Performance issues of for each statements in Java 7

Performance issues of for each statements in Java 7

Performance of for each statements in Java 7

TodayLeetCodeI made a fully-arranged algorithm question. At that time, I had obtained the correct result in eclipse. However, when I submitted it to Leetcode, I reported"Time Limit Exceeded".

Permutation on Leetcode

I was puzzled at the beginning. I searched for many similar problems in Leetcode, but I did not find the same problems as I encountered. Many of them say that they finally found an error in their own code. As a result, I checked my code issues carefully. Later I found that after removing one of my circular print List codes, I submitted the code again. At that time, I felt very strange, however, if you are not sure about the reason, write it down for now.

Many people said at nightEvernoteVery easy to use. I already gave up Evernote and switchedNoteBut curiosity drove me to try Evernote again, so I went to my Evernote and suddenly saw a previously added blog.Performance Comparison of Three Java list traversal methodsI think of a similar problem today. Then I went to the page and tested it myself. I found that, as the blogger said, the new syntax supported by Java 7 is the most concise code, but the performance is indeed the worst; then think about the problems encountered during the day.

The following code tests the List traversal performance:

package org.phn;import java.util.ArrayList;import java.util.Iterator;import java.util.List;public class TestListPrintTime {    public static void main(String[] args) {        List
  
    list = new ArrayList
   
    ();        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
    
      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!!!!!!!!");    }}
    
   
  

Result:

It can be seen that the for each statement in Java 7 is very convenient and clear, but the performance is several times worse. You should use it with caution. Leave your mind blank. Some problems may arise in the future.