Java动态、重写 理解

相关类：

class A ...{
         public String show(D obj)...{
                return ("A and D");
         }
         public String show(A obj)...{
                return ("A and A");
         }
}
class B extends A...{
         public String show(B obj)...{
                return ("B and B");
         }
         public String show(A obj)...{
                return ("B and A");
         }
}
class C extends B...{}
class D extends B...{}

问题：以下输出结果是什么？

A a1 = new A();
        A a2 = new B();
        B b = new B();
        C c = new C();
        D d = new D();
        System.out.println(a1.show(b));   ①
        System.out.println(a1.show(c));   ②
        System.out.println(a1.show(d));   ③
        System.out.println(a2.show(b));   ④
        System.out.println(a2.show(c));   ⑤
        System.out.println(a2.show(d));   ⑥
        System.out.println(b.show(b));    ⑦
        System.out.println(b.show(c));    ⑧
        System.out.println(b.show(d));    ⑨

结果：

①   A and A
②   A and A
③   A and D
④   B and A
⑤   B and A
⑥   A and D
⑦   B and B
⑧   B and B
⑨   A and D