The question has been answered, but one good reason to use a nested interface is if its function is directly related to the class it is in. A good example of this is a Listener. If you had a class Foo and you wanted other classes to be able to listen for events on it, you could declare an interface named FooListener, which is ok, but it would probably be more clear to declare a nested interface and have those other classes implement Foo.Listener (a nested class Foo.Event isn't bad along with this).

内部接口必须是静态的，才能被访问。该接口与类的实例无关，而是与类本身相关，因此可以通过Foo访问它。酒吧，像这样:

public class Baz implements Foo.Bar {
   ...
}

在大多数情况下，这与静态内部类没有什么不同。

Jesse's answer is close, but I think that there is a better code to demonstrate why an inner interface may be useful. Look at the code below before you read on. Can you find why the inner interface is useful? The answer is that class DoSomethingAlready can be instantiated with any class that implements A and C; not just the concrete class Zoo. Of course, this can be achieved even if AC is not inner, but imagine concatenating longer names (not just A and C), and doing this for other combinations (say, A and B, C and B, etc.) and you easily see how things go out of control. Not to mention that people reviewing your source tree will be overwhelmed by interfaces that are meaningful only in one class.So to summarize, an inner interface enables the construction of custom types and improves their encapsulation.

class ConcreteA implements A {
 :
}

class ConcreteB implements B {
 :
}

class ConcreteC implements C {
 :
}

class Zoo implements A, C {
 :
}

class DoSomethingAlready {
  interface AC extends A, C { }

  private final AC ac;

  DoSomethingAlready(AC ac) {
    this.ac = ac;
  }
}

上面例子中的static关键字是多余的(嵌套的接口是自动“静态”的)，可以删除而不影响语义;我建议将其删除。接口方法上的“public”和接口字段上的“public final”也是如此——修饰符是多余的，只会给源代码增加混乱。

不管怎样，开发人员只是声明了一个名为Foo.Bar的接口。除了不能访问Foo的代码将不能访问Foo之外，与外围类没有进一步的关联。酒吧。(从源代码-字节码或反射可以访问Foo。即使Foo是包私有的!)

如果您希望只从外部类使用嵌套接口，那么以这种方式创建嵌套接口是可以接受的，这样就不会创建新的顶级名称。例如:

public class Foo {
    public interface Bar {
        void callback();
    }
    public static void registerCallback(Bar bar) {...}
}
// ...elsewhere...
Foo.registerCallback(new Foo.Bar() {
    public void callback() {...}
});

要非常直接地回答你的问题，请查看Map.Entry。

地图。条目

这也可能是有用的

静态嵌套Inerfaces博客条目

The question has been answered, but one good reason to use a nested interface is if its function is directly related to the class it is in. A good example of this is a Listener. If you had a class Foo and you wanted other classes to be able to listen for events on it, you could declare an interface named FooListener, which is ok, but it would probably be more clear to declare a nested interface and have those other classes implement Foo.Listener (a nested class Foo.Event isn't bad along with this).