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 array
21 for (int i = size; i > 1; i--)
22 swap(arr, i - 1, rnd.nextInt(i));
23
24 // Dump array back into list
25 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<E> {
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<Card> cards = new ArrayList<Card>();
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 00000011
132 // rank=10=00000000 00000000 00000000 00000011
133 // suit<<16=00000000 00000011 00000000 00000000
134 // 00000000 00000011 00000000 00000011
135 return (suit << 16) + rank;// (x<<16)+y
136 }
137 }