問題描述
在遍歷 ArrayList、HashMap 和其他集合時，比較傳統(tǒng)的 for 循環(huán)與 Iterator 是否有任何性能測試結(jié)果?
Is there any performance testing results available in comparing traditional for loop vs Iterator while traversing a ArrayList,HashMap and other collections?
或者只是為什么我應(yīng)該使用迭代器而不是 for 循環(huán)，反之亦然?
Or simply why should I use Iterator over for loop or vice versa?
推薦答案
假設(shè)這就是你的意思:
// 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
}
對于沒有隨機訪問的集合(例如 TreeSet、HashMap、LinkedList)，迭代器更快.對于數(shù)組和 ArrayList，性能差異應(yīng)該可以忽略不計.
Iterator is faster for collections with no random access (e.g. TreeSet, HashMap, LinkedList). For arrays and ArrayLists, performance differences should be negligible.
我相信微基準測試是萬惡之源，就像早期優(yōu)化一樣.但話又說回來，我認為對這些微不足道的事情的含義有一種感覺是件好事.因此我運行了一個小測試:
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:
- 分別遍歷 LinkedList 和 ArrayList
- 包含 100,000 個隨機"字符串
- 總結(jié)它們的長度(只是為了避免編譯器優(yōu)化整個循環(huán))
- 使用所有 3 種循環(huán)樣式(迭代器、for each、for with counter)
結(jié)果與 LinkedList 的for with counter"不同.其他五個都用了不到 20 毫秒的時間來遍歷整個列表.在 LinkedList 上使用 list.get(i) 100,000 次需要超過 2 分鐘 (!) 才能完成(慢 60,000 倍).哇！:) 因此，最好使用迭代器(顯式或隱式地使用每個迭代器)，特別是如果您不知道要處理的列表的類型和大小.
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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