在Java中迭代列表的方法 [英] Ways to iterate over a list in Java
问题描述
对于Java语言有点新，我试图让自己熟悉一个可能遍历列表（或者可能是其他集合）的所有方式（或者至少是非病态方法）以及它的优点或缺点。每。
Being somewhat new to the Java language I'm trying to familiarize myself with all the ways (or at least the non-pathological ones) that one might iterate through a list (or perhaps other collections) and the advantages or disadvantages of each.
给定列表< E> list object，我知道以下循环遍历所有元素的方法：
Given a List<E> list object, I know of the following ways to loop through all elements:
// Not recommended (see below)!
for (int i = 0; i < list.size(); i++) {
E element = list.get(i);
// 1 - can call methods of element
// 2 - can use 'i' to make index-based calls to methods of list
// ...
}
注意：正如@amarseillan指出的那样，这个表格是一个糟糕的选择
用于迭代 List s，因为实际实现
get 方法可能效率不高当使用 Iterator 时。
例如， LinkedList 实现必须遍历
之前的元素以获取第i个元素。
Note: As @amarseillan pointed out, this form is a poor choice
for iterating over Lists, because the actual implementation of
the get method may not be as efficient as when using an Iterator.
For example, LinkedList implementations must traverse all of
the elements preceding i to get the i-th element.
在上面的例子中， List 实现没有办法将
保存其位置以使未来的迭代更有效。
对于 ArrayList ，这并不重要，因为 get 的复杂性/成本是恒定时间（O（1））而对于 LinkedList ，它与列表的大小（O（n））成比例。
In the above example there's no way for the List implementation to
"save its place" to make future iterations more efficient.
For an ArrayList it doesn't really matter, because the complexity/cost of get is constant time (O(1)) whereas for a LinkedList is it proportional to the size of the list (O(n)).
有关内置集合实现的计算复杂性的更多信息，请查看这个问题。
For more information about the computational complexity of the built-in Collections implementations, check out this question.
for (E element : list) {
// 1 - can call methods of element
// ...
}
迭代器
Iterator
for (Iterator<E> iter = list.iterator(); iter.hasNext(); ) {
E element = iter.next();
// 1 - can call methods of element
// 2 - can use iter.remove() to remove the current element from the list
// ...
}
ListIterator
ListIterator
for (ListIterator<E> iter = list.listIterator(); iter.hasNext(); ) {
E element = iter.next();
// 1 - can call methods of element
// 2 - can use iter.remove() to remove the current element from the list
// 3 - can use iter.add(...) to insert a new element into the list
// between element and iter->next()
// 4 - can use iter.set(...) to replace the current element
// ...
}
功能性Java
Functional Java
list.stream().map(e -> e + 1); // Can apply a transformation function for e
Iterable.forEach ， Stream.forEach ，...
（来自Java 8的Stream API的地图方法（请参阅@ i_am_zero的回答）。）
Iterable.forEach, Stream.forEach, ...
(A map method from Java 8's Stream API (see @i_am_zero's answer).)
在实现 Iterable 的Java 8集合类中（例如，所有 List s）现在有一个 forEach 方法，可以用来代替 for loop statement 。 （这里有另一个问题，提供了一个很好的比较。）
In Java 8 collection classes that implement Iterable (for example, all Lists) now have a forEach method, which can be used instead of the for loop statement demonstrated above. (Here is another question that provides a good comparison.)
Arrays.asList(1,2,3,4).forEach(System.out::println);
// 1 - can call methods of an element
// 2 - would need reference to containing object to remove an item
// (TODO: someone please confirm / deny this)
// 3 - functionally separates iteration from the action
// being performed with each item.
Arrays.asList(1,2,3,4).stream().forEach(System.out::println);
// Same capabilities as above plus potentially greater
// utilization of parallelism
// (caution: consequently, order of execution is not guaranteed,
// see [Stream.forEachOrdered][stream-foreach-ordered] for more
// information about this).
还有哪些其他方式？
（顺便说一下，我的兴趣并非源于对优化性能;我只是想知道作为开发人员可以使用哪些表单。）
(BTW, my interest does not stem at all from a desire to optimize performance; I just want to know what forms are available to me as a developer.)
推荐答案
三种形式的循环几乎完全相同。 的增强循环：
The three forms of looping are nearly identical. The enhanced for loop:
for (E element : list) {
. . .
}
是根据 Java语言规范，相同对显式使用用于循环的传统的迭代器。在第三种情况下，您只能通过删除当前元素来修改列表内容，然后才能通过迭代器本身的 remove 方法执行此操作。使用基于索引的迭代，您可以以任何方式自由修改列表。但是，添加或删除当前索引之前的元素可能会导致循环跳过元素或多次处理相同的元素;进行此类更改时，需要正确调整循环索引。
is, according to the Java Language Specification, identical in effect to the explicit use of an iterator with a traditional for loop. In the third case, you can only modify the list contents by removing the current element, and then only if you do it through the remove method of the iterator itself. With index-based iteration, you are free to modify the list in any way. However, adding or removing elements that come before the current index risks having your loop skipping elements or processing the same element multiple times; you need to adjust the loop index properly when you make such changes.
在所有情况下，元素是一个参考到实际的列表元素。没有迭代方法可以复制列表中的任何内容。将始终在列表中相应元素的内部状态中看到对元素的内部状态的更改。
In all cases, element is a reference to the actual list element. None of the iteration methods makes a copy of anything in the list. Changes to the internal state of element will always be seen in the internal state of the corresponding element on the list.
实质上，只有两种方法可以迭代列表：使用索引或使用迭代器。增强的for循环只是Java 5中引入的一个语法快捷方式，以避免显式定义迭代器的繁琐。对于这两种样式，您可以使用获取，，而或执行基本上微不足道的变体而阻止，但它们都归结为相同的东西（或者更确切地说，两件事）。
Essentially, there only two ways to iterate over a list: by using an index or by using an iterator. The enhanced for loop is just a syntactic shortcut introduced in Java 5 to avoid the tedium of explicitly defining an iterator. For both styles, you can come up with essentially trivial variations using for, while or do while blocks, but they all boil down to the same thing (or, rather, two things).
编辑：正如@ iX3指出的那样在注释中，您可以使用 ListIterator 在迭代时设置列表的当前元素。您需要使用 列表#listIterator（） 而不是 List＃iterator（） 初始化循环变量（显然，必须将其声明为 ListIterator 而不是 Iterator ）。
As @iX3 points out in a comment, you can use a ListIterator to set the current element of a list as you are iterating. You would need to use List#listIterator() instead of List#iterator() to initialize the loop variable (which, obviously, would have to be declared a ListIterator rather than an Iterator).
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