為什么第一次將對象添加到ArrayList或LinkedList需要更多時間？

Question

為什么第一次將對象添加到ArrayList或LinkedList需要更多時間？

我正在測試ArrayList和LinkedList的性能。 結果是我所期望的。 ArrayList通常更適合隨機訪問，每次插入LinkedList都需要相似的時間。 但是我發現第一次將對象添加到ArrayList或LinkedList 所需的時間比其他時間要多得多。

我已經在Mac和Linux上測試了代碼，情況相似。 您可以從此處下載代碼和完整結果。

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：它們以納秒為單位測量。

/* 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

我想這個問題的主要原因是從這里開始的規則4，即新類的初始化效果。 我首先創建了另一個對象來進行初始化操作。 第一次使用的類始終需要更長的時間進行初始化。 此后，操作時間將大大縮短。

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

   PlayArrayList playAList = new PlayArrayList();

然后結果將如下所示：

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