Detailed Explanation of Traversal Instance of List Set in Java

1. There are three ways to traverse List

 List<String>  list  =  new  ArrayList<String>();  
  list.add("testone");  
  list.add( " testtwo " );  
  ...  

Type 1:

for(Iterator<String>  it  =  list.iterator();  it.hasNext();  )  {  
    ....  
  } 

In this way, data locking will be carried out during the loop execution, and the performance is slightly poor. At the same time, if you want to remove an element during the fun-seeking process, you can only call the it. remove method, but not the list. remove method, otherwise, there will be a concurrent access error.

Type 2:

 for(String  data  :  list)  {  
    .....  
  } 

Calling the first kind internally, it is slower than Iterator. This loop has other limitations, so it is not recommended.

Type 3:

 for(int  i=0;  i<list.size();  i++)  {  
    A  a  =  list.get(i);  
    ...  
  }  

Internal locking is the most efficient, but when writing multi-threads, we should consider the problem of concurrent operation.

2. Test examples

package com.inspur.Test;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

/**
 *@author WHD
 *2015-3-5
 */
@SuppressWarnings("unused")
public class MapTest {
    private static List<String> list= new ArrayList<String>();
    public static void main(String[]args){
        MapTest  mapTest = new  MapTest();
        mapTest.initList(list);
        mapTest.foreach(list);
        mapTest.forlist(list);
        mapTest.iteratorList(list);
    }
    
    //list  Add to the collection 10 Ten thousand pieces of data 
    public List initList(List<String> list){
        int i=0;
        int num=6000000;
        for(i=0;i<num;i++){
            list.add("list"+i);
        }
        return list;
    }
    //list  Set traversal  foreach
    
    public void  foreach(List<String> list){
        long start= System.currentTimeMillis();
        for(String data:list){
            String value=data;
        }
        
        long end=System.currentTimeMillis();
        long count=end-start;
        System.out.println("foreach  Cycle time "+count);
    }
    // list Set traversal   for
    public void forlist(List<String> list){
        long start=System.currentTimeMillis();
        int i=0;
        for( i=0;i<list.size();i++){
            String value=list.get(i);
        }
        long end=System.currentTimeMillis();
        long count=end-start;
        System.out.println("for list.size()  Traversal time "+count);
    }
    
    // Iterator  Traversal loop 
    public void iteratorList(List<String> list){
        long start= System.currentTimeMillis();
        for(Iterator<String>  it=list.iterator();it.hasNext();){
            String value=it.next();
        }
        long end=System.currentTimeMillis();
        long count=end-start;
        System.out.println("iterator  Traversal time "+count);
    }

}

3. Test results:

(1), the first time

foreach traversal time: 55
for list. size () Traversal time: 47
iterator traversal time: 51

(2), 2nd time

foreach traversal time: 54
for list. size () Traversal time: 44
iterator traversal time: 50

(3), 3rd time

foreach traversal time: 48
for list. size () Traversal time: 43
iterator traversal time: 44

From the test results, we can clearly see the efficiency!