Collections.shuffle()源码分析
源代码展示：
1 public class Collections { 2 private static Random r; 3 private static final int SHUFFLE_THRESHOLD = 5; 4 5 public static void shuffle(List list) { 6 if (r == null) { 7 r = new Random(); 8 } 9 shuffle(list, r);10 }11 12 public static void shuffle(List list, Random rnd) {13 int size = list.size();14 if (size < SHUFFLE_THRESHOLD || list instanceof RandomAccess) {15 for (int i = size; i > 1; i--)16 swap(list, i - 1, rnd.nextInt(i));17 } else {18 Object arr[] = list.toArray();19 20 // Shuffle array21 for (int i = size; i > 1; i--)22 swap(arr, i - 1, rnd.nextInt(i));23 24 // Dump array back into list25 ListIterator it = list.listIterator();26 for (int i = 0; i < arr.length; i++) {27 it.next();28 it.set(arr[i]);29 }30 }31 }32 33 public static void swap(List list, int i, int j) {34 final List l = list;35 l.set(i, l.set(j, l.get(i)));36 }37 38 private static void swap(Object[] arr, int i, int j) {39 Object tmp = arr[i];40 arr[i] = arr[j];41 arr[j] = tmp;42 }43 44 } 1 public class ArrayList{ 2 private transient Object[] elementData; 3 private int size; 4 5 public E set(int index, E element) { 6 RangeCheck(index); 7 8 E oldValue = (E) elementData[index]; 9 elementData[index] = element;10 return oldValue;11 }12 13 private void RangeCheck(int index) {14 if (index >= size)15 throw new IndexOutOfBoundsException("Index: " + index + ", Size: "16 + size);17 }18 }
经典示例：洗牌算法
1 import java.util.ArrayList; 2 import java.util.List; 3 import java.util.Random; 4 5 public class CollectionsShuffle { 6 public static void main(String[] args) { 7 List cards = new ArrayList (); 8 // 生成一副牌 9 for (int rank = Card.THREE; rank <= Card.DEUCE; rank++) { 10 cards.add(new Card(Card.DIAMOND, rank)); 11 cards.add(new Card(Card.CLUB, rank)); 12 cards.add(new Card(Card.HEART, rank)); 13 cards.add(new Card(Card.SPADE, rank)); 14 } 15 cards.add(new Card(Card.JOKER, Card.BLACK)); 16 cards.add(new Card(Card.JOKER, Card.COLOR)); 17 System.out.println(cards.toString()); 18 /* 19 * [方块3, 梅花3, 红桃3, 黑桃3, 方块4, 梅花4, 红桃4, 黑桃4, 方块5, 梅花5, 红桃5, 黑桃5, 方块6, 20 * 梅花6, 红桃6, 黑桃6, 方块7, 梅花7, 红桃7, 黑桃7, 方块8, 梅花8, 红桃8, 黑桃8, 方块9, 梅花9, 红桃9, 21 * 黑桃9, 方块10, 梅花10, 红桃10, 黑桃10, 方块J, 梅花J, 红桃J, 黑桃J, 方块Q, 梅花Q, 红桃Q, 黑桃Q, 22 * 方块K, 梅花K, 红桃K, 黑桃K, 方块A, 梅花A, 红桃A, 黑桃A, 方块2, 梅花2, 红桃2, 黑桃2, 小王, 大王] 23 */ 24 // 经典洗牌算法 25 Random random = new Random(); 26 for (int i = cards.size(); i > 1; i--) { 27 int m = random.nextInt(i); 28 swap(cards, i - 1, m); 29 } 30 System.out.println(cards.toString()); 31 /* 32 * [黑桃7, 黑桃A, 梅花A, 红桃9, 梅花4, 红桃K, 方块5, 梅花7, 梅花6, 方块A, 黑桃Q, 梅花5, 红桃10, 33 * 梅花Q, 梅花J, 方块J, 梅花K, 方块8, 方块6, 方块10, 红桃7, 方块K, 红桃6, 黑桃2, 黑桃K, 梅花10, 34 * 红桃8, 方块Q, 红桃Q, 大王, 梅花3, 梅花2, 方块7, 方块9, 方块4, 红桃3, 梅花9, 红桃J, 黑桃8, 红桃2, 35 * 黑桃6, 红桃A, 黑桃9, 黑桃4, 黑桃J, 黑桃10, 小王, 黑桃3, 黑桃5, 红桃5, 红桃4, 方块2, 方块3, 梅花8] 36 */ 37 38 } 39 40 public static void swap(List list, int i, int j) { 41 final List l = list; 42 l.set(i, l.set(j, l.get(i))); 43 } 44 } 45 46 class Card { 47 48 public static final int DIAMOND = 0; // 方块(钻石) 49 public final static int CLUB = 1; // 梅花 50 public static final int HEART = 2; // 红桃(红心) 51 public static final int SPADE = 3; // 黑桃(花锄) 52 public static final int JOKER = 4; // 王 53 54 public final static int THREE = 0; 55 public final static int FOUR = 1; 56 public final static int FIVE = 2; 57 public final static int SIX = 3; 58 public final static int SEVEN = 4; 59 public final static int EIGHT = 5; 60 public final static int NINE = 6; 61 public final static int TEN = 7; 62 public final static int JACK = 8;// J 63 public final static int QUEEN = 9;// Q 64 public final static int KING = 10;// K 65 public final static int ACE = 11;// A 66 public final static int DEUCE = 12; // 2 67 public final static int BLACK = 13; // 小王 68 public final static int COLOR = 14;// 大王 69 70 /** 花色 0代表方块, 1代表梅花, 2代表红桃, 3代表黑桃,4:王 */ 71 private int suit; 72 /** 点数 规定: 0代表3, 1代表4, 2代表5,... */ 73 private int rank; 74 75 public Card() { 76 } 77 78 public Card(int suit, int rank) { 79 // this.rank = rank; 80 // this.suit = suit; 81 setRank(rank); 82 setSuit(suit); 83 } 84 85 public int getSuit() { 86 return suit; 87 } 88 89 public void setSuit(int suit) { 90 if (suit < DIAMOND || suit > JOKER) 91 throw new RuntimeException("花色超过范围!"); 92 this.suit = suit; 93 } 94 95 public int getRank() { 96 return rank; 97 } 98 99 public void setRank(int rank) {100 if (rank < THREE || rank > COLOR) {101 throw new RuntimeException("点数超过范围!");102 }103 this.rank = rank;104 }105 106 private static final String[] RANK_NAMES = { "3", "4", "5", "6", "7", "8",107 "9", "10", "J", "Q", "K", "A", "2", "小王", "大王" };108 private static final String[] SUIT_NAMES = { "方块", "梅花", "红桃", "黑桃", "" };109 110 // 覆盖Object 类的toStirng() 方法. 实现对象的文本描述111 public String toString() {112 return SUIT_NAMES[suit] + RANK_NAMES[rank];113 }114 115 public boolean equals(Object obj) {116 if (obj == null) {117 return false;118 }119 if (this == obj) {120 return true;121 }122 if (obj instanceof Card) {123 Card other = (Card) obj;124 return this.rank == other.rank && this.suit == other.suit;125 }126 return false;127 }128 129 public int hashCode() {130 // return suit*100+rank;131 // suit=3= 00000000 00000000 00000000 00000011132 // rank=10=00000000 00000000 00000000 00000011133 // suit<<16=00000000 00000011 00000000 00000000134 // 00000000 00000011 00000000 00000011135 return (suit << 16) + rank;// (x<<16)+y136 }137 }