Iterator 类和 foreach 构造之间的性能差异

发布于 2024-09-08 03:45:36 字数 387 浏览 10 评论 0原文

我正在运行以下代码，但有时在运行它时会出现某种并发异常。

ArrayList<Mob> carriers = new ArrayList<Mob>();
ArrayList<Mob> mobs = new ArrayList<Mob>();
...
for (Mob carrier : carriers){
    for (Mob mob : mobs){
        checkInfections (carrier, mob);
    } 
}

我重构它来解决并发问题，但这确实让我产生了一个问题。如果我将 for 构造更改为迭代器模式，性能是否会有所不同？ foreach 构造和 Iterator 类之间的访问级别差异是什么？

原文

I have the following code running, but I sometimes get some sort of concurrency exception when running it.

ArrayList<Mob> carriers = new ArrayList<Mob>();
ArrayList<Mob> mobs = new ArrayList<Mob>();
...
for (Mob carrier : carriers){
    for (Mob mob : mobs){
        checkInfections (carrier, mob);
    } 
}

I refactored it to solve the concurrency problem, but it did lead me to a question. Would there be a difference in performance if I change the for construct to an Iterator pattern? What's the access level difference between the foreach construct and the Iterator class?

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橙幽之幻 2024-09-15 03:45:36

区别主要在于语法糖，除了 Iterator 可以从它正在迭代的 Collection 中删除项目。从技术上讲，增强的 for 循环允许您循环遍历任何 Iterable 的内容，其中至少包括 Collection 和数组。

不用担心性能差异。这种微观优化是一种无关紧要的干扰。如果您需要随时删除项目，请使用迭代器。否则 for 循环往往被更多地使用，因为它们更具可读性，即：

for (String s : stringList) { ... }

vs:

for (Iterator<String> iter = stringList.iterator(); iter.hasNext(); ) {
  String s = iter.next();
  ...
}

The difference is largely syntactic sugar except that an Iterator can remove items from the Collection it is iterating. Technically, enhanced for loops allow you to loop over anything that's Iterable, which at a minimum includes both Collections and arrays.

Don't worry about performance differences. Such micro-optimization is an irrelevant distraction. If you need to remove items as you go, use an Iterator. Otherwise for loops tend to be used more just because they're more readable ie:

for (String s : stringList) { ... }

vs:

for (Iterator<String> iter = stringList.iterator(); iter.hasNext(); ) {
  String s = iter.next();
  ...
}

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咆哮 2024-09-15 03:45:36

在幕后，新样式 for 是由编译器以迭代器的形式实现的，因此如果您自己这样做，不会有什么区别。

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北方的韩爷 2024-09-15 03:45:36

您所说的“某种并发异常”很可能是java.util.ConcurrentModificationException。之所以会出现这种情况，是因为在迭代列表时无法更改列表；如果这样做，迭代器会注意到并抛出此异常。

如果您需要在迭代列表时从列表中删除元素，请通过迭代器上的 remove() 方法来完成，例如：（

List<String> list = ...; // wherever you get this

for (Iterator<String> iter = list.iterator(); iter.hasNext(); ) {
    String s = iter.next();
    if (...) {
        iter.remove(); // Remove element through iterator
    }
}

注意：您不能将 foreach 语法用于在这种情况下循环，因为您需要显式访问迭代器）。

The "some sort of concurrency exception" you're talking about is most likely java.util.ConcurrentModificationException. You get this because you cannot change the list while you are iterating over it; if you do that, the iterator will notice and throw this exception.

If you need to remove elements from a list while iterating over it, then do it through the remove() method on the iterator, for example:

List<String> list = ...; // wherever you get this

for (Iterator<String> iter = list.iterator(); iter.hasNext(); ) {
    String s = iter.next();
    if (...) {
        iter.remove(); // Remove element through iterator
    }
}

(Note: You can't use the foreach syntax for the loop in this case, because you need explicit access to the iterator).

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