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! :)

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! — Unai P. Mendizabal, Jun 29 '15 at 19:33

Louis Wasserman · Accepted Answer · 2015-06-29T19:37:45.687

11

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:

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

edited Jun 29 '15 at 19:37

answered Jun 29 '15 at 19:27

Louis Wasserman

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This answer would be better if you explained the reason. Even if it is complicated, it's better than "seeming to work." – Atreys Jun 29 '15 at 19:34
I agree with that. I'd love to know why this happens. – Unai P. Mendizabal Jun 29 '15 at 19:36
The accepted answer here explains it pretty well: http://stackoverflow.com/questions/4492950/java-generics-arraylist-initialization – mstelz Jun 29 '15 at 19:37
Added an explanation. – Louis Wasserman Jun 29 '15 at 19:37
The reason for this is particularly complicated. https://en.wikipedia.org/wiki/Covariance_and_contravariance_(computer_science) – Falmarri Jun 29 '15 at 19:41
That's some complex stuff! I guess I'll have to re-read it some times. Thanks for all the help! – Unai P. Mendizabal Jun 29 '15 at 19:51

won't accept A's child classes

1 Answers1