需求：我們需要在三個不同的數字段產生不同數量的隨機數。
比如：[0, 10) 6個；[10, 20) 2個；[20, 30) 2個。
以下為測試代碼：
1 package com.homeland.myapp;
2
3 import java.util.Random;
4
5
6 public class RandomSequence {
7
8 public static void main(String[] args) {
9 RandomGen d1 = new RandomGen(0, 10, 6);
10 RandomGen d2 = new RandomGen(10, 20, 2);
11 RandomGen d3 = new RandomGen(20, 30, 2);
12 Thread t1 = new Thread(d1);
13 Thread t2 = new Thread(d2);
14 Thread t3 = new Thread(d3);
15 t1.start();
16 t2.start();
17 t3.start();
18 }
19
20 }
21
22 class RandomGen implements Runnable {
23 Random r = new Random();
24 int start, end, num;
25
26 public RandomGen(int end) {
27 this.end = end;
28 }
29
30 public RandomGen(int start, int end) {
31 this.start = start;
32 this.end = end;
33 }
34
35 public RandomGen(int start, int end, int num) {
36 this.start = start;
37 this.end = end;
38 this.num = num;
39 }
40
41 @Override
42 public void run() {
43 int c = 0;
44 for (int i = 0; c < num; i++) {
45 int a = r.nextInt(end);
46 if (a >= start) {
47 System.out.println("Number " + i + " is: " + a);
48 c++;
49 }
50 }
51 System.out.println("-----------------------");
52 }
53 }
以上三個不同的數字段都只包括第一個數字，不包括最后一個。也即，不包含上限。
以下為測試結果：
1 Number 0 is: 1
2 Number 1 is: 5
3 Number 2 is: 0
4 Number 3 is: 7
5 Number 4 is: 6
6 Number 5 is: 8
7 -----------------------
8 Number 12 is: 26
9 Number 13 is: 28
10 -----------------------
11 Number 0 is: 10
12 Number 4 is: 11
13 -----------------------
考慮到去掉重復的數字，代碼作如下修改：
1 package com.homeland.myapp;
2
3 import java.util.HashSet;
4 import java.util.Random;
5 import java.util.Set;
6
7
8 public class RandomSequence {
9
10 public static void main(String[] args) {
11 RandomGen d1 = new RandomGen(0, 10, 6);
12 RandomGen d2 = new RandomGen(10, 20, 2);
13 RandomGen d3 = new RandomGen(20, 30, 2);
14 Thread t1 = new Thread(d1);
15 Thread t2 = new Thread(d2);
16 Thread t3 = new Thread(d3);
17 t1.start();
18 t2.start();
19 t3.start();
20 }
21
22 }
23
24 class RandomGen implements Runnable {
25 Random r = new Random();
26 int start, end, num;
27
28 public RandomGen(int end) {
29 this.end = end;
30 }
31
32 public RandomGen(int start, int end) {
33 this.start = start;
34 this.end = end;
35 }
36
37 public RandomGen(int start, int end, int num) {
38 this.start = start;
39 this.end = end;
40 this.num = num;
41 }
42
43 @Override
44 public void run() {
45 Set<Integer> s = new HashSet<Integer>();
46 for (int i = 0; s.size() < num; i++) {
47 int a = r.nextInt(end);
48 if (a >= start) {
49 s.add(a);
50 }
51 }
52 System.out.println("there are "+ num +" numbers in range: [" + start + ", " + end + "]:\n");
53 System.out.println(s.toString());
54 System.out.println("-----------------------");
55 }
56 }
其中一組結果如下：
1 there are 6 numbers in range: [0, 10]:
2
3 [0, 2, 5, 6, 8, 9]
4 -----------------------
5 there are 2 numbers in range: [20, 30]:
6
7 [22, 26]
8 -----------------------
9 there are 2 numbers in range: [10, 20]:
10
11 [11, 13]
12 -----------------------
13
我修改了代碼，在某個位置上加入了syncronized關鍵字，還有用循環來控制線程的產生：
1 package com.homeland.myapp;
2
3 import java.util.HashSet;
4 import java.util.Random;
5 import java.util.Set;
6
7 public class RandomSequence {
8
9 public static void main(String[] args) {
10
11 int a[][] = {
12 {0, 10, 6}, {10, 20, 2}, {20, 30, 4},
13 {30, 40, 3}, {40, 50, 5}, {50, 60, 7},
14 {60, 70, 8}, {70, 80, 9}, {80, 90, 4}, {90, 100, 7}};
15
16 for (int i = 0; i < a.length; i++) {
17 RandomGen d = new RandomGen(a[i][0], a[i][1], a[i][2]);
18 Thread t = new Thread(d);
19 t.start();
20 }
21 }
22
23 }
24
25 class RandomGen implements Runnable {
26 Random r = new Random();
27 int start, end, num;
28 Set<Integer> s = new HashSet<Integer>();
29
30 public RandomGen(int end) {
31 this.end = end;
32 }
33
34 public RandomGen(int start, int end) {
35 this.start = start;
36 this.end = end;
37 }
38
39 public RandomGen(int start, int end, int num) {
40 this.start = start;
41 this.end = end;
42 this.num = num;
43 }
44
45 @Override
46 public void run() {
47 synchronized (s) {
48 for (int i = 0; s.size() < num; i++) {
49 int a = r.nextInt(end);
50 if (a >= start) {
51 s.add(a);
52 }
53 }
54 System.out.println("there are "+ num +" numbers in range: [" + start + ", " + end + "]:\n");
55 System.out.println(s.toString());
56 System.out.println("-----------------------");
57 }// sync
58 }
59 }
但是，我得到如下結果：
1 there are 4 numbers in range: [20, 30]:
2
3 there are 3 numbers in range: [30, 40]:
4
5 [35, 39, 30]
6 -----------------------
7 there are 6 numbers in range: [0, 10]:
8
9 [0, 1, 2, 6, 7, 9]
10 -----------------------
11 there are 2 numbers in range: [10, 20]:
12
13 [16, 12]
14 -----------------------
15 there are 7 numbers in range: [50, 60]:
16
17 [51, 50, 55, 53, 52, 59, 58]
18 -----------------------
19 [21, 20, 24, 26]
20 -----------------------
21 there are 9 numbers in range: [70, 80]:
22
23 [70, 71, 76, 77, 79, 72, 73, 74, 75]
24 -----------------------
25 there are 5 numbers in range: [40, 50]:
26
27 [48, 43, 40, 46, 45]
28 -----------------------
29 there are 7 numbers in range: [90, 100]:
30
31 [98, 99, 96, 93, 95, 94, 90]
32 -----------------------
33 there are 8 numbers in range: [60, 70]:
34
35 [68, 69, 64, 65, 66, 67, 62, 60]
36 -----------------------
37 there are 4 numbers in range: [80, 90]:
38
39 [84, 87, 83, 89]
40 -----------------------
看起來某些部分的輸出時交錯的。看來我需要另外一種控制機制。
這個控制機制就是：
1 Thread.sleep(5000);
把這句放到上面代碼中的main()方法的循環中去。
上面的Thread.sleep(5000)不是沒有問題的。你可以試想一下，如果數字的范圍很大呢？
所以，問題的關鍵是：如何保證線程同步或者說線程對共享資源的互斥訪問呢？把代碼做如下修改：
1 package com.homeland.myapp;
2
3 import java.util.HashSet;
4 import java.util.Random;
5 import java.util.Set;
6
7 public class RandomSequence {
8
9 public static void main(String[] args) throws Exception {
10
11 int a[][] = {
12 {0, 10, 6}, {10, 20, 2}, {20, 30, 4},
13 {30, 40, 3}, {40, 50, 5}, {50, 60, 7},
14 {60, 70, 8}, {70, 80, 9}, {80, 90, 4}, {90, 100, 7}};
15
16 for (int i = 0; i < a.length; i++) {
17 RandomGen d = new RandomGen(a[i][0], a[i][1], a[i][2]);
18 Thread t = new Thread(d);
19 t.start();
20 Thread.sleep(1);
21 }
22 }
23
24 }
25
26 class RandomGen implements Runnable {
27 Random r = new Random();
28 int start, end, num;
29 private Set<Integer> s = new HashSet<Integer>();
30 private static Object lock = new Object();
31
32 public RandomGen(int end) {
33 this.end = end;
34 }
35
36 public RandomGen(int start, int end) {
37 this.start = start;
38 this.end = end;
39 }
40
41 public RandomGen(int start, int end, int num) {
42 this.start = start;
43 this.end = end;
44 this.num = num;
45 }
46
47
48 @Override
49 public void run() {
50 synchronized (lock) {
51 for (int i = 0; s.size() < num; i++) {
52 int a = r.nextInt(end);
53 if (a >= start) {
54 s.add(a);
55 }
56 }
57 System.out.println("there are "+ num +" numbers in range: [" + start + ", " + end + "]:\n");
58 System.out.println(s.toString());
59 System.out.println("-----------------------");
60 }// sync
61 }
62 }
你木有看錯！靜態類成員被所有類實例共享。關鍵的一句：
1 private static Object lock = new Object();
當然，你也可以這么改：
1 package com.homeland.myapp;
2
3 import java.util.HashSet;
4 import java.util.Random;
5 import java.util.Set;
6
7 public class RandomSequence {
8
9 public static void main(String[] args) throws Exception {
10
11 int a[][] = {
12 {0, 10, 6}, {10, 20, 2}, {20, 30, 4},
13 {30, 40, 3}, {40, 50, 5}, {50, 60, 7},
14 {60, 70, 8}, {70, 80, 9}, {80, 90, 4}, {90, 100, 7}};
15
16 for (int i = 0; i < a.length; i++) {
17 RandomGen d = new RandomGen(a[i][0], a[i][1], a[i][2]);
18 Thread t = new Thread(d);
19 t.start();
20 Thread.sleep(1);
21 }
22 }
23
24 }
25
26 class RandomGen implements Runnable {
27 static final Random r = new Random();
28 static int start, end, num;
29 private static Set<Integer> s = new HashSet<Integer>();
30
31 public RandomGen(int end) {
32 this.end = end;
33 }
34
35 public RandomGen(int start, int end) {
36 this.start = start;
37 this.end = end;
38 }
39
40 public RandomGen(int start, int end, int num) {
41 this.start = start;
42 this.end = end;
43 this.num = num;
44 }
45
46 private static synchronized void printSequence() {
47 for (int i = 0; s.size() < num; i++) {
48 int a = r.nextInt(end);
49 if (a >= start) {
50 s.add(a);
51 }
52 }
53 System.out.println("there are "+ num +" numbers in range: [" + start + ", " + end + "]:\n");
54 System.out.println(s.toString());
55 System.out.println("-----------------------");
56 }
57
58 @Override
59 public void run() {
60 printSequence();
61 }
62 }
需要注意下，后面的靜態方法會產生類似這樣的結果：
1 there are 2 numbers in range: [10, 20]:
2
3 [18, 3]
4 -----------------------
5 there are 4 numbers in range: [20, 30]:
6
7 [18, 3, 20, 23]
8 -----------------------
9 there are 4 numbers in range: [20, 30]:
10
11 [18, 3, 20, 23]
12 -----------------------
13 there are 3 numbers in range: [30, 40]:
14
15 [18, 3, 20, 23]
16 -----------------------
17 there are 5 numbers in range: [40, 50]:
18
19 [18, 3, 20, 23, 42]
20 -----------------------
21 there are 7 numbers in range: [50, 60]:
22
23 [18, 3, 20, 23, 42, 58, 57]
24 -----------------------
25 there are 8 numbers in range: [60, 70]:
26
27 [18, 3, 65, 20, 23, 42, 58, 57]
28 -----------------------
29 there are 9 numbers in range: [70, 80]:
30
31 [18, 3, 65, 20, 23, 76, 42, 58, 57]
32 -----------------------
33 there are 4 numbers in range: [80, 90]:
34
35 [18, 3, 65, 20, 23, 76, 42, 58, 57]
36 -----------------------
37 there are 7 numbers in range: [90, 100]:
38
39 [18, 3, 65, 20, 23, 76, 42, 58, 57]
40 -----------------------
41
所以，你需要這么修改代碼：
1 package com.homeland.myapp;
2
3 import java.util.HashSet;
4 import java.util.Random;
5 import java.util.Set;
6
7 public class RandomSequence {
8
9 public static void main(String[] args) throws Exception {
10
11 int a[][] = {
12 {0, 10, 6}, {10, 20, 2}, {20, 30, 4},
13 {30, 40, 3}, {40, 50, 5}, {50, 60, 7},
14 {60, 70, 8}, {70, 80, 9}, {80, 90, 4}, {90, 100, 7}};
15
16 for (int i = 0; i < a.length; i++) {
17 RandomGen d = new RandomGen(a[i][0], a[i][1], a[i][2]);
18 Thread t = new Thread(d);
19 t.start();
20 Thread.sleep(1);
21 }
22 }
23
24 }
25
26 class RandomGen implements Runnable {
27 static final Random r = new Random();
28 int start, end, num;
29
30 public RandomGen(int end) {
31 this.end = end;
32 }
33
34 public RandomGen(int start, int end) {
35 this.start = start;
36 this.end = end;
37 }
38
39 public RandomGen(int start, int end, int num) {
40 this.start = start;
41 this.end = end;
42 this.num = num;
43 }
44
45 private static synchronized void printSequence(int start, int end, int num) {
46 final Set<Integer> s = new HashSet<Integer>();
47 for (; s.size() < num;) {
48 int a = r.nextInt(end);
49 if (a >= start) {
50 s.add(a);
51 }
52 }
53 System.out.println("there are "+ num +" numbers in range: [" + start + ", " + end + "]:\n");
54 System.out.println(s.toString());
55 System.out.println("-----------------------");
56 }
57
58 @Override
59 public void run() {
60 printSequence(start, end, num);
61 }
62 }
我得到的結果類似這種：
1 there are 6 numbers in range: [0, 10]:
2
3 [0, 3, 4, 6, 7, 8]
4 -----------------------
5 there are 2 numbers in range: [10, 20]:
6
7 [17, 13]
8 -----------------------
9 there are 4 numbers in range: [20, 30]:
10
11 [20, 25, 26, 28]
12 -----------------------
13 there are 3 numbers in range: [30, 40]:
14
15 [33, 38, 30]
16 -----------------------
17 there are 5 numbers in range: [40, 50]:
18
19 [43, 40, 46, 47, 45]
20 -----------------------
21 there are 7 numbers in range: [50, 60]:
22
23 [55, 54, 53, 59, 58, 57, 56]
24 -----------------------
25 there are 8 numbers in range: [60, 70]:
26
27 [68, 69, 64, 66, 63, 62, 61, 60]
28 -----------------------
29 there are 9 numbers in range: [70, 80]:
30
31 [70, 71, 76, 77, 78, 79, 72, 73, 74]
32 -----------------------
33 there are 4 numbers in range: [80, 90]:
34
35 [86, 80, 82, 88]
36 -----------------------
37 there are 7 numbers in range: [90, 100]:
38
39 [96, 97, 93, 92, 95, 94, 90]
40 -----------------------
41
留個小尾巴：用HashSet去重效率如何呢？