Java的Collections.shuffle是做什么的？ [英] Java's Collections.shuffle is doing what?

问题描述

我最近发现自己需要确保我的列表不正常。 Hibernate很好，以完美的顺序返回它。愚蠢的休眠，不读我的心。

I recently found myself needing to be sure my list wasn't in order. Hibernate was nice enough to return it in perfect order. Silly hibernate, not reading my mind.

我看了看我的Java API，它告诉我它的shuffle方法这样：

I looked at my Java API and it tells me its shuffle method does this:

使用默认的随机性随机排列指定的列表。

作为我的好奇乔治，我想知道这是什么意思。有一个数学课程我可以学习这个吗？我可以看到代码吗？ Java，你在为我的ArrayList做什么？！？！？

Being the curious george that I am, I want to know what exactly this means. Is there a math course I can take to learn this? Can I see the code? Java, what are you doing to my ArrayList?!?!?

更具体地说，这里使用了哪些数学概念？

To be more specific, which math concepts are being used here?

推荐答案

是的，你可以看看代码;它基本上会执行 Fisher-Yates shuffle 。这里是（感谢OpenJDK和yay的开源:-P）：

Yes, you can look at the code; it basically does a Fisher-Yates shuffle. Here it is (thanks OpenJDK, and yay for open source :-P):

public static void shuffle(List<?> list, Random rnd) {
    int size = list.size();
    if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {
        for (int i=size; i>1; i--)
            swap(list, i-1, rnd.nextInt(i));
    } else {
        Object arr[] = list.toArray();

        // Shuffle array
        for (int i=size; i>1; i--)
            swap(arr, i-1, rnd.nextInt(i));

        // Dump array back into list
        ListIterator it = list.listIterator();
        for (int i=0; i<arr.length; i++) {
            it.next();
            it.set(arr[i]);
        }
    }
}

交换方法： p>

The swap method:

 private static void swap(Object[] x, int a, int b) {
    Object t = x[a];
    x[a] = x[b];
    x[b] = t;
}

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