There are four internal classes, and why?

There are four internal classes:
1. static internal classes can only use static members in external classes, which is of little use.
2. The member internal class is similar to the external class attributes and methods in parallel. It can call the attributes and methods of the external class and is fully useful.
3. Internal class of the method, defined in the method, can access attributes and methods in the peripheral class, and final constants in the method. It's useless, because the fourth one is easier than him.
4. Anonymous internal classes, which are most used, are implemented through extended interfaces or other classes. The usage is the same as that of internal classes of methods, which is easier to write.
Why use internal classes? In short, it makes sense when the class and its class or method are together. If it is separate, it does not make sense. Therefore, it is defined in the class or method.