Java: Interface generic of a superclass

Question

Imagine the following code:

class A {}
class B extends A {}

interface IA extends Iterable<A> {}
interface IB extends Iterable<B> {}

Ideally, I would like the interface IB to be able to also extend IA because it does in fact allow you to retrieve As.

interface IB extends Iterable<B>, IA {}

or even

interface IB extends Iterable<B> implements IA {}

However, the compiler really dislikes both of those and it would make my code much better if this was allowed as conceptually B can be used as A without up-casting everywhere

What solutions are available for me to solve this problem?

Answer 1

The non-covariance of generics means that what you want isn't viable (at least not in the general case).

However, perhaps wildcards would solve your specific problem? e.g.

void methodThatOperatesOnA(Iterable<? extends A> it) {
    ...
}

This will allow you to extract elements from it as if they were As, but the compiler will prevent you from inserting objects,^* because it can't guarantee that invariants would be maintained.

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_{* other than null and a few other wacky cases}