Using a nested type as a generic type argument in the enclosing class' supertype declaration

Question

I basically have a question about why the following does not work. I have an abstract class:

public abstract class Abstrct <T> {

}

I then define a class that makes use of that class with a public inner class defined that I want to use for the generic parameter, as follows:

public class Outer extends Abstrct<Inner> {
    public class Inner {

    }
}

As I am learning Java still, I am more interested in why it does not work. Not so much as to how to make it work, but I would be interested in that also.

Answer 1

Inner is not in scope for the class declaration of Outer. It's not a known type name when used in the extends clause. Use a qualified reference:

class Outer extends Abstract<Outer.Inner>

Or import it:

import com.example.Outer.Inner;

From the specification, regarding Scope:

The scope of a declaration of a member m declared in or inherited by a class type C (§8.1.6) is the entire body of C, including any nested type declarations.

The extends clause is part of the class' Superclass declaration, as described in the Specification. It's not part of the class body.

The scope of a type used in an import statement, however,

[..] is all the class and interface type declarations (§7.6) in the compilation unit in which the import declaration appears, as well as any annotations on the package declaration (if any) of the compilation unit .

Answer 2

You can use Abstrct<Outer.Inner> making the type unambiguous and valid.