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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).

Typically I see static inner classes. Static inner classes cannot reference the containing classes wherease non-static classes can. Unless you're running into some package collisions (there already is an interface called Bar in the same package as Foo) I think I'd make it it's own file. It could also be a design decision to enforce the logical connection between Foo and Bar. Perhaps the author intended Bar to only be used with Foo (though a static inner interface won't enforce this, just a logical connection)

如果你将类Foo改为接口Foo，上面例子中的“public”关键字也将是多余的，因为

在另一个接口中定义的接口将隐式公开 静态的。

成员接口是隐式静态的。可以在不改变代码语义的情况下删除示例中的静态修饰符。请参见Java语言规范8.5.1。静态成员类型声明

要非常直接地回答你的问题，请查看Map.Entry。

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这也可能是有用的

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