Java HashMap 两种遍历方法性能比较

	HashMap<String, Integer> map = new HashMap<>();

		map.put("1", 111);
		map.put("2", 222);
		map.put("3", 333);
		map.put("4", 444);
		map.put("5", 555);

		// This is the first way to get all the map element via Iterator with keySet.

		long startTime = System.nanoTime();
		Iterator it = map.keySet().iterator();
		while (it.hasNext()) {
			String key = (String) it.next();
			System.out.print(key + ", ");
			System.out.println(map.get(key));
		}
		long interval = System.nanoTime() - startTime;
		System.out.println("interval1: " + interval);
		
		// ======================================================================================================================

		// This is the second way to get all the map element via Entry

		long startTime2 = System.nanoTime();
		Set<Entry<String, Integer>> sets = map.entrySet();
		for (Entry<String, Integer> entry : sets) {
			System.out.print(entry.getKey() + ", ");
			System.out.println(entry.getValue());
		}
		long interval2 = System.nanoTime() - startTime2;
		System.out.println("interval2: " + interval2);

First way result:

3, 333

2, 222
1, 111
5, 555
4, 444

interval1: 1152533

Second way result:

3, 333
2, 222
1, 111
5, 555
4, 444
interval2: 521673

It is obvious that the second way with EntrySet is better than the first one with keySet!