初级类部类

class A{ 
     public String show(D obj){ 
         return ("A and D"); 
     }  
     publiiuc String show(A obj){
  return ("A and A"); 
     }  
     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)); // new A(). show(b)new 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));    
     }
 }
 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{}