JAVA多态问题

public class Test1{
public static void main(String arg[]){

        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));   //A-D
    System.out.println(b.show(b));     
    System.out.println(b.show(c));     
    System.out.println(b.show(d));  
}
}

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

A and A
A and A
A and D
B and A
B and A
A and D
B and B
B and B
A and D
为什么第4个输出不是B and B而是 B and A明明入参就是B类