keySet 与entrySet 遍历HashMap性能差别

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对于Java中Map的遍历方式，很多文章都推荐使用entrySet，认为其比keySet的效率高很多。理由是：entrySet方法一次拿到所有key和value的集合；而keySet拿到的只是key的集合，针对每个key，都要去Map中额外查找一次value，从而降低了总体效率。那么实际情况如何呢？

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.

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