How to get different random numbers in java

Three generation methods:

1. Get a long type of current time milliseconds through System.currentTimeMillis() number.

2. Return a double value between 0 and 1 through Math.random().

3. Generate a random number through the Random class. This is a professional Random tool class with powerful functions.

First type:

Get random numbers through System.currentTimeMillis(). It actually gets the current time in milliseconds, which is of type long. The usage method is as follows:

final long l = System.currentTimeMillis();

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If you want to get an integer of type int, you only need to convert the above result to int Just type. For example, get an integer between [0, 100). The method is as follows:

final long l = System.currentTimeMillis();
final int i = (int)( l % 100 );

Second:

Get random numbers through Math.random(). In fact, it returns a double value between 0 (inclusive) and 1 (exclusive). The usage method is as follows:

final double d = Math.random();

To obtain an integer of type int, you only need to convert the above result to type int. For example, get an integer between [0, 100). The method is as follows:

final double d = Math.random();
final int i = (int)(d*100);

The third method:

Get random numbers through the Random class. The usage method is as follows:

1. Create a Random object. There are two methods to create Random objects, as follows:

Random random = new Random();//默认构造方法
Random random = new Random(1000);//指定种子数字

(02) Get random numbers through Random objects. Random value types supported by Random include: boolean, byte, int, long, float, double.

For example, get the int integer between [0, 100). The method is as follows:

int i2 = random.nextInt(100);

Code example:

 1 import java.util.Random;
 2 import java.lang.Math;
 3 
 4 /**
 5  * java 的随机数测试程序。共3种获取随机数的方法：
 6  *   (01)、通过System.currentTimeMillis()来获取一个当前时间毫秒数的long型数字。
 7  *   (02)、通过Math.random()返回一个0到1之间的double值。
 8  *   (03)、通过Random类来产生一个随机数，这个是专业的Random工具类，功能强大。
 9  *
10  * @author skywang
11  * @email kuiwu-wang@163.com
12  */
13 public class RandomTest{
14 
15     public static void main(String args[]){
16 
17         // 通过System的currentTimeMillis()返回随机数
18         testSystemTimeMillis();
19 
20         // 通过Math的random()返回随机数
21         testMathRandom();
22 
23         // 新建“种子为1000”的Random对象，并通过该种子去测试Random的API
24         testRandomAPIs(new Random(1000), " 1st Random(1000)");
25         testRandomAPIs(new Random(1000), " 2nd Random(1000)");
26         // 新建“默认种子”的Random对象，并通过该种子去测试Random的API
27         testRandomAPIs(new Random(), " 1st Random()");
28         testRandomAPIs(new Random(), " 2nd Random()");
29     }
30 
31     /**
32      * 返回随机数-01：测试System的currentTimeMillis()
33      */
34     private static void testSystemTimeMillis() {
35         // 通过
36         final long l = System.currentTimeMillis();
37         // 通过l获取一个[0, 100)之间的整数
38         final int i = (int)( l % 100 );
39 
40         System.out.printf("\n---- System.currentTimeMillis() ----\n l=%s i=%s\n", l, i);
41     }
42 
43 
44     /**
45      * 返回随机数-02：测试Math的random()
46      */
47     private static void testMathRandom() {
48         // 通过Math的random()函数返回一个double类型随机数，范围[0.0, 1.0)
49         final double d = Math.random();
50         // 通过d获取一个[0, 100)之间的整数
51         final int i = (int)(d*100);
52 
53         System.out.printf("\n---- Math.random() ----\n d=%s i=%s\n", d, i);
54     }
55 
56 
57     /**
58      * 返回随机数-03：测试Random的API
59      */
60     private static void testRandomAPIs(Random random, String title) {
61         final int BUFFER_LEN = 5;
62 
63         // 获取随机的boolean值
64         boolean b = random.nextBoolean();
65         // 获取随机的数组buf[]
66         byte[] buf = new byte[BUFFER_LEN];
67         random.nextBytes(buf);
68         // 获取随机的Double值，范围[0.0, 1.0)
69         double d = random.nextDouble();
70         // 获取随机的float值，范围[0.0, 1.0)
71         float f = random.nextFloat();
72         // 获取随机的int值
73         int i1 = random.nextInt();
74         // 获取随机的[0,100)之间的int值
75         int i2 = random.nextInt(100);
76         // 获取随机的高斯分布的double值
77         double g = random.nextGaussian();
78         // 获取随机的long值
79         long l = random.nextLong();
80 
81         System.out.printf("\n---- %s ----\nb=%s, d=%s, f=%s, i1=%s, i2=%s, g=%s, l=%s, buf=[",
82                 title, b, d, f, i1, i2, g, l);
83         for (byte bt:buf) 
84             System.out.printf("%s, ", bt);
85         System.out.println("]");
86     }
87 }

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