Java的Collections.shuffle是做什么的？(Java&'s Collections.shuffle is doing what?)
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.
I looked at my Java API and it tells me its shuffle method does this:
Randomly permutes the specified list using a default source of randomness.
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?!?!?
To be more specific, which math concepts are being used here?
解决方案
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]);
}
}
}
The swap method:
private static void swap(Object[] x, int a, int b) {
Object t = x[a];
x[a] = x[b];
x[b] = t;
}
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。
我看了看我的Java API，它告诉我它的shuffle方法这样：
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？！？！？
更具体地说，这里使用了哪些数学概念？
解决方案
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）：
public static void shuffle（List<？> ; list，Random rnd）{
int size = list.size（）;
if（size< SHUFFLE_THRESHOLD || RandomAccess的实例列表）{
for（int i = size; i> 1; i--）
swap（list，i-1， nextInt（i））;
} else {
Object arr [] = list.toArray（）;
// shuffle数组
for（int i = size; i> 1; i--）
swap（arr，i-1，rnd.nextInt（i）） ;
//将数组转储回列表
ListIterator it = list.listIterator（）;
for（int i = 0; iit.next（）;
it.set（arr [i]）;
}
}
}
交换方法： p>
private static void swap（Object [] x，int a，int b）{
Object t = x [a] ;
x [a] = x [b];
x [b] = t;
}