Why can't I use generics in an inner interface?

Question

I tried to compile the following code:

public interface Foo<T> {
    public interface Bar {
        public void bar(T t);
    }

    void foo(T t);
}

But I get this error: "Foo.this cannot be referenced from a static context."

Specifically, I get it on the "T" in bar(T t). However foo(T t) does not produce the same error. I don't understand why that's a static context and what the error really means.

Similar with this: http://stackoverflow.com/questions/10963601/java-generics-non-static-type-variable-t-cannot-be-referenced-from-a-static-con — Alexey Semenyuk, Dec 18 '14 at 18:07
@brso05 this would redefine `T` rather than reuse it, so `T` might have a different value which I assume is not what the OP intends. — Thomas, Dec 18 '14 at 18:10
@brso05 of course but the compiler would still treat both `T`'s as having potentially different values - it's still parameter shadowing/hiding. — Thomas, Dec 18 '14 at 18:17
@BarryFruitman it should work just be careful when you use it... — brso05, Dec 18 '14 at 18:26
@BarryFruitman I'd probably use a different name for the inner `T` so that you see the difference the compiler sees. Hiding the outer `T` might cause hard to track bugs. — Thomas, Dec 18 '14 at 18:30

score 12 · Accepted Answer · answered Dec 18 '14 at 18:06

12

A "nested" interface (Bar in your example) is implicitly static. So it can't access instance specific information related to Foo, such as its generic type.

See for example JLS #8.5.1:

A member interface is implicitly static

answered Dec 18 '14 at 18:06

assylias

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Why can't I use generics in an inner interface?

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