<? extends A> won't accept A's child classes

Question

I'm working with Android Studio and I keep getting a problem I don't know how to solve. I don't know whether it's a problem with Android Studio, with Java or a mistake a make.

I have a class whose constructor is the following:

public MakeQuery(Callable<ArrayList<? extends A>) {
     ...
}

I try to create an object of that class with the following lines:

Callable<ArrayList<B>> callable = new Callable<ArrayList<B>>() {...};
MakeQuery makeQuery = new MakeQuery(callable);

(Of course, class B extends A. Double checked)

But when I call the constructor the IDE tells me that it expects another type of argument.

What mistake am I making? Thanks for all the help! :)

Johns

show all code of Callable class – Lives

Does the IDE tell you or the java compiler ? – Ascham

The IDE tells me. I don't get to compile. And by the way, Callable is an interface that exists since JDK v 1.5. Thanks for helping! – Johns

you may try Callable<ArrayList<? extends A>> callable = new Callable<ArrayList<? extends A>>() {...}; – Valadez

It may work but that's not what I wanted. – Johns

Answer 1

Write Callable<? extends ArrayList<? extends A>>.

The reasons for this are complicated, but the code you wrote will only work if you pass in exactly Callable<ArrayList<? extends A>>.

The rule is that even if B extends A, Foo<B> does not extend Foo<A>. It does, however, extend Foo<? extends A>.

So apply that rule twice:

1. List<B> doesn't extend List<A>, but it extends List<? extends A>.
2. Callable<List<B>> doesn't extend Callable<List<? extends A>>, but it extends Callable<? extends List<? extends A>>.

Downtown

This answer would be better if you explained the reason. Even if it is complicated, it's better than "seeming to work." – Kurman

I agree with that. I'd love to know why this happens. – Johns

The accepted answer here explains it pretty well: #4493450 – Pentastich

Added an explanation. – Downtown

The reason for this is particularly complicated. en.wikipedia.org/wiki/… – Thermy

That's some complex stuff! I guess I'll have to re-read it some times. Thanks for all the help! – Johns