Map map = new HashMap(); 
Iterator iter = map.entrySet().iterator(); 
while (iter.hasNext()) { 
 Map.Entry entry = (Map.Entry) iter.next(); 
 Object key = entry.getKey(); 
 Object val = entry.getValue(); 
}

二、遍歷Map方法B

Map map = new HashMap(); 
Iterator iter = map.keySet().iterator(); 
while (iter.hasNext()) { 
 Object key = iter.next(); 
 　Object val = map.get(key); 
}

三、分析遍歷方法

方法A：在遍歷中一次讀取Map.Entry，然后直接獲取的值。

方法B: 基于keySet則是，先遍歷，然后再從Map中讀取信息。

四、性能測試

import java.util.HashMap; 
import java.util.Iterator; 
import java.util.Map; 
 
import org.junit.BeforeClass; 
import org.junit.Test; 
 
public class MapLoopA { 
 private static Map<Integer, String> infos = new HashMap<Integer, String>(); 
 
 @BeforeClass 
 public static void setUp() { 
 for (int i=0; i<1000000; i++) { 
  infos.put(i, "test information" + i); 
 } 
 System.out.println("setUp is done."); 
 } 
 
 @Test 
 public void testMapLoopA() { 
 Iterator<Map.Entry<Integer, String>> iterator = infos.entrySet().iterator(); 
 long startTime = System.currentTimeMillis(); 
 while (iterator.hasNext()) { 
  Map.Entry<Integer, String> entry = iterator.next(); 
  int key = entry.getKey(); 
  String val = entry.getValue(); 
 } 
  
 System.out.println("A solution takes in looping Map with 1000000 entries:" 
   + (System.currentTimeMillis()-startTime) + " milli seconds"); 
 } 
 
 @Test 
 public void testMapLoopB() { 
 Iterator<Integer> iterator = infos.keySet().iterator(); 
 long startTime = System.currentTimeMillis(); 
 while (iterator.hasNext()) {  
  int key = iterator.next(); 
  String val = infos.get(key); 
 } 
  
 System.out.println("B solution takes in looping Map with 1000000 entries:" + 
   (System.currentTimeMillis()-startTime) + " milli seconds"); 
 } 
}

測試結(jié)果：