ensureCapacity()方法提高ArrayList的初始化速度

import java.util.ArrayList;   
  
public class EnsureCapacityTest {   
    public static void main(String[] args){   
        final int N = 1000000;   
        Object obj = new Object();   
           
        //没用调用ensureCapacity()方法初始化ArrayList对象   
        ArrayList list = new ArrayList();   
        long startTime = System.currentTimeMillis();   
        for(int i=0;i<=N;i++){   
            list.add(obj);   
        }   
        long endTime = System.currentTimeMillis();   
        System.out.println("没有调用ensureCapacity()方法所用时间：" + (endTime - startTime) + "ms");   
           
        //调用ensureCapacity()方法初始化ArrayList对象   
        list = new ArrayList();   
        startTime = System.currentTimeMillis();   
        list.ensureCapacity(N);//预先设置list的大小   
        for(int i=0;i<=N;i++){   
            list.add(obj);   
        }   
        endTime = System.currentTimeMillis();   
        System.out.println("调用ensureCapacity()方法所用时间：" + (endTime - startTime) + "ms");   
    }   
}