深入理解java多态性

贴子中部分源代码摘自csdn的一篇帖子， 参考了他们的理解，说说自己的一些见解：
//源代码：
class A {
         public String show(D obj){
                return ("A and D");
         }
         public String show(A obj){
                return ("A and A");
         }
         public String show(C obj){
               return ("A and C");
         }
}
class B extends A{
         public String show(B obj){
                return ("B and B");
         }
         public String show(A obj){
                return ("B and A");
         }
         public String show(C obj){
                return ("B and C");
  }
}
class C extends B{}
class D extends B{}
class Test{     
   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));  &nb