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

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.