JAVA 多态遇到的题目

  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(A obj){
        return "b and a";
    }   
    public String show(B obj){
        return "b and b";
    }
}
class C extends B{}
class D extends B{}
public class Test1{
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));
    System.out.println(a1.show(c));
    System.out.println(a1.show(d));
    System.out.println(a2.show(b));
    System.out.println(a2.show(c));
    System.out.println(a2.show(d));
    System.out.println(b.show(b));
    System.out.println(b.show(c));
    System.out.println(b.show(d));
}
}

特别是a2.show( )输出的结果不理解。