java - 如何在 Java 中使用内部类实例化递归绑定(bind)泛型？

Question

我想要以内部类作为参数的通用外部类。我期待的是，我将从基类派生并使用它的内部，或者也派生内部。在每个级别上，我都希望从当前派生级别开始限制内部类的使用。

不幸的是，我在使用该模式时遇到了各种错误和警告，以至于我无法想象如何使用它。

package tests.java;

public class Try_GenericInnerRecursion {

    // base class, consisting of outer and inner parts
    public static class Outer1<E extends Outer1<E>.Inner1> {
        public class Inner1 {
        }

        public void addElement(E e) {
            System.out.println("Added " + e.toString());
        }
    }

    // extending outer, but not inner
    public static class Outer2<E extends Outer1<E>.Inner1> extends Outer1<E>{

    }

    // extending both outer and inner
    public static class Outer3<E extends Outer3<E>.Inner3> extends Outer1<E>{
        public class Inner3 extends Inner1 {
        }
    }

    // extending both outer and inner and stopping extension
    public static class Outer4 extends Outer1<Outer4.Inner4> {
        public class Inner4 extends Outer1<Inner4>.Inner1 {
        }
    }



    // instantiating
    public static void main(String[] args) {

        Outer1<Outer1.Inner1> a1; // WARNING: Outer1.Inner1 is a raw type

        a1 = new Outer1<Outer1.Inner1>(); // WARNING: Outer1.Inner1 is a raw type

        Outer1<?> a2; // OK 

        a2 = new Outer1<?>(); // ERROR: Cannot instantiate the type Outer1<?>

        Outer1<Outer1<?>.Inner1> a3; // ERROR: Bound mismatch: The type Outer1<?>.Inner1 is not a valid substitute for the bounded parameter <E extends Outer1<E>.Inner1> of the type Outer1<E>

    Outer1<? extends Outer1<?>.Inner1> a4; // OK

    a4 = new Outer1<Outer1.Inner1>(); // ERROR: Type mismatch

            Outer2<Outer1.Inner1> b1; // WARNING: Outer1.Inner1 is a raw type

            b1 = new Outer2<Outer1.Inner1>(); // WARNING: Outer1.Inner1 is a raw type

        // and so on
    }

}

如何正确使用这个模式？

Answer 1

我相信你可以简单地做

    DerivedCorpus1<?>

在通配符捕获期间，它变为

    DerivedCorpus1<x> where x extends Corpus<x>.Element

正确界定。

在你的例子中

    DerivedCorpus1<? extends Corpus<?>.Element>

在通配符捕获期间，它变为

    DerivedCorpus1<x> where x extends Corpus<x>.Element
                        and x extends Corpus<?>.Element

显然，extends Corpus<?>.Element条款是多余的。