为什么第一次将对象添加到 ArrayList 或 LinkedList 需要更多时间？

Question

为什么第一次将对象添加到 ArrayList 或 LinkedList 需要更多时间？

我正在测试 ArrayList 和 LinkedList 的性能。结果是我所期望的。 ArrayList 通常更适合随机访问，每次插入 LinkedList 都需要相似的时间。但是我发现第一次将对象添加到 ArrayList 或 LinkedList 需要比其他更多的时间。

我在 Mac 和 Linux 上测试过代码，情况类似。您可以从here 下载代码和完整结果。

java PlayArrayList       
0 to 10, elapsedTime: 716362
10 to 20, elapsedTime: 19765
20 to 30, elapsedTime: 10895

$ java PlayLinkedList 
0 to 10, elapsedTime: 704209
10 to 20, elapsedTime: 5867
20 to 30, elapsedTime: 5378

PS：它们以纳秒为单位。

/* Untilty Class for measuring elapsed time */
public class BenmarkTimer {
    private long startTime;
    private long endTime;

    /* Getter */
    public long getStartTime(){
        return startTime;
    }

    public long getEndTime(){
        return endTime;
    }

    /* Setter */
    private void setStartTime(long t){
        startTime = t;
    }

    private void setEndTime(long t){
        endTime = t;
    }

    /* Method */
    public void start(){
        setStartTime(System.nanoTime());
    } 

    public long end(){
        setEndTime(System.nanoTime());
        return getDuration();
    }

    public void cancel(){
        setStartTime(0);
        setEndTime(0);
    }

    public long getDuration(){
        return getEndTime() - getStartTime();
    }

    /* Unit testing */
    public static void main (String [] args){
        BenmarkTimer timer = new BenmarkTimer();
        timer.start();
        System.out.println("Hello, World for timer");
        timer.end();
        long t = timer.getDuration();
        System.out.println("Start time "+ timer.getStartTime());
        System.out.println("End time "+ timer.getEndTime());
        System.out.println("Elaped time "+ t);
    }
}


import java.util.*;

public class PlayArrayList{
    private int itemCount;
    private BenmarkTimer timer;
    private List<Integer> list; 

    public PlayArrayList(){
        itemCount = 0;
        timer = new BenmarkTimer();
        list = getList();
    }

    public long addTenIntegers(){
        timer.start();
        for (int i=itemCount; i<(itemCount + 10); i++ ) {
            list.add(i);
        }
        long elapsedTime = timer.end();
        itemCount += 10;

        return elapsedTime;
    }

    public long randomRemove(){
        Random r = new Random();
        int s = list.size();
        int i = r.nextInt(s);

        timer.start();
        list.remove(i);
        return timer.end();
    }

    public String toString(){
        return list.toString();
    }

    /* Factory method */
    protected List<Integer> getList(){
        return new ArrayList<Integer>();
    }

    public static void main (String [] args){
        PlayArrayList playAList = new PlayArrayList();

        /* Add 99 integers */
        long elapsedTime;
        for (int i=0; i<10; i++){
            elapsedTime = playAList.addTenIntegers();
            System.out.print(i*10 + " to " + (i*10+10));
            System.out.println(", elapsedTime: "+ elapsedTime);
        }

        System.out.println("Array content");
        System.out.println(playAList.toString());


        /* Remove 99 integer */
        for (int i=0; i<99; i++){
            elapsedTime = playAList.randomRemove();
            System.out.print("Remove a integer");
            System.out.println(", elapsedTime: "+ elapsedTime);
        }

    }
} 

import java.util.*;

public class PlayLinkedList extends PlayArrayList{
    /* Factory method */
    @Override
    protected List<Integer> getList(){
        return new LinkedList<Integer>();
    }

    public static void main (String [] args){
        PlayLinkedList playAList = new PlayLinkedList();

        /* Add 99 integers */
        long elapsedTime;
        for (int i=0; i<10; i++){
            elapsedTime = playAList.addTenIntegers();
            System.out.print(i*10 + " to " + (i*10+10));
            System.out.println(", elapsedTime: "+ elapsedTime);
        }

        System.out.println("Array content");
        System.out.println(playAList.toString());


        /* Remove 99 integer */
        for (int i=0; i<99; i++){
            elapsedTime = playAList.randomRemove();
            System.out.print("Remove a integer");
            System.out.println(", elapsedTime: "+ elapsedTime);
        }

    }
} 

Answer 1

我猜这个问题的主要原因是来自here的规则4，新类的初始化效果。我首先创建了另一个对象来进行初始化操作。第一次使用的类总是需要更长的时间进行初始化。之后，手术时间会短很多。

   PlayArrayList playAListInit = new PlayArrayList();
   long firstTime = playAListInit.addTenIntegers();
   System.out.println("First time: "+ firstTime);

   PlayArrayList playAList = new PlayArrayList();

那么结果会是这样的：

java PlayArrayList
First time: 710000
0 to 10, elapsedTime: 6000
10 to 20, elapsedTime: 12000
20 to 30, elapsedTime: 9000