什么时候使用嵌套的 Java 类真的有用? [英] When is really useful to use a nested Java class?

问题描述

你能给我一个具体的例子，说明嵌套的 java 类是有用的吗?我正在研究它，我了解它的工作原理，但我无法想象真正有必要使用它的真实情况.

Could you give me a concrete example where a nested java class is useful? I am studying it, I understand how it works but I can not imagine a real situation where its use is really necessary.

非常感谢

马可

推荐答案

关于何时使用嵌套类的最简洁的总结是:当该类在逻辑上属于外部类的 API 时，或者何时封装特定于外部类的行为.

The most concise summary of when to use a nested class is: when that class is logically part of the outer class' API, or when it encapsulates behavior specific to the outer class.

例如，Map.Entry:它是映射中的一个条目.它在逻辑上是 Map 的 API 的一部分，所以把它放在 Map 中是有意义的.

For example, Map.Entry: it's an entry in a mapping. It's logically part of Map's API, so it makes sense to put it inside Map.

另一个常见示例是 Builder 类:您使用 Builder 制作一个东西，所以把它放在那个东西里面是有意义的.

Another common example is Builder classes: you use a Builder to make a thing, so it makes sense to put it inside that thing.

这些是您仅在使用外部类的上下文中真正使用的类:有时您可能会将 Map.Entry 本身用作某种配对类;有时您可能想单独使用 Builder ，例如作为方法的参数，该方法将事物放入该构建器中，但实际上并不构建本身.这些情况可能与也使用外部类的代码相去甚远:您仍在使用外部类的 API 的某些方面，因此它们在逻辑上仍然属于该 API.

These are classes that you only really use in the context of using the outer classes: sometimes you might use a Map.Entry by itself as some sort of pair class; and sometimes you might want to use a Builder by itself, for example as a parameter to a method which puts things into that builder, but doesn't actually do the building itself. These cases are probably rarely far from code that also uses the outer class too: you're still using some aspect the outer class' API, so they do still logically belong to that API.

您可以将此类类与主"类一起放在顶层.您可能不想这样做的原因有几个:

You could put such classes at the top level, alongside the "main" class. There are a few reasons why you might not want to do this:

• 它使命名空间变得混乱.如果你有 Foo、Bar、Baz 等类，还有 FooBuilder、BarBuilder, BazBuilder 在顶层只会让我们更难看到有用的"顶层类.
  • Google 的协议缓冲区 为每个生成一个 Builder 类消息类型.考虑到 Google 代码中使用的协议缓冲区的数量，这种混乱会太麻烦.
  • It clutters the namespace. If you've got classes Foo, Bar, Baz etc, having FooBuilder, BarBuilder, BazBuilder at the top level just makes it harder to see the "useful" top level classes.
    • Google's protocol buffers generate a Builder class for every message type. Given the number of protocol buffers used in Google code, this clutter would be far too cumbersome.

    您还可以使用它们来封装类中的内部逻辑或中间状态:例如，当我发现自己在类中的私有方法之间传递相同的 N 个参数时，我喜欢定义私有嵌套类.这是您不希望班级以外的人关心的事情；只是在课堂上方便.

    You can also use them to encapsulate internal logic or intermediate state in the class: for example, I like to define private nested classes when I find myself passing around the same N parameters between private methods in a class. This is stuff that you don't want people outside your class to care about; it's merely convenient inside the class.

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