一道Java程序输出题(继承-重载-覆盖-向上转型的问题)

 1 class A {
 2     public String show(D obj) { // func1
 3         return ("A and D");
 4     }
 5     public String show(A obj) { // func2
 6         return ("A and A");
 7     }
 8 }
 9 class B extends A {
10     public String show(B obj) { // func3
11         System.out.println("1");
12         return ("B and B");
13     }
14     public String show(A obj) { // func4
15         System.out.println("2");
16         return ("B and A");
17     }
18 }
19 class C extends B {}
20 class D extends B {}
21 
22 public class Main {
23     public static void main(String[] args) {
24         A a1 = new A();
25         A a2 = new B();
26         A b = new B();
27         C c = new C();
28         D d = new D();
29         System.out.println(a2.show(b));
30     }
31 }