关于java中Map遍历效率的测试

无意中看见代码扫描出的一些performance警告，大意是建议使用entrySet 代替KeySet对Map进行遍历。

 

 

测试了一下两者效率，发现确实使用entrySet更高效 大概高出30%-40%,原因是KeySet多了一次从Map中get的操作，虽然时间复杂度为常量级,但是对于大规模Map的遍历，还是有很大影响的。

 1 public class Test {
 2 
 3     public static void main(String[] args) {
 4         Map<String,String> entity = createTestMap(1000000 , 5);
 5         traversalBykeySet(entity);
 6         traversalByEntry(entity);
 7        
 8        
 9     }
10    
11     private static void traversalByEntry(Map<String,String> entity){
12         long startTime = Calendar.getInstance().getTimeInMillis();  
13         Iterator<Entry<String, String>> iter = entity.entrySet().iterator();
14         while (iter.hasNext()) {
15             Map.Entry<String, String> entry = (Map.Entry<String, String>) iter.next();
16             entry.getKey();
17             entry.getValue();
18         }
19         System.out.println("CostTime:" + (Calendar.getInstance().getTimeInMillis() - startTime));
20     }
21    
22     private static void traversalBykeySet(Map<String,String> entity){
23         long startTime = Calendar.getInstance().getTimeInMillis();  
24         for(String key : entity.keySet()){
25             entity.get(key);
26         }
27         System.out.println("CostTime:" + (Calendar.getInstance().getTimeInMillis() - startTime));
28     }
29    
30     private static Map<String,String> createTestMap(int size , int keyLength){
31         Map<String,String> entity = new HashMap<String,String>();
32         for(int i=0 ; i<size ; i++){
33             String randomString = getRandomString(keyLength);
34             entity.put(randomString, randomString);
35         }
36         return entity;
37     }
38    
39     public static String getRandomString(int length) {
40         StringBuffer buffer = new StringBuffer("0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ");
41         StringBuffer sb = new StringBuffer();
42         Random r = new Random();
43         int range = buffer.length();
44         for (int i = 0; i < length; i ++) {
45             sb.append(buffer.charAt(r.nextInt(range)));
46         }
47         return sb.toString();
48     }
49 
50 }