JAVA:Collections類的shuffle()方法
阿新 • • 發佈:2019-02-18
JAVA中Collections類的shuffle()方法的作用是將List中的內容隨機打亂順序。
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);//給定隨機種子 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()); } } }
輸出結果:
[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()方法的原始碼如下:
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]); } } }