in C:

A a = new A(this);

That will force class C to implement interface B.