Java generics - override a function that returns a set<T>

Question

This works :

public interface A {
     A doSomething(); 
}

public interface B extends A{
     B doSomething(); 
}

This doesn't :

public interface A {
     Collection<A> doSomething(); 
}

public interface B extends A{
     Collection<B> doSomething(); 
}

Why? and what can I do to get the functionality I want in the second example?

Answer 1

The reason is that Collection<B> is not a sub-type of Collection<A>.

Not every operation that's possible with a Collection<A> is also possible with a Collection<B>. Case in point: calling add(new A()) works with a Collection<A> but must fail with a Collection<B> (even if that's just checked at compile-time).

If you don't care about adding elements to the returned Collection (often you only care about iterating over them, effectively treating them as if they were read-only), then you can do this:

public interface A {
     Collection<? extends A> doSomething();
}

public interface B extends A{
     Collection<? extends B> doSomething();
}

You can even let B.doSomething() return a Collection<B> if you want, but then you won't be able to extend B in the same way again.

Answer 2

Change it to:

public interface A {
    Collection<? extends A> doSomething();
}

public interface B extends A{
    Collection<B> doSomething(); 
}

and it'll work properly.

It's because "a collection of B" does not extend a collection of A", even though B extends A. A needs to define the return type as a "collection of items that extends A" than you can narrow it further as "a collection of B". Confusing, yes ... but that's just how it is.