在封闭类的超类型声明中使用嵌套类型作为泛型类型参数

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

您可以使用Abstrct<Outer.Inner>使类型明确且有效。

Answer 3

You have to be explicit since the inner class you defined is not static so it only exists within the scope of the containing class. (See https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html )

In your case try following:

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

    }
}