Java interfaces and return types

Question

Consider I have the following interface:

public interface A { public void b(); }

However I want each of the classes that implement it to ha

Answer 1

Generics.

public interface A{
    public E b();
}

public class C implements A{
    public C b(){
        return new C();
    }
}

public class D implements A{
    public D b(){
        return new D();
    }
}

Search up generics for more details, but (very) basically, what's happening is that A leaves E's type up to the implementing clases (C and D).

So basically A doesn't know (and doesn't have to know) what E might be in any given implementation.