java 中的instanceof 运算符

 1 package com.instanceoftest;
 2  
 3  
 4  
 5  interface A{}
 6  class B implements A{
 7   
 8  }
 9  class C extends B {
10   
11  }
12  
13  class instanceoftest {
14   public static void main(String[] args){
15      A a=null;
16      B b=null;
17      boolean res; 
18      
19      System.out.println("instanceoftest test case 1: ------------------");
20        res = a instanceof A; 
21        System.out.println("a instanceof A: " + res);
22        
23        res = b instanceof B;
24        System.out.println("b instanceof B: " + res);
25        
26      System.out.println("/ninstanceoftest test case 2: ------------------");   
27      a=new B();
28      b=new B();
29      
30      res = a instanceof A; 
31      System.out.println("a instanceof A: " + res);
32      
33      res = a instanceof B;
34      System.out.println("a instanceof B: " + res);
35  
36      res = b instanceof A;
37      System.out.println("b instanceof A: " + res);
38      
39      res = b instanceof B;
40      System.out.println("b instanceof B: " + res);
41     
42      System.out.println("/ninstanceoftest test case 3: ------------------");
43      B b2=(C)new C();
44      
45      res = b2 instanceof A;
46      System.out.println("b2 instanceof A: " + res);
47      
48      res = b2 instanceof B;
49      System.out.println("b2 instanceof B: " + res);
50      
51      res = b2 instanceof C;
52      System.out.println("b2 instanceof C: " + res);
53   }
54 }
55  
56 
57 /*
58 result:
59  
60 
61 instanceoftest test case 1: ------------------
62 a instanceof A: false
63 b instanceof B: false
64  
65 instanceoftest test case 2: ------------------
66 a instanceof A: true
67 a instanceof B: true
68 b instanceof A: true
69 b instanceof B: true
70  
71 instanceoftest test case 3: ------------------
72 b2 instanceof A: true
73 b2 instanceof B: true
74 b2 instanceof C: true
75  
76  
77  
78 */