面对ArrayList以及LinkedList的循环遍历,使用forEach、for，Iterator谁的效率会好一些呢？

前言

日常生活中，循环遍历是少不了的，那么对于Java的几种遍历方式都很熟悉嘛？

• 在刚接触Java的时候我们知道循环分为 for 循环、 while以及 do while循环，那么除了这三种遍历还有别的方式嘛？
• 没错，还有forEach，慢慢地学习中我们发现，jdk1.5版本之后更新的forEach太好用了！比for、while，do while书写方面快多了！
• 那么在效率上，哪个好点呢？
• 本文简单的从ArrayList，LinkedList角度进行测试。

提示：这里单纯说明测试结果以及对于一些使用方式的比较说明，对于java循环讲解暂时略过

一、forEach与Iterator

1.发行时间

forEach是jdk5(jdk1.5)版本之后更新产物；Iterator则是jdk8(jdk1.8)的产物，Iterator是collection 的一个迭代器。

2.区别

1. forEach通常用于一次性遍历整个集合，大大提升了代码的简洁性和可阅读性，但期间无法控制遍历的过程。而Iterator通过hasNext()，next() 更好地控制便利过程的每一步。
2. forEach在遍历过程中严禁改变集合的长度，且无法对集合的删除或添加等操作。而Iterator可以在遍历过程中对集合元素进行删除操作。
3. 在设计模式中迭代器模式搭配组合模式一起使用，可以递归方式控制树状结构的遍历。
4. Map虽然不能使用forEach，但可以通过Map.Entry来对Map进行forEach遍历

    for(Map.Entry entry ：map.entrySet()){
		  // code...
    }

二、测试

测试须知

• 测试容器：本次是对ArrayList以及LinkedList的循环遍历
• 测试项：forEach、for，Iterator
• 赋值量：
• • 100000条长度为10的字符串
• • 10000000条长度为10的字符串
• • 1000条长度为10的字符串
• 运算时间方法: 使用的是 System.nanoTime()

并不绝对，测试环境也要考虑

结果普遍如下

································here for loop······················ArrayList····················
execution time：   0.6844 ms.
································here forEach loop··················ArrayList····················
execution time：   1.9834 ms.
································here iterator loop·················ArrayList····················
execution time：   0.8786 ms.
································································································
·····After many tests, in most cases, the for the most efficiency, followed by the iterator.····
································································································
································here for loop·····················LinkedList····················
execution time：3035.3940 ms.
································here forEach loop·················LinkedList····················
execution time：   1.4215 ms.
································here iterator loop················LinkedList····················
execution time：   0.8353 ms.

↑ 在100000条数据测试中，for对于ArrayList的遍历效率略胜一筹，与iterator差不多；对于LinkedList，for效率最差，iterator最高。

································here for loop······················ArrayList····················
execution time：  56.7496 ms.
································here forEach loop··················ArrayList····················
execution time：  83.0643 ms.
································here iterator loop·················ArrayList····················
execution time：  58.5370 ms.
································································································
·····After many tests, in most cases, the for the most efficiency, followed by the iterator.····
································································································
································here for loop·····················LinkedList····················
pass for loop
execution time：   0.0060 ms.
································here forEach loop·················LinkedList····················
execution time： 694.5913 ms.
································here iterator loop················LinkedList····················
execution time： 648.3043 ms.

↑ 在10000000条数据测试中，for对于ArrayList的遍历效率略胜一筹，与iterator差不多；对于LinkedList，for效率最差直接卡死，iterator略高。

································here for loop······················ArrayList····················
execution time：   0.0275 ms.
································here forEach loop··················ArrayList····················
execution time：   0.1711 ms.
································here iterator loop·················ArrayList····················
execution time：   0.0382 ms.
································································································
·····After many tests, in most cases, the for the most efficiency, followed by the iterator.····
································································································
································here for loop·····················LinkedList····················
execution time：   1.0022 ms.
································here forEach loop·················LinkedList····················
execution time：   0.2920 ms.
································here iterator loop················LinkedList····················
execution time：   0.0428 ms.

↑ 在1000条数据测试中，for对于ArrayList的遍历效率略胜一筹，与iterator差不多；对于LinkedList，for效率最差，iterator最高。

三、总结

经过上述实验，ArrayList推荐 for ， iterator；LinkedList 推荐 iterator，for循环会很慢。

最后

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