Java wildcards don't allow type-casting

Question

interface Message<T, L> {
}

interface Foo<T> {
    void frob(Message<T, Foo<?>> message);
}

class AuxiliaryFoo implements Foo<Integer> {
    @Override
    public void frob(Message<Integer, Foo<?>> message) { }
}

class MainFoo implements Foo<Object> {
    @Override
    public void frob(Message<Object, Foo<?>> message) {
        new AuxiliaryFoo().frob(new Message<Integer, MainFoo>() {});
    }
}

The Java compiler tells me that actual argument < anonymous Message< Integer, MainFoo >> cannot be converted to Message< Integer, Foo< ?>> by method invocation conversion.

Why?

And what can be converted to Message< Integer, Foo< ?>> ?

score 10 · Accepted Answer · edited May 23 '17 at 11:46

10

You should use <? extends Foo<?>>

interface Foo<T> {     
    void frob(Message<T, ? extends Foo<?>> message); 
}

Also, something worthy to keep in mind when dealing with generics is the PECS rule: Producer Extends, Consumer Super though it doesn't directly belong here, but I can't say that it doesn't belong here at all..

edited May 23 '17 at 11:46

Community

1
1

answered Oct 16 '12 at 12:14

ppeterka

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Oh, a double wildcard. It worked, thanks! Also, I doubt PECS quite applies here. – Joker_vD Oct 16 '12 at 12:21
I just mentioned PECS because it's worth keeping in mind, I'll edit the answer to reflect this. – ppeterka Oct 16 '12 at 12:22

Java wildcards don't allow type-casting

1 Answers1