HashMap的2中遍历方式比较-优快云博客

[url]http://smallnetvisitor.iteye.com/blog/1729381[/url]

首先我们准备数据，准备一个map

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

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


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：

  
   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;  
   }