Implements or extends from nestedclasses or interfaces.
实施或者扩展固定类或者界面。
2
The protected modifier cannot be used for classes at package scope, but it can be used for nestedclasses.
受保护的修饰符不能对包范围的类使用,但可以对嵌套的类使用。
3
If a class has one or more private constructors and no public constructors, other classes (except nestedclasses) cannot create instances of this class.