解析ArrayList与LinkedList的遍历方法

ArrayList与LinkedList都是实现List接口，前者底层是数据存储，而LinkedList则是以链表的形式存储，因此两个都各有其独自的特性。在两者中，遍历方法也随着改变而有所不同。

遍历方法

通用的List的遍历方法有三种：

1、for get循环遍历

2、foreach遍历

3、for Iterator迭代遍历

遍历结果

arrayList length: 360000

linkedList length: 360000

First: for get

arrayList need time: 15ms

linkedList need time: 239653ms

Second: foreach

arrayList need time: 22ms

linkedList need time: 20ms

Thrid: foreach itertor

arrayList need time: 17 ms

linkedList need time: 8 ms

这是在三十六万条数据下的结果。可以得出一个初步结论，linkedList使用get方法的效率极低，而linkedList使用foreach和itertor的表现是优于arraylist的。如果再把数据扩大十倍，让数据达到三百六十万条数据，在该情况下的速度结构更加体现了两者在不同遍历方法下的区别。

源码分析

ArrayList get:

public E get(int index) {
    rangeCheck(index); //越界检查

    return elementData(index);
}

因为ArrayList存储为数组存储，所以可以直接用下表来访问，速度极快。

LinkedList get:

public E get(int index) {
    checkElementIndex(index);
    return node(index).item;
}
Node<E> node(int index) {
    // assert isElementIndex(index);

    if (index < (size >> 1)) {
        Node<E> x = first;
        for (int i = 0; i < index; i++)
            x = x.next;
        return x;
    } else {
        Node<E> x = last;
        for (int i = size - 1; i > index; i--)
            x = x.prev;
        return x;
    }
}

LinkedList存储为Node，也就是双向链表方式存储，虽然两头都可以进行遍历，但是每次都要重复进行从头遍历元素，效率自然就慢了。

而foreach和迭代器方式两者的区别不大。

总结

1、ArrayList使用Get来遍历元素；Linkedlist多用foreach或迭代器方式遍历。

2、java开发当中为了能够尽可能的挖掘优化的点，应当对代码中的结构有深刻的理解，了解其中的构造，那么自然能够优化出速度快优美的代码。

附：遍历代码

 public static void main(String[] args) throws Exception{
    ArrayList<String> arrayList = new ArrayList<String>();
    LinkedList<String> linkedList = new LinkedList<String>();
    int num = 360000;

    Random random = new Random();
    String randomStr = "";
    for (int i = 0; i < num; i++){
        randomStr = String.valueOf(random.nextFloat());
        arrayList.add(randomStr);
        linkedList.add(randomStr);
    }

    System.out.println("------------------");
    System.out.println("arrayList length: "+arrayList.size());
    System.out.println("linkedList length: "+linkedList.size());
    System.out.println("------------------");

    System.out.println("------The way------");
    System.out.println("First: for get");
    long time1 = System.currentTimeMillis();
    for (int i =0;i<arrayList.size();i++) {
        String temp = arrayList.get(i);
    }
    long time2 = System.currentTimeMillis();
    System.out.println("arrayList need time: "+(time2-time1)+"ms");
    long time11 = System.currentTimeMillis();
    for (int i =0;i<linkedList.size();i++) {
        String temp = linkedList.get(i);
    }
    long time22 = System.currentTimeMillis();
    System.out.println("linkedList need time: "+(time22-time11)+"ms");

    System.out.println("
Second: foreach");
    long time3 = System.currentTimeMillis();
    for (String temp:arrayList) {
        String t = new String();
    }
    long time4 = System.currentTimeMillis();
    System.out.println("arrayList need time: "+(time4-time3)+"ms");
    long time33 = System.currentTimeMillis();
    for (String temp:linkedList) {
        String t = new String();
    }
    long time44 = System.currentTimeMillis();
    System.out.println("linkedList  need time: "+(time44-time33)+"ms");

    System.out.println("
Thrid: foreach itertor");
    long time5 = System.currentTimeMillis();
    Iterator<String> iterator1 = arrayList.iterator();
    while ( iterator1.hasNext()) {
        String t = iterator1.next();
    }
    long time6 = System.currentTimeMillis();
    System.out.println("arrayList need time: "+(time6-time5)+" ms");
    long time55 = System.currentTimeMillis();
    Iterator<String> iterator2 = linkedList.iterator();
    while ( iterator2.hasNext()) {
        String t = iterator2.next();
    }
    long time66 = System.currentTimeMillis();
    System.out.println("linkedList need time: "+(time66-time55)+" ms");
}//main