[JAVA 基础] hashcode() &　equals()

时间：2016-04-30 11:32:11 阅读：196 评论：0 收藏：0 [点我收藏+]

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平时使用map时都是用JAVA原生的类型，所以很少关注到hashcode()和equals()的方法的内部实现。近期实现一个小工具，涉及到自己写的类的查找比对，又再次重温了相关的知识。

上简单示例代码，比对自己覆写和不覆写的差异

不覆写hashcode()和equals()：

 1 import java.util.HashMap;
 2 import java.util.Map;
 3 
 4 public class HashCode_Equals {
 5 
 6     public int a;
 7     public int b;
 8 
 9     public static void main(String[] args) {
10 
11         Map<HashCode_Equals, String> map = new HashMap<HashCode_Equals, String>();
12 
13         HashCode_Equals obj1 = new HashCode_Equals();
14         obj1.a = 1;
15         obj1.b = 1;
16         map.put(obj1, "test");
17 
18         HashCode_Equals obj2 = new HashCode_Equals();
19         obj2.a = 1;
20         obj2.b = 1;
21 
22         System.out.println("obj1.equals(obj2)=" + obj1.equals(obj2));
23         System.out.println("map.get(obj2)=" + map.get(obj2));
24     }
25 }

View Code

执行结果:

obj1.equals(obj2)=false
map.get(obj2)=null

仅覆写equals（）方法

 1 import java.util.HashMap;
 2 import java.util.Map;
 3 
 4 public class HashCode_Equals {
 5 
 6     public int a;
 7     public int b;
 8 
 9     @Override
10     public boolean equals(Object obj) {
11         if (this == obj)
12             return true;
13         if (obj == null)
14             return false;
15         if (getClass() != obj.getClass())
16             return false;
17         HashCode_Equals other = (HashCode_Equals) obj;
18         if (a != other.a)
19             return false;
20         if (b != other.b)
21             return false;
22         return true;
23     }
24 
25     public static void main(String[] args) {
26 
27         Map<HashCode_Equals, String> map = new HashMap<HashCode_Equals, String>();
28 
29         HashCode_Equals obj1 = new HashCode_Equals();
30         obj1.a = 1;
31         obj1.b = 1;
32         map.put(obj1, "test");
33 
34         HashCode_Equals obj2 = new HashCode_Equals();
35         obj2.a = 1;
36         obj2.b = 1;
37 
38         System.out.println("obj1.equals(obj2)=" + obj1.equals(obj2));
39         System.out.println("map.get(obj2)=" + map.get(obj2));
40     }
41 
42 }

View Code

执行结果：

obj1.equals(obj2)=true
map.get(obj2)=null

同时覆写hashcode()和equals()（借用eclipse集成的功能自动生成）

 1 import java.util.HashMap;
 2 import java.util.Map;
 3 
 4 public class HashCode_Equals {
 5 
 6     public int a;
 7     public int b;
 8 
 9     @Override
10     public int hashCode() {
11         final int prime = 31;
12         int result = 1;
13         result = prime * result + a;
14         result = prime * result + b;
15         return result;
16     }
17 
18     @Override
19     public boolean equals(Object obj) {
20         if (this == obj)
21             return true;
22         if (obj == null)
23             return false;
24         if (getClass() != obj.getClass())
25             return false;
26         HashCode_Equals other = (HashCode_Equals) obj;
27         if (a != other.a)
28             return false;
29         if (b != other.b)
30             return false;
31         return true;
32     }
33 
34     public static void main(String[] args) {
35 
36         Map<HashCode_Equals, String> map = new HashMap<HashCode_Equals, String>();
37 
38         HashCode_Equals obj1 = new HashCode_Equals();
39         obj1.a = 1;
40         obj1.b = 1;
41         map.put(obj1, "test");
42 
43         HashCode_Equals obj2 = new HashCode_Equals();
44         obj2.a = 1;
45         obj2.b = 1;
46 
47         System.out.println("obj1.equals(obj2)=" + obj1.equals(obj2));
48         System.out.println("map.get(obj2)=" + map.get(obj2));
49     }
50 
51 }