1 class 用public 修饰时，只能与abstract & final 联用

Illegal modifier for the class InnerDemo04; only public, abstract & final are permitted

2 方法中的内部类只能用abstract or final

Illegal modifier for the local class Inner05; only abstract or final is permitted

3 外部类不能用static，内部类(pulic static class、static class)可以用

Illegal modifier for the class Outer08; only public, abstract & final are permitted