HashMap的四种访问方式

第一种：通过Map.entrySet使用iterator遍历key和value

1 public void visit_1(HashMap<String,Integer> hm){
2 Iterator<Map.Entry<String,Integer>> it = hm.entrySet().iterator();
3 while(it.hasNext()){
4 Map.Entry<String ,Integer> entry = it.next();
5 String key = entry.getKey();
6 Integer value = entry.getValue();
7 }
8 }

第二种：通过Key来遍历value

1 public void visit_2(HashMap<String,Integer> hm){
2 for (String key:hm.keySet()){
3 Integer value = hm.get(key);
4 }
5 }

第三种：通过Map.Entry遍历key和value

1 public void visit_3(HashMap<String,Integer> hm){
2 for(Map.Entry<String,Integer> entry:hm.entrySet()){
3 String key = entry.getKey();
4 Integer value = entry.getValue();
5 }
6 }

第四种：通过Map.keySet使用iterator遍历key和value

 1 public void visit_4(HashMap<Integer,String> hm){
 2 long startTime = System.currentTimeMillis();
 3 Iterator<Integer> it = hm.keySet().iterator();
 4 while(it.hasNext()){
 5 Integer key = it.next();
 6 String value = hm.get(key);
 7 }
 8 System.out.println("visit_4 10000000 entry:"
 9 + (System.currentTimeMillis()-startTime) + " milli seconds");
10 }

四种方法比较：

 1 package chapter08.c86.c862;
 2 
 3 import java.util.HashMap;
 4 import java.util.Iterator;
 5 import java.util.Map;
 6 
 7 /**
 8 * Created by ceoicac on 2017/8/12 10:22.
 9 */
10 public class HashMapVisitTest {
11 public void visit_1(HashMap<Integer,String> hm){
12 long startTime = System.currentTimeMillis();
13 Iterator<Map.Entry<Integer,String>> it = hm.entrySet().iterator();
14 while(it.hasNext()){
15 Map.Entry<Integer,String> entry = it.next();
16 Integer key = entry.getKey();
17 String value = entry.getValue();
18 }
19 System.out.println("visit_1 10000000 entry:"
20 + (System.currentTimeMillis()-startTime) + " milli seconds");
21 }
22 public void visit_2(HashMap<Integer,String> hm){
23 long startTime = System.currentTimeMillis();
24 for (Integer key:hm.keySet()){
25 String value = hm.get(key);
26 }
27 System.out.println("visit_1 10000000 entry:"
28 + (System.currentTimeMillis()-startTime) + " milli seconds");
29 }
30 public void visit_3(HashMap<Integer,String> hm){
31 long startTime = System.currentTimeMillis();
32 for(Map.Entry<Integer,String> entry : hm.entrySet()){
33 Integer key = entry.getKey();
34 String value = entry.getValue();
35 }
36 System.out.println("visit_1 10000000 entry:"
37 + (System.currentTimeMillis()-startTime) + " milli seconds");
38 }
39 public void visit_4(HashMap<Integer,String> hm){
40 long startTime = System.currentTimeMillis();
41 Iterator<Integer> it = hm.keySet().iterator();
42 while(it.hasNext()){
43 Integer key = it.next();
44 String value = hm.get(key);
45 }
46 System.out.println("visit_1 10000000 entry:"
47 + (System.currentTimeMillis()-startTime) + " milli seconds");
48 }
49 public static void main(String [] args){
50 HashMap<Integer,String> hm = new HashMap<>();
51 for(int i = 1;i <= 10000000;++i){
52 hm.put(i,"num: " + i);
53 }
54 new HashMapVisitTest().visit_1(hm);
55 new HashMapVisitTest().visit_2(hm);
56 new HashMapVisitTest().visit_3(hm);
57 new HashMapVisitTest().visit_4(hm);
58 
59 }
60 }