关于for循环与for-each的效率比较

在翻看Java相关的面试题时看到一个问题，普通for循环和for-each的效率孰优孰劣？

于是写了一个简单的例子做了一些对比，如下：

package com.yummy.nowcoder;

import java.util.ArrayList;
import java.util.Calendar;
import java.util.Iterator;
import java.util.LinkedList;

/**
* Created by qq985 on 2017/3/13.
*/
public class TestForTime {
public static void main(String[] args) {

ArrayList<Integer> arrayList = new ArrayList<>();
for (int i = 0; i < 100000000; i++) {
arrayList.add(1);
}
long startTime;
long endTime;

System.out.println("=======对ArrayList操作=======");

startTime = Calendar.getInstance().getTimeInMillis();
int size = arrayList.size();
for (int i = 0; i < size; i++) {

}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("空循环耗时:" + (endTime - startTime)+"ms");

startTime = Calendar.getInstance().getTimeInMillis();
int size2 = arrayList.size();
for (int i = 0; i < size2; i++) {
int iValue = arrayList.get(i);
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("普通for循环，使用get()耗时:" + (endTime - startTime)+"ms");

startTime = Calendar.getInstance().getTimeInMillis();
for (int iValue : arrayList) {

}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("for-each耗时:" + (endTime - startTime)+"ms");

startTime = Calendar.getInstance().getTimeInMillis();
Iterator iterator = arrayList.iterator();
while (iterator.hasNext()) {
int iValue = (int) iterator.next();
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("使用Iterator耗时:" + (endTime - startTime)+"ms");

LinkedList<Integer> linkedList = new LinkedList<>();
for (int i = 0; i < 100000; i++) {
linkedList.add(1);
}

System.out.println("=======对LinkedList操作=======");

startTime = Calendar.getInstance().getTimeInMillis();
int size3 = linkedList.size();
for (int i = 0; i < size3; i++) {

}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("空循环耗时:" + (endTime - startTime)+"ms");

startTime = Calendar.getInstance().getTimeInMillis();
int size4 = linkedList.size();
for (int i = 0; i < size4; i++) {
int iValue = linkedList.get(i);
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("普通for循环，使用get()耗时:" + (endTime - startTime)+"ms");

startTime = Calendar.getInstance().getTimeInMillis();
for (int iValue : linkedList) {

}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("for-each耗时:" + (endTime - startTime)+"ms");

startTime = Calendar.getInstance().getTimeInMillis();
Iterator iterator2 = linkedList.iterator();
while (iterator2.hasNext()) {
int iValue = (int) iterator2.next();
}
endTime = Calendar.getInstance().getTimeInMillis();
System.out.println("使用Iterator耗时:" + (endTime - startTime)+"ms");
}
}

实验中，分别使用了ArrayList和LinkedList作为操作对象，结果如下：

=======对ArrayList操作=======

空循环耗时:27ms

普通for循环，使用get()耗时:126ms

for-each耗时:257ms

使用Iterator耗时:180ms

=======对LinkedList操作=======

空循环耗时:2ms

普通for循环，使用get()耗时:3495ms

for-each耗时:3ms

使用Iterator耗时:3ms

Process finished with exit code 0

由运行结果可以看出，若以ArrayList作为操作对象，for-each的效率比普通for循环低许多，但是若以LinkedList作为操作对象，for-each的效率与普通for循环的效率不在一个量级，原因也是因为对LinkedList使用了get(index)，导致每次访问数据需从链表头开始遍历，耗时增加。