Java中ArrayList和LinkedList之间的区别 - 性能的原因

Question

我认为理解上我理解ArrayList和LinkedList之间的区别很好.然而,这是第一次,我把它做了一点测试,测试结果出来了,与我的期望完全不同.

期望 :

1. 在开头插入时,Arraylist会比LinkedList慢,因为它必须"移动"元素,对于链表,它只是更新2个引用.

   现实:在大多数迭代中都是相同的.对于选择的几次迭代,它更慢.

2. 在开头删除时,Arraylist会比LinkedList慢,因为它必须"移位"元素,对于Linkedlist,它只是使一个元素无效.

   现实:从beg中删除时性能相同.

测试用例:1,000,000个元素

public static void main(String[] args) {
    int n = 1000000;

    List arrayList = new ArrayList(n+10);
    long milis = System.currentTimeMillis();
    for(int i= 0 ;i<n;i++){
        arrayList.add(i);
    }
    System.out.println("insert arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    List linkedList = new LinkedList();
    milis = System.currentTimeMillis();
    for(int i= 0 ;i<n;i++){
        linkedList.add(i);
    }
    System.out.println("insert linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    //System.out.println("Adding at end");
    milis = System.currentTimeMillis();
    arrayList.add(n-5,n+1);
    System.out.println("APPEND arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.add(n-5,n+1);
    System.out.println("APPEND linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    //add at front
    milis = System.currentTimeMillis();
    arrayList.add(0,0);
    System.out.println("INSERT BEG arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.add(0,0);
    System.out.println("INSERT BEG linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    //add at middle
    milis = System.currentTimeMillis();
    arrayList.add(n/2,n/2);
    System.out.println("INSERT MIDDLE arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.add(n/2,n/2);
    System.out.println("INSERT MIDDLE linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    //get from front, mid, end
    milis = System.currentTimeMillis();
    arrayList.get(0);
    System.out.println("get front arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.get(0);
    System.out.println("get front linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    arrayList.get(n/2);
    System.out.println("get MIDDLE arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.get(n/2);
    System.out.println("get MIDDLE linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    arrayList.get(n-4);
    System.out.println("get last arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.get(n-4);
    System.out.println("get last linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    //delete from front, mid, end.
    milis = System.currentTimeMillis();
    arrayList.remove(0);
    System.out.println("del front arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.remove(0);
    System.out.println("del front linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    arrayList.remove(n/2);
    System.out.println("del mid arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.remove(n/2);
    System.out.println("del mid linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    arrayList.remove(n-4);
    System.out.println("del end arraylist takes "+(System.currentTimeMillis()-milis)+" ms");

    milis = System.currentTimeMillis();
    linkedList.remove(n-4);
    System.out.println("del end linkedlist takes "+(System.currentTimeMillis()-milis)+" ms");

}

输出日志

insert arraylist takes 141 ms
insert linkedlist takes 312 ms
APPEND arraylist takes 0 ms
APPEND linkedlist takes 0 ms
INSERT BEG arraylist takes 0 ms
INSERT BEG linkedlist takes 0 ms
INSERT MIDDLE arraylist takes 0 ms
INSERT MIDDLE linkedlist takes 0 ms
get front arraylist takes 0 ms
get front linkedlist takes 0 ms
get MIDDLE arraylist takes 0 ms
get MIDDLE linkedlist takes 16 ms
get last arraylist takes 0 ms
get last linkedlist takes 0 ms
del front arraylist takes 0 ms
del front linkedlist takes 0 ms
del mid arraylist takes 0 ms
del mid linkedlist takes 15 ms
del end arraylist takes 0 ms
del end linkedlist takes 0 ms

那是什么原因？使用JDK 1.6.

编辑:使用System.nanotime()后,它确实给了我预期的答案.同意,它只是一次试验,应该是平均的.

insert arraylist takes 137076082 ns
insert linkdlist takes 318985917 ns
APPEND arraylist takes 69751 ns
APPEND linkdlist takes 98126 ns
**INSERT BEG arraylist takes 2027764 ns
INSERT BEG linkdlist takes 53522 ns**
INSERT MIDDLE arraylist takes 1008253 ns
INSERT MIDDLE linkdlist takes 10395846 ns
get front arraylist takes 42364 ns
get front linkdlist takes 77473 ns
get MIDDLE arraylist takes 39499 ns
get MIDDLE linkdlist takes 9645996 ns
get last arraylist takes 46165 ns
get last linkdlist takes 43446 ns
**del front arraylist takes 1720329 ns
del front linkdlist takes 108063 ns**
del mid arraylist takes 1157398 ns
del mid linkdlist takes 11845077 ns
del end arraylist takes 54149 ns
del end linkdlist takes 49744 ns

Answer 1

前两个(奇怪的)测试数字的解释是:

插入ArrayList通常较慢,因为一旦达到其边界就必须增长.它必须创建一个新的更大的数组,并从原始数组中复制数据.

但是当你创建一个已足够大的ArrayList 以适应你所有的插件时(你的情况就是这样new ArrayList(n+10)) - 它显然不会涉及任何数组复制操作.添加到它将比使用LinkedList更快,因为LinkedList必须处理它的"链接"(指针),而巨大的ArrayList只是在给定(最后)索引处设置值.

此外,您的测试并不好,因为每次迭代都涉及自动装箱(从int转换为整数) - 这将花费额外的时间来完成此操作,并且还会因为在第一次传递时将填充的整数缓存而搞砸结果.