覆盖嵌套在Java中参数化外部类中的非参数化类

Question

Can someone please help with the syntax of subclassing a non-parameterized nested class inside a parameterized outer class, like the following?

public abstract class Foo<T> {

    public class Bar {
        Set<T> aSet;
    }

    abstract class Baz {
        abstract void doSomething(Map<? extends Bar, T> aMap);
    } 
}

Then in a separate file, I'm not entirely sure how to define the type variable S here without parameterizing ConcreteBaz . And I can't use a wildcard variable like extends Foo<?>.Baz (I think) because I need access to that generic type variable in the doSomething method:

public class ConcreteBaz extends Foo<S>.Baz {    // compilation error

    public ConcreteBaz(Foo<S> foo) { foo.super(); }

    public void doSomething(Map<? extends Foo<S>.Bar, S> aMap) { ... }

}

Can someone please help me figure out a way around this? Thanks.

Answer 1

Your problem is that the nested class is nonstatic. I'll check it out, but I'm pretty sure you cannot subclass such classes, except maybe if nested in the same class, or when creating an anonymous type.

Is declaring the nested class static viable? Them it would definitely work.

Edit: scratch all that. I don't have a compilation error. What is S in your case? You do realize you have to pass a concrete class to Foo, and cannot leave it parametrized with an unknown parameter S?

Answer 2

如果ConcreteBaz需要引用类型参数，则意味着它需要类型参数本身：

public class ConcreteBaz<S> extends Foo<S>.Baz {

Answer 3

Declare S as a type parameter:

public class ConcreteBaz<S> extends Foo<S>.Baz {

Otherwise the compiler will think S is a concrete type instead of a type parameter.

Complete example:

public class ConcreteBaz<S> extends Foo<S>.Baz {
    public ConcreteBaz(Foo<S> foo) {
        foo.super();
    }

    @Override
    void doSomething(Map<? extends Foo<S>.Bar, S> aMap) {
        // ...
    }
}