从c/c++语言转向java开发,学习java语言list遍历的三种方法,顺便测试各种遍历方法的性能,测试方法为在ArrayList中插入1千万条记录,然后遍历ArrayList,发现了一个奇怪的现象,测试代码如下:
package com.hisense.tiger.list; import java.util.ArrayList; import java.util.Iterator; import java.util.List; public class ListTest { public static void main(String[] args) { List<String> list = new ArrayList<String>(); long t1,t2; for(int j = 0; j < 10000000; j++) { list.add("aaaaaa" + j); } System.out.println("List first visit method:"); t1=System.currentTimeMillis(); for(String tmp:list) { //System.out.println(tmp); } t2=System.currentTimeMillis(); System.out.println("Run Time:" + (t2 -t1) + "(ms)"); System.out.println("List second visit method:"); t1=System.currentTimeMillis(); for(int i = 0; i < list.size(); i++) { list.get(i); //System.out.println(list.get(i)); } t2=System.currentTimeMillis(); System.out.println("Run Time:" + (t2 -t1) + "(ms)"); System.out.println("List Third visit method:"); Iterator<String> iter = list.iterator(); t1=System.currentTimeMillis(); while(iter.hasNext()) { iter.next(); //System.out.println(iter.next()); } t2=System.currentTimeMillis(); System.out.println("Run Time:" + (t2 -t1) + "(ms)"); System.out.println("Finished!!!!!!!!"); } }
List first visit method:
Run Time:170(ms)
List second visit method:
Run Time:10(ms)
List Third visit method:
Run Time:34(ms)
Finished!!!!!!!!
测试的结论很奇怪,第一种方法是java语言支持的新语法,代码最简洁,但是在三种方法中,性能确是最差的,取size进行遍历性能是最高的,求牛人解释?