传统的绩效循环VS Java中的Iterator /的foreach [英] Performance of traditional for loop vs Iterator/foreach in Java

问题描述

有没有在循环迭代器VS在遍历一个ArrayList中，HashMap和其他收藏品相比传统的使用性能测试的结果？

Is there any performance testing results available in comparing traditional for loop vs Iterator while traversing a ArrayList,HashMap and other collections?

或者干脆我为什么要使用迭代器循环反之亦然？

Or simply why should I use Iterator over for loop or vice versa?

推荐答案

假设这是你的意思：

// traditional for loop
for (int i = 0; i < collection.size(); i++) {
  T obj = collection.get(i);
  // snip
}

// using iterator
Iterator<T> iter = collection.iterator();
while (iter.hasNext()) {
  T obj = iter.next();
  // snip
}

// using iterator internally (confirm it yourself using javap -c)
for (T obj : collection) {
   // snip
}

的Iterator是对于没有随机访问（例如TreeSet中，HashMap的，链表）的集合速度更快。对于数组和的ArrayList，性能差异可以忽略不计。

Iterator is faster for collections with no random access (e.g. TreeSet, HashMap, LinkedList). For arrays and ArrayLists, performance differences should be negligible.

编辑：我认为，微基准是pretty多的邪恶根源，就像早期的优化。但话又说回来，我认为这是件好事，对于这样的相当琐碎的事情的影响的感觉。因此，我已经运行小试：

I believe that micro-benchmarking is root of pretty much evil, just like early optimization. But then again, I think it's good to have a feeling for the implications of such quite trivial things. Hence I've run a small test:

• 遍历一个链表和一个ArrayList respecively


• 10万随机字符串


• 总结其长度（只是要避免编译器优化掉整个循环）


• 使用所有3环样式（迭代器，对于每一个，对于计数器）

结果是所有与计数器与LinkedList的相似，但。所有其他五只花了不到20毫秒来遍历整个列表。使用 list.get（我）在LinkedList的100,000次时间超过2分钟（！），完成（慢6万倍）。哇！ :)因此，最好使用一个迭代器（或明或暗地使用每个），特别是如果你不知道你处理什么类型和大小的列表。

Results are similar for all but "for with counter" with LinkedList. All the other five took less than 20 milliseconds to iterate over the whole list. Using list.get(i) on a LinkedList 100,000 times took more than 2 minutes (!) to complete (60,000 times slower). Wow! :) Hence it's best to use an iterator (explicitly or implicitly using for each), especially if you don't know what type and size of list your dealing with.

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