HashMap遍历效率测试

测试环境:

win8.1 64bit 

CPU:Intel 奔腾3560M 2.4GHz

内存:8G 1600MHz

硬盘:三星SSD 256G

1.遍历测试,分别用keySet和entrySet做测试,测试代码如下:

@Testpublic void test() {int i, N = 2000001, testTimes = 100;long start, end, total_keySet, total_entrySet;double avg_keySet = 0, avg_entrySet = 0;HashMap<Integer, String> hashMap = new HashMap<Integer, String>();for (i = 0; i < N; i++) {hashMap.put(i, i + "");}for (i = 0; i < testTimes; i++) {//=========keySet==========start = System.nanoTime();Iterator<Integer> it_ks = hashMap.keySet().iterator();while (it_ks.hasNext()) {int key = it_ks.next();String value = hashMap.get(key);//System.out.println(key + "-" + value);}end = System.nanoTime();total_keySet = end - start;//=========keySet==========//*********entrySet*****************start = System.nanoTime();Iterator<Entry<Integer, String>> it_et = hashMap.entrySet().iterator();while (it_et.hasNext()) {Entry<Integer, String> entry = it_et.next();//System.out.println(entry.getKey() + "-" + entry.getValue());}end = System.nanoTime();total_entrySet = end - start;//*********entrySet*****************avg_keySet += total_keySet;avg_entrySet += total_entrySet;}System.out.println("The average time of keySet:     "+ BigDecimal.valueOf((long) (avg_keySet / 100.0), 9));System.out.println("The average time of entrySet:   "+ BigDecimal.valueOf((long) (avg_entrySet / 100.0), 9));}

</pre><p></p><pre>

100组测试结果平均耗时:

The average time of keySet:     0.054277459The average time of entrySet:   0.020779901

HashMap中keySet和entrySet的部分源码:

 public V get(Object key) {        if (key == null)            return getForNullKey();        Entry<K,V> entry = getEntry(key);        return null == entry ? null : entry.getValue();    }

  public Set<K> keySet() {        Set<K> ks = keySet;        return (ks != null ? ks : (keySet = new KeySet()));    }

public Set<Map.Entry<K,V>> entrySet() {        return entrySet0();    }    private Set<Map.Entry<K,V>> entrySet0() {        Set<Map.Entry<K,V>> es = entrySet;        return es != null ? es : (entrySet = new EntrySet());    }

根据源码的意思,利用keySet获取K,V需要经过2个步骤(获取K,再获取V),而entrySet只需一个步骤.

结论:迭代遍历取HashMap值时,使用entrSet比keySet快.