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!!!!!!!!");
-
-
- }
- }
测试结果如下:
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?
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