Compelling reasons for using nested classes include the following:
It is a way of logically grouping classes that are only used in one
place: If a class is useful to only one other class, then it is
logical to embed it in that class and keep the two together. Nesting
such "helper classes" makes their package more streamlined.
It increases encapsulation: Consider two top-level classes, A and B,
where B needs access to members of A that would otherwise be declared
private. By hiding class B within class A, A's members can be declared
private and B can access them. In addition, B itself can be hidden
from the outside world.
It can lead to more readable and maintainable code: Nesting small
classes within top-level classes places the code closer to where it is
used.
# 1 楼答案
Here是嵌套类的完整文档
如果您想知道何时使用内部类（从link）：
是的，您可以从其他类访问内部类。（阅读controlling access to members of a class）
Here您可以找到一个关于如何从类的“外部”访问内部类的示例