java多态

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 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("1- "+a1.show(b));   
        System.out.println("2- "+a1.show(c));    
        System.out.println("3- "+a1.show(d));    
        System.out.println("4- "+a2.show(b));   
        System.out.println("5- "+a2.show(c));
        System.out.println("6- "+a2.show(d));    
        System.out.println("7- "+b.show(b));    
        System.out.println("8- "+b.show(c));     
        System.out.println("9- "+b.show(d)); 
        System.out.println("10- "+a2.show(a1)); 
        System.out.println("11- "+a1.show(c)); 
        System.out.println("12- "+a1.show(d)); 
    }