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: stackoverflow.com/a/74400/697449

Paul Bellora
– Paul Bellora

2012-06-09 20:35:02 +00:00
Commented Jun 9, 2012 at 20:35 — Paul Bellora
– Paul Bellora, Commented Jun 9, 2012 at 20:35

alisianoi · Accepted Answer · 2016-10-22 19:43:34Z

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, 2016 at 19:43

alisianoi

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answered Jun 9, 2012 at 18:48

Jigar Joshi

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3 Comments

auser Over a year ago

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?

Paul Bellora Over a year ago

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.

Jigar Joshi Over a year ago

well inner interface is same as inner field

Akhi · Accepted Answer · 2012-06-09 18:57:31Z

3

How about something like this.

public interface A<T> {

     interface B<T> extends A<T>{

       T foo(); 
    }

}

answered Jun 9, 2012 at 18:57

Akhi

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Comments

Dodd10x · Accepted Answer · 2020-02-05 18:41:05Z

0

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

interface A<T> {

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

}

answered Feb 5, 2020 at 18:41

Dodd10x

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1 Comment

Josie Thompson Over a year ago

Isn't the T in B<T> a different T than in A<T> here?

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