public class A {
    public static class B {
        public void fun() {
            System.out.println("OK");
        }
    }
    public class C {
        public void fun() {
            System.out.println("OK");
        }
    }
    public static void main(String[] args) {
        A a = new A();
        A.B ab = new A.B();
        A.C b = a.new C();//成功
        A.C c = new A().new C();//成功
    }
}