Java shuffle 算法
Fisher–Yates shuffle 基本思想（Knuth shuffle ）：
To shuffle an array a of n elements (indices 0..n-1):
for i from n − 1 downto 1 do
j ← random integer with 0 ≤ j ≤ i
exchange a[j] and a[i]
JDK源代码如下：
/** * Moves every element of the List to a random new position in the list. * * @param list * the List to shuffle * * @throws UnsupportedOperationException * when replacing an element in the List is not supported */ public static void shuffle(List<?> list) { shuffle(list, new Random()); } /** * Moves every element of the List to a random new position in the list * using the specified random number generator. * * @param list * the List to shuffle * @param random * the random number generator * * @throws UnsupportedOperationException * when replacing an element in the List is not supported */ @SuppressWarnings("unchecked") public static void shuffle(List<?> list, Random random) { if (!(list instanceof RandomAccess)) { Object[] array = list.toArray(); for (int i = array.length - 1; i > 0; i--) { int index = random.nextInt(i + 1); if (index < 0) { index = -index; } Object temp = array[i]; array[i] = array[index]; array[index] = temp; } int i = 0; ListIterator<Object> it = (ListIterator<Object>) list .listIterator(); while (it.hasNext()) { it.next(); it.set(array[i++]); } } else { List<Object> rawList = (List<Object>) list; for (int i = rawList.size() - 1; i > 0; i--) { int index = random.nextInt(i + 1); if (index < 0) { index = -index; } rawList.set(index, rawList.set(i, rawList.get(index))); } } }
测试代码，为了确保每种情况的初始化一样，使用了多个容器：
public class javaShuffle { public static int temp = 0; public static long start; public static long end; public static void main(final String args[]) { Object changeTemp; List<Integer> numList = new ArrayList<Integer>(); List<Integer> firstList = new ArrayList<Integer>(); List<Integer> secondList = new ArrayList<Integer>(); List<Integer> thirdList = new ArrayList<Integer>(); List<Integer> fourthList = new ArrayList<Integer>(); for (int i = 1; i <= 100000; i++) { numList.add(i); firstList.add(i); secondList.add(i); thirdList.add(i); fourthList.add(i); } // first shuffle,use changeTemp getStartTime(); int randInt = 0; for (int i = 0, length = firstList.size(); i < length; i++) { randInt = getRandom(i, firstList.size()); changeTemp = firstList.get(i); firstList.set(i, firstList.get(randInt)); firstList.set(randInt, javaShuffle.temp); } getEndTime("first shuffle run time "); // second shuffle,exchange list getStartTime(); for (int i = 0, length = secondList.size(); i < length; i++) { randInt = getRandom(i, secondList.size()); secondList.set(i, secondList.set(randInt, secondList.get(i))); } getEndTime("second shuffle run time"); // third shuffle, change generate random int getStartTime(); Object[] tempArray = thirdList.toArray(); Random rand = new Random(); int j = 0; for (int i = tempArray.length - 1; i > 0; i--) { j = rand.nextInt(i + 1); thirdList.set(i, thirdList.set(j, thirdList.get(i))); } getEndTime("third shuffle run time "); // fourth shuffle, simulate java shuffle getStartTime(); Random random = new Random(); if (!(fourthList instanceof RandomAccess)) { Object[] array = fourthList.toArray(); for (int i = array.length - 1; i > 0; i--) { int index = random.nextInt(i + 1); if (index < 0) { index = -index; } Object temp = array[i]; array[i] = array[index]; array[index] = temp; } int i = 0; ListIterator<Integer> it = (ListIterator<Integer>) fourthList.listIterator(); while (it.hasNext()) { it.next(); it.set((Integer) array[i++]); } } else { List<Integer> rawList = (List<Integer>) fourthList; for (int i = rawList.size() - 1; i > 0; i--) { int index = random.nextInt(i + 1); if (index < 0) { index = -index; } rawList.set(index, rawList.set(i, rawList.get(index))); } } getEndTime("fourth shuffle run time"); // java shuffle getStartTime(); Collections.shuffle(numList); getEndTime("java shuffle run time "); } public static void swap(int a, int b) { javaShuffle.temp = a; a = b; b = javaShuffle.temp; } public static int getRandom(final int low, final int high) { return (int) (Math.random() * (high - low) + low); } public static void getStartTime() { javaShuffle.start = System.nanoTime(); } public static void getEndTime(final String s) { javaShuffle.end = System.nanoTime(); System.out.println(s + ": " + (javaShuffle.end - javaShuffle.start) + "ns"); } } 如果数值较小，例如100000级别，则输出大概是： first shuffle run time : 85029499ns second shuffle run time: 80909474ns third shuffle run time : 71543926ns fourth shuffle run time: 76520595ns java shuffle run time : 61027643ns first shuffle run time : 82326239ns second shuffle run time: 78575611ns third shuffle run time : 95009632ns fourth shuffle run time: 105946897ns java shuffle run time : 90849302ns first shuffle run time : 84539840ns second shuffle run time: 85965575ns third shuffle run time : 101814998ns fourth shuffle run time: 113309672ns java shuffle run time : 35089693ns first shuffle run time : 87679863ns second shuffle run time: 79991814ns third shuffle run time : 73720515ns fourth shuffle run time: 78353061ns java shuffle run time : 64146465ns first shuffle run time : 84314386ns second shuffle run time: 80074803ns third shuffle run time : 74001283ns fourth shuffle run time: 79931321ns java shuffle run time : 86427540ns first shuffle run time : 84315523ns second shuffle run time: 81468386ns third shuffle run time : 75052284ns fourth shuffle run time: 79461407ns java shuffle run time : 66607729ns
多次运行结果可能都不一样，但是基本java自带 shuffle速度最快，第三种方式次之。而第一种方法耗时最久。
如果是10000000级别，大概如下：
first shuffle run time : 2115703288ns second shuffle run time: 3114045871ns third shuffle run time : 4664426798ns fourth shuffle run time: 2962686695ns java shuffle run time : 3246883026ns
first shuffle run time : 2165398466ns second shuffle run time: 3129558913ns third shuffle run time : 4147859664ns fourth shuffle run time: 2911849942ns java shuffle run time : 4311703487ns
first shuffle run time : 2227462247ns second shuffle run time: 3279548770ns third shuffle run time : 4704344954ns fourth shuffle run time: 2942635980ns java shuffle run time : 3933172427ns
first shuffle run time : 2200158789ns second shuffle run time: 3172666791ns third shuffle run time : 4715631517ns fourth shuffle run time: 2950817535ns java shuffle run time : 3387417676ns
first shuffle run time : 2201124449ns second shuffle run time: 3203823874ns third shuffle run time : 4179926278ns fourth shuffle run time: 2913690411ns java shuffle run time : 3571313813ns
first shuffle run time : 2163053190ns second shuffle run time: 3073889926ns third shuffle run time : 4493831518ns fourth shuffle run time: 2852713887ns java shuffle run time : 3773602415ns
可以看出，第一种方法速度最快，而第四种最慢。java自带 shuffle速度也不理想。
在进行大数据处理的时候，如果使用java库效率较低时，可以考虑使用其他方式。