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