JAVA：Collections类的shuffle()方法

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JAVA中Collections类的shuffle()方法的作用是将List中的内容随机打乱顺序。

import java.util.ArrayList;import java.util.Collections;import java.util.HashSet;import java.util.List;import java.util.Random;/* *Created on 2015年12月23日 *Copyright 2015 Yong Cai Limited crop. All Rights Reserved * */public class CollectionShuffle {public static void main(String[] args) {List<Integer> li = new ArrayList<Integer>();HashSet<Integer> set = new HashSet<Integer>();for(int i=0;i < 10;i++){li.add(i);}System.out.println(li.toString());Random rnd = new Random(2);//给定随机种子for(int j=1;j <= 3;j++){Collections.shuffle(li);System.out.println(li.toString());Collections.shuffle(li, rnd);System.out.println("add rnd:"+li.toString());}}}

输出结果：

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9][7, 3, 8, 4, 0, 5, 1, 2, 9, 6]add rnd:[8, 3, 6, 5, 7, 4, 0, 2, 1, 9][2, 3, 8, 9, 7, 6, 5, 0, 1, 4]add rnd:[4, 6, 3, 0, 7, 5, 2, 9, 8, 1][9, 2, 8, 7, 0, 5, 3, 6, 1, 4]add rnd:[2, 0, 6, 7, 8, 3, 5, 4, 1, 9]

shuffle()方法的源代码如下：

   public static void shuffle(List<?> list) {        if (r == null) {            r = new Random();        }        shuffle(list, r);    }    private static Random r;    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]);            }        }    }

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