Shulou(Shulou.com)06/01 Report--

In this issue, the editor will bring you about the difference between for and foreach when traversing the collection. The article is rich in content and analyzes and narrates it from a professional point of view. I hope you can get something after reading this article.

Although there are many ways to write traversal, but the bottom is nothing more than for loop and foreach (iterator) two ways! Both of these two traversal methods can achieve the effect of traversing sets, but which is better or worse in efficiency? Let's look at a piece of code first:

1public static void main (String [] args) {

2 List linkedList = new LinkedList ()

3 for (int I = 0; I

< 100000; i++) { 4 linkedList.add("L1"); 5 } 6 long time1 = System.currentTimeMillis(); 7 //for遍历 8 for (int i = 0; i < linkedList.size(); i++) { 9 System.out.println(linkedList.get(i)); 10 } 11 long time2 = System.currentTimeMillis(); 12 //foreach遍历 13 for (String s : linkedList) { 14 System.out.println(s); 15 } 16 long time3 = System.currentTimeMillis(); 17 System.out.println("for执行时间：" + (time2 - time1)); 18 System.out.println("foreach执行时间：" + (time3 - time2)); 19}   这次我们创建的是一个LinkedList，通过循环往集合里面写入了十万个元素，然后分别通过foreach（迭代器）和for的方式对集合进行遍历输出，并且记录了两种方式的耗时。那我们先思考一下哪种方式的耗时更长呢？我们看下结果：      我们能看到使用for遍历的耗时是foreach的好几倍！如果想知道为什么会有这么大的差距，我们只能通过源码找答案了！其实差距不是在循环本身，而是在循环内获取元素的时候！我们先看下使用for循环时的linkedList.get(i) 的实现： 1public E get(int index) { 2 checkElementIndex(index); 3 return node(index).item; 4}   LinkedList的get方法内部先调用了checkElementIndex方法检查了index参数的合法性，然后调用node(index)方法获取元素。我们进入node方法: 1Node node(int index) { 2// assert isElementIndex(index); 3if (index < (size >

> 1) {

4 Node x = first

5 for (int I = 0; I

< index; i++) 6 x = x.next; 7 return x; 8} else { 9 Node x = last; 10 for (int i = size - 1; i >

Index; iMurt -)

11 x = x.prev

12 return x

13}

}

   from the above code, we can analyze the following features

1. The bottom layer of LinkedListd is a two-way linked list, which can be traversed from the head or tail, which can improve the efficiency of getting elements!

2. When the index of the fetched element is less than half of the number of collection elements, it traverses from the queue header to the target element! If it is more than half, then traverse from the end of the queue to the target element!

3. Each time you get an element, you need to traverse from the head of the queue or the end of the queue one by one until you find the target element. That's why it's slow to use for loops to traverse LinkedList! Also, the greater the number of elements in the collection, the greater the gap between for traversal and foreach!

   so if our collection type is ArrayList, is there a big gap between using for and foreach? Because the underlying layer of ArrayList is an array, the efficiency of getting an array through an index is very high, so there is not much difference in efficiency between using for and foreach!

These are the differences between for and foreach shared by the editor when traversing the collection. If you happen to have similar doubts, you might as well refer to the above analysis to understand. If you want to know more about it, you are welcome to follow the industry information channel.

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