keySet 与entrySet 遍历HashMap性能差别

keySet 与entrySet 遍历HashMap性能差别
博客分类： Java

一.问题发现
今天,在写完代码后用Find Bugs扫锚了一下,发现类中一处代码中有提示如下内容:
        Java代码 
Map<String, EventChain> map = ContextHolder.getContext().getEventChains();  
        for (Iterator<String> iter = map.keySet().iterator(); iter.hasNext();) {  
            String key = iter.next();  
            EventChain eventChain = eventChains.get(key);  
         } 

Map<String, EventChain> map = ContextHolder.getContext().getEventChains();
        for (Iterator<String> iter = map.keySet().iterator(); iter.hasNext();) {
            String key = iter.next();
            EventChain eventChain = eventChains.get(key);
         }
makes inefficient use of keySet iterator instead of entrySet iterator
意思是说用keySet 方式遍历Map的性能不如entrySet性能好.起初想不明白,索性仔细看下代码:

二.常用的遍历HashMap的两种方法

1.第一种方式
Java代码 
Iterator<String> keySetIterator = keySetMap.keySet().iterator();  
        while (keySetIterator.hasNext()) {  
            String key = keySetIterator.next();  
            String value = keySetMap.get(key);  
              
        } 

Iterator<String> keySetIterator = keySetMap.keySet().iterator();
  while (keySetIterator.hasNext()) {
   String key = keySetIterator.next();
   String value = keySetMap.get(key);
  
  }


2.第二种方式
Java代码 
Iterator<Entry<String, String>> entryKeyIterator = entrySetMap.entrySet()  
                .iterator();  
        while (entryKeyIterator.hasNext()) {  
            Entry<String, String> e = entryKeyIterator.next();  
            String value=e.getValue();  
        } 

Iterator<Entry<String, String>> entryKeyIterator = entrySetMap.entrySet()
    .iterator();
  while (entryKeyIterator.hasNext()) {
   Entry<String, String> e = entryKeyIterator.next();
   String value=e.getValue();
  }

三.性能比较

    到底第二种方式的性能比第一种方式的性能高多少呢,通过一个简单的测试类可以看一下,测试代码如下:
Java代码 
public class HashMapTest {  
    public static void main(String[] args) {  
 
        HashMap<String, String> keySetMap = new HashMap<String, String>();  
        HashMap<String, String> entrySetMap = new HashMap<String, String>();  
 
        for (int i = 0; i < 1000; i++) {  
            keySetMap.put("" + i, "keySet");  
        }  
        for (int i = 0; i < 1000; i++) {  
            entrySetMap.put("" + i, "entrySet");  
        }  
 
        long startTimeOne = System.currentTimeMillis();  
        Iterator<String> keySetIterator = keySetMap.keySet().iterator();  
        while (keySetIterator.hasNext()) {  
            String key = keySetIterator.next();  
            String value = keySetMap.get(key);  
            System.out.println(value);  
        }  
 
        System.out.println("keyset spent times:" 
                + (System.currentTimeMillis() - startTimeOne));  
 
        long startTimeTwo = System.currentTimeMillis();  
 
        Iterator<Entry<String, String>> entryKeyIterator = entrySetMap  
                .entrySet().iterator();  
        while (entryKeyIterator.hasNext()) {  
            Entry<String, String> e = entryKeyIterator.next();  
            System.out.println(e.getValue());  
        }  
        System.out.println("entrySet spent times:" 
                + (System.currentTimeMillis() - startTimeTwo));  
 
    }  
} 

public class HashMapTest {
public static void main(String[] args) {

  HashMap<String, String> keySetMap = new HashMap<String, String>();
  HashMap<String, String> entrySetMap = new HashMap<String, String>();

  for (int i = 0; i < 1000; i++) {
   keySetMap.put("" + i, "keySet");
  }
  for (int i = 0; i < 1000; i++) {
   entrySetMap.put("" + i, "entrySet");
  }

  long startTimeOne = System.currentTimeMillis();
  Iterator<String> keySetIterator = keySetMap.keySet().iterator();
  while (keySetIterator.hasNext()) {
   String key = keySetIterator.next();
   String value = keySetMap.get(key);
   System.out.println(value);
  }

  System.out.println("keyset spent times:"
    + (System.currentTimeMillis() - startTimeOne));

  long startTimeTwo = System.currentTimeMillis();

  Iterator<Entry<String, String>> entryKeyIterator = entrySetMap
    .entrySet().iterator();
  while (entryKeyIterator.hasNext()) {
   Entry<String, String> e = entryKeyIterator.next();
   System.out.println(e.getValue());
  }
  System.out.println("entrySet spent times:"
    + (System.currentTimeMillis() - startTimeTwo));

}
}

通过测试发现,第二种方式的性能通常要比第一种方式高一倍.

四.原因分析:

  通过查看源代码发现,调用这个方法keySetMap.keySet()会生成KeyIterator迭代器,其next方法只返回其key值.
Java代码 
private class KeyIterator extends HashIterator<K> {  
       public K next() {  
           return nextEntry().getKey();  
       }  
   } 

private class KeyIterator extends HashIterator<K> {
        public K next() {
            return nextEntry().getKey();
        }
    }
而调用entrySetMap.entrySet()方法会生成EntryIterator 迭代器,其next方法返回一个Entry对象的一个实例,其中包含key和value.
Java代码 
private class EntryIterator extends HashIterator<Map.Entry<K,V>> {  
       public Map.Entry<K,V> next() {  
           return nextEntry();  
       }  
} 

private class EntryIterator extends HashIterator<Map.Entry<K,V>> {
        public Map.Entry<K,V> next() {
            return nextEntry();
        }
  }

二者在此时的性能应该是相同的,但方式一再取得key所对应的value时,此时还要访问Map的这个方法,这时,方式一多遍历了一次table.

Java代码 
public V get(Object key) {  
        Object k = maskNull(key);  
        int hash = hash(k);  
        int i = indexFor(hash, table.length);  
        Entry<K,V> e = table[i];   
        while (true) {  
            if (e == null)  
                return null;  
            if (e.hash == hash && eq(k, e.key))   
                return e.value;  
            e = e.next;  
        }  
    } 

public V get(Object key) {
        Object k = maskNull(key);
        int hash = hash(k);
        int i = indexFor(hash, table.length);
        Entry<K,V> e = table[i];
        while (true) {
            if (e == null)
                return null;
            if (e.hash == hash && eq(k, e.key))
                return e.value;
            e = e.next;
        }
    }

这个方法就是二者性能差别的主要原因.