public static void main(String[] args) {
traverseMap();
}
public static void traverseMap() {
long start = 0;
long finish = 0;
long sum = 0;
Map<String, Integer> map = createTestMap();
start = System.currentTimeMillis();
Set<Map.Entry<String, Integer>> entrySet = map.entrySet();
for (Map.Entry<String, Integer> entry : entrySet){
sum += entry.getValue();
}
finish = System.currentTimeMillis();
System.out.println("entrySet | sum:" + sum + ", cost:" + (finish - start) + "ms");
sum = 0;
start = System.currentTimeMillis();
Set<String> keySet = map.keySet();
for (String key : keySet){
sum += map.get(key);
}
finish = System.currentTimeMillis();
System.out.println("keySet | sum:" + sum + ", cost:" + (finish - start) + "ms");
sum = 0;
start = System.currentTimeMillis();
Iterator itr = map.entrySet().iterator();
Map.Entry entry = null;
while (itr.hasNext()) {
entry = (Map.Entry) itr.next();
sum += (Integer) entry.getValue();
}
finish = System.currentTimeMillis();
System.out.println("iterator | sum:" + sum + ", cost:" + (finish - start) + "ms");
}
private static Map<String, Integer> createTestMap() {
Map<String, Integer> map = new HashMap<>();
for (int i = 0; i < 10000000; i++) {
map.put(Integer.toString(i), i);
}
return map;
}
从实测结果看,大多情况下使用entrySet迭代器是最快的,直接遍历entrySet次之,但相差非常小,遍历keySet再通过key找value是最慢的,其实很容易理解:通过key去Map里取value需要经过hash的计算,和Hash表的查询,这之间必然会消耗性能,特别是如果key的hashCode方法又实现不好或者较复杂的话,情况会更糟糕。
其中某一次的测试输出:
entrySet | sum:49999995000000, cost:238ms
keySet | sum:49999995000000, cost:361ms
iterator | sum:49999995000000, cost:232ms