java 多态输出 问题求解

Question

有如下代码：
public static void main(String[] args) {

    A a2 = new B();  
    B b = new B();  
    C c = new C();   
    D d = new D();   
    System.out.println(" 1:"+a2.show(b));    
    System.out.println(" 2:"+a2.show(c));    
    System.out.println(" 3:"+a2.show(d));    
    System.out.println(" 4:"+b.show(c));     
    System.out.println(" 5:"+b.show(d));  
}

static class A {  
    public String show(D obj){  
           return ("A and D");  
    }   
    public String show(A obj){  
           return ("A and A");  
    }   

}

static class B extends A{

public String show(B obj){

return ("B and B");

}

public String show(A obj){

return ("B and A");

}

}

static class C extends B{}

static class D extends B{}

运行后结果是：
1:B and A

2:B and A
3:A and D
4:B and B
5:A and D

小弟非常不理解这样的输入结果，望各位大神指教

Answer 1

同样不太懂，但是查了一下有一个帖子回答了，你可以看一下
http://ask.csdn.net/questions/250227