一道多态题

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 {}

public class Main {
    public static void main(String[] args) {
        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
    }
}