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 node(int index) {
// assert isElementIndex(index);

if (index < (size >> 1)) {
Node x = first;
for (int i = 0; i < index; i++)
x = x.next;
return x;
} else {
Node 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 arrayList = new ArrayList();
LinkedList linkedList = new LinkedList();
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("\nSecond: 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("\nThrid: foreach itertor");
long time5 = System.currentTimeMillis();
Iterator 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 iterator2 = linkedList.iterator();
while ( iterator2.hasNext()) {
String t = iterator2.next();
}
long time66 = System.currentTimeMillis();
System.out.println("linkedList need time: "+(time66-time55)+" ms");
}//main