hashMap的2中遍历方式比较

首先我们准备数据，准备一个map

Java代码

1. Map<String, String> map = new HashMap<String, String>(); 
2. for (int i =0; i < 10; i++) { 
3.     map.put(i + "", "value" + i); 
4. } 

Map<String, String> map = new HashMap<String, String>();for (int i = 0; i < 10; i++) {map.put(i + "", "value" + i);}

然后我们采用传说中的key遍历：

步骤：先弄成key set，然后遍历key set 通过key从map中获取value

Java代码

1. Iterator<String> iterator = map.keySet().iterator(); 
2. while (iterator.hasNext()) { 
3.     String key = iterator.next(); 
4.     String val = map.get(key); 
5.     System.out.println(key + "->" + val); 
6. } 
7. System.out.println("*********************"); 

Iterator<String> iterator = map.keySet().iterator();while (iterator.hasNext()) {String key = iterator.next();String val = map.get(key);System.out.println(key + "->" + val);}System.out.println("*********************");

然后我们在采用entrySet的方式遍历下：

步骤：先弄成entrySet 然后遍历他，获取key和value

Java代码

1. Set<Entry<String, String>> set = map.entrySet(); 
2. Iterator<Entry<String, String>> s = set.iterator(); 
3. while (s.hasNext()) { 
4.     Entry<String, String> en = s.next(); 
5.     String key = en.getKey(); 
6.     String val = en.getValue(); 
7.     System.out.println(key + "->" + val); 
8. } 

Set<Entry<String, String>> set = map.entrySet();Iterator<Entry<String, String>> s = set.iterator();while (s.hasNext()) {Entry<String, String> en = s.next();String key = en.getKey();String val = en.getValue();System.out.println(key + "->" + val);}

孰优孰劣？

看JDK源码，对比两种访问方式：

首先看KeySet访问方式：

Java代码

1.    public Set<K> keySet() { 
2. if (keySet == null) { 
3.     keySet = new AbstractSet<K>() { 
4.     public Iterator<K> iterator() { 
5.         return new Iterator<K>() { 
6.         private Iterator<Entry<K,V>> i = entrySet().iterator(); 
7.  
8.         public boolean hasNext() { 
9.             return i.hasNext(); 
10.         } 
11.  
12.         public K next() { 
13.             return i.next().getKey(); 
14.         } 
15.  
16.         public void remove() { 
17.             i.remove(); 
18.         } 
19.                    }; 
20.     } 
21.  
22.     public int size() { 
23.         return AbstractMap.this.size(); 
24.     } 
25.  
26.     public boolean contains(Object k) { 
27.         return AbstractMap.this.containsKey(k); 
28.     } 
29.     }; 
30. } 
31. return keySet; 
32.    } 

    public Set<K> keySet() {if (keySet == null) {    keySet = new AbstractSet<K>() {public Iterator<K> iterator() {    return new Iterator<K>() {private Iterator<Entry<K,V>> i = entrySet().iterator();public boolean hasNext() {    return i.hasNext();}public K next() {    return i.next().getKey();}public void remove() {    i.remove();}                    };}public int size() {    return AbstractMap.this.size();}public boolean contains(Object k) {    return AbstractMap.this.containsKey(k);}    };}return keySet;    }

也就是调用entrySet()从entryset中获取key，也就是说是在entry set的基础上来做的，貌似多次一举

结论：

通过上述代码我们就知道，采用entrySet方式要优于keySet，因为keySet首先要访问entrySet来组建一个keySet，重复工作不言而喻