java - 使用增强的 For 循环语法输出较慢的结果

Question

当我看到这篇文章时，我正在阅读 Google 文档页面 http://developer.android.com/training/articles/perf-tips.html

谁能解释为什么我得到相反的结果？

我的结果是:(平均) 结果 1:76 结果 2:73 结果 3:143

我使用以下代码执行了一些测试(循环执行相同的测试):

package TestPack;

import java.util.ArrayList;


public class Test
{
static ArrayList<Integer> mTheArray = new ArrayList<Integer>();
static
{
    for(int i = 0; i < 10000000; i++)
    {
        mTheArray.add(1234234223);
    }
}

static long mTimeStarted;
public static void main(String args[])
{
    //Test 1.
    mTimeStarted = System.currentTimeMillis();
    One();
    Fn.Out("Result 1: " + String.valueOf(System.currentTimeMillis() - mTimeStarted));

    //Test 2.
    mTimeStarted = System.currentTimeMillis();
    Two();
    Fn.Out("Result 2: " + String.valueOf(System.currentTimeMillis() - mTimeStarted));

    //Test 3
    mTimeStarted = System.currentTimeMillis();
    Three();
    Fn.Out("Result 3: " + String.valueOf(System.currentTimeMillis() - mTimeStarted));

}

//Slowest (But not...).
public static int One()
{
    int sum = 0;
    for (int i = 0; i < mTheArray.size(); ++i) 
    {
        sum += mTheArray.get(i);
    }
    return sum;
}

public static int Two()
{
    int sum = 0;
    ArrayList<Integer> localArray = mTheArray;
    int len = localArray.size();

    for (int i = 0; i < len; ++i) {
        sum += localArray.get(i);
    }
    return sum;
}

//Fastest (But actually slowest in this test).
public static int Three()
{
    int sum = 0;
    for (Integer a : mTheArray) {
        sum += a;
    }
    return sum;
}

Answer 1

您链接的页面明确指出:

With an ArrayList, a hand-written counted loop is about 3x faster (with or without JIT), but for other collections the enhanced for loop syntax will be exactly equivalent to explicit iterator usage.

So, you should use the enhanced for loop by default, but consider a hand-written counted loop for performance-critical ArrayList iteration.

您使用的是 ArrayList，因此您得到的结果似乎与此声明相当一致。