Java从内部接口访问外部类的泛型参数

发布于 2024-12-11 11:49:18 字数 451 浏览 1 评论 0原文

考虑一下代码：

public class A<T extends X> {
  public static interface Delegate {
    void doMagic(T t); // why can't I access "T" here?
  } 

  public A(Delegate delegate) { ... }
}
...
public class TheDelegate implements A<Y> { ... }
...
A<Y> a = new A<Y>(new A<Y>.Delegate() {
  @Override
  public void doMagic(Y y) {
    ...
  }
});

为什么我无法从 Delegate 接口访问 T？

原文

Consider the code:

public class A<T extends X> {
  public static interface Delegate {
    void doMagic(T t); // why can't I access "T" here?
  } 

  public A(Delegate delegate) { ... }
}
...
public class TheDelegate implements A<Y> { ... }
...
A<Y> a = new A<Y>(new A<Y>.Delegate() {
  @Override
  public void doMagic(Y y) {
    ...
  }
});

Why can't I access T from Delegate interface?

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好倦 2024-12-18 11:49:18

这是因为你的内部接口是静态的。泛型参数仅适用于 A 的实例，而不适用于类，因此 T 的范围是 A 的非静态范围。

如果您不知道，所有接口和枚举在 Java 中是静态的，即使它们没有声明为静态并且位于另一个类中。因此，无法通过接口来解决这个问题。

另请参阅此答案。

编辑：史蒂文的答案是正确的。但是，您的用户代码将如下所示：

// Note the extra declaration of the generic type on the Delegate.
A<Integer> a = new A<Integer>(new A.Delegate<Integer>() {
  @Override
  public Integer myMethod() {
    return null;
  }
});

It's because your inner interface is static. The generic parameter only applies to an instance of A as opposed to applying to the class, so the scope of T is the non-static scope of A.

In case you didn't know, all interfaces and enumerations are static in Java, even if they are not declared as static and are inside another class. Therefore there is no way to work around this with an interface.

See this answer also.

EDIT: Steven's answer is correct. However, your user code will look like this:

// Note the extra declaration of the generic type on the Delegate.
A<Integer> a = new A<Integer>(new A.Delegate<Integer>() {
  @Override
  public Integer myMethod() {
    return null;
  }
});

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