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 have a different return type for the method b().

Examples:

public class C { 
  public C b() {} 
}

public class D { 
  public D b() {} 
}

How would I define my interface so that this was possible?

Answer 1

If the return type must be the type of the class that implements the interface, then what you want is called an F-bounded type:

public interface A<T extends A<T>>{ public T b(); }

public class C implements A<C>{
  public C b() { ... }
}

public class D implements A<D>{
  public D b() { ... }
}

Answer 2

Generics.

public interface A<E>{
    public E b();
}

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

public class D implements A<D>{
    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.

Answer 3

Since Java supports covariant return types (since Java 1.5), you can do:

public interface A { public Object b(); }