Java Generics: non-static type variable T cannot be referenced from a static context

Question

interface A<T> {

    interface B {
       // Results in non-static type variable T cannot
       // be referenced from a static context
       T foo(); 
    }

}

Is there anyway round this? Why is T seen as static when referenced from A.B?

related: http://stackoverflow.com/a/74400/697449 – Paul Bellora Jun 09 '12 at 20:35 — Paul Bellora, Jun 09 '12 at 20:35

score 10 · Accepted Answer · edited Oct 22 '16 at 19:43

10

All member fields of an interface are by default public, static and final.

Since inner interface is static by default, you can't refer to T from static fields or methods.

Because T is actually associated with an instance of a class, if it were associated with a static field or method which is associated with class then it wouldn't make any sense

edited Oct 22 '16 at 19:43

alisianoi

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answered Jun 09 '12 at 18:48

jmj

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Thanks, that answers the first part of the question. How do I get the type T seen in the inner interface to be the same type at that of the containing interface? – auser Jun 09 '12 at 18:51
2

I would remove the first sentence as it's unnecessary and misleading - yes interface fields are implicitly `public static final` but this has to do with interface *methods* which are implicitly `public abstract`. The fact that inner interfaces are implicitly `static` themselves is what's important. – Paul Bellora Jun 09 '12 at 22:12
well inner interface is same as inner field – jmj Jun 10 '12 at 05:27

score 3 · Answer 2 · answered Jun 09 '12 at 18:57

3

How about something like this.

public interface A<T> {

     interface B<T> extends A<T>{

       T foo(); 
    }

}

answered Jun 09 '12 at 18:57

Akhi

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score 0 · Answer 3 · answered Feb 05 '20 at 18:41

0

Your inner interface doesn't know what T is. Try this.

interface A<T> {

    interface B<T> {
       T foo(); 
    }

}

answered Feb 05 '20 at 18:41

Dodd10x

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Isn't the T in B a different T than in A here? – Josie Thompson May 15 '20 at 03:35

Java Generics: non-static type variable T cannot be referenced from a static context

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