随机数生成器（RNG, random number generator）

Java 8提供了4种生成随机数的方法，如下所示：

java.util.Random class
Math.random method : Can Generate Random Numbers of double type.
ThreadLocalRandom class
java.Security.SecureRandom

1) java.util.Random

Random使用的算法是linear congruential generator (LCG 线性同余发生器).
要使用此类生成随机数，我们必须首先创建此类的实例，然后使用该实例调用 nextInt()、nextDouble()、nextLong() 等方法。
我们可以使用这个类生成整数、浮点数、双精度数、长整数、布尔值类型的随机数。
我们可以将参数传递给在要生成的数字范围上设置上限的方法。例如，nextInt(6) 将生成 0 到 5 范围内的数字，包括两者。

// A Java program to demonstrate random number generation
// using java.util.Random;
import java.util.Random;public class generateRandom{public static void main(String args[]){// create instance of Random classRandom rand = new Random();// Generate random integers in range 0 to 999int rand_int1 = rand.nextInt(1000);int rand_int2 = rand.nextInt(1000);// Print random integersSystem.out.println("Random Integers: "+rand_int1);System.out.println("Random Integers: "+rand_int2);// Generate Random doublesdouble rand_dub1 = rand.nextDouble();double rand_dub2 = rand.nextDouble();// Print random doublesSystem.out.println("Random Doubles: "+rand_dub1);System.out.println("Random Doubles: "+rand_dub2);}
}

Random Integers: 547
Random Integers: 126
Random Doubles: 0.8369779739988428
Random Doubles: 0.5497554388209912

2) Math.random()

类 Math 包含各种数值运算的方法，例如计算幂、对数等。其中一种方法是 random()，该方法返回一个双精度值，带正号，大于或等于 0.0 且小于 1.0 . 返回值是伪随机选择的。此方法只能生成 Doubles 类型的随机数。下面的程序解释了如何使用这种方法：


// Java program to demonstrate working of
// Math.random() to generate random numbers
import java.util.*;public class generateRandom
{public static void main(String args[]){// Generating random doublesSystem.out.println("Random doubles: " + Math.random());System.out.println("Random doubles: " + Math.random());}
}

Random doubles: 0.13077348615666562
Random doubles: 0.09247016928442775

3) java.util.concurrent.ThreadLocalRandom class

这个类是在 java 1.7 中引入的，用于生成整数、双精度、布尔值等类型的随机数。下面的程序解释了如何使用这个类来生成随机数

// Java program to demonstrate working of ThreadLocalRandom
// to generate random numbers.
import java.util.concurrent.ThreadLocalRandom;public class generateRandom
{public static void main(String args[]){// Generate random integers in range 0 to 999int rand_int1 = ThreadLocalRandom.current().nextInt();int rand_int2 = ThreadLocalRandom.current().nextInt();// Print random integersSystem.out.println("Random Integers: " + rand_int1);System.out.println("Random Integers: " + rand_int2);// Generate Random doublesdouble rand_dub1 = ThreadLocalRandom.current().nextDouble();double rand_dub2 = ThreadLocalRandom.current().nextDouble();// Print random doublesSystem.out.println("Random Doubles: " + rand_dub1);System.out.println("Random Doubles: " + rand_dub2);// Generate random booleansboolean rand_bool1 = ThreadLocalRandom.current().nextBoolean();boolean rand_bool2 = ThreadLocalRandom.current().nextBoolean();// Print random BooleansSystem.out.println("Random Booleans: " + rand_bool1);System.out.println("Random Booleans: " + rand_bool2);}
}

Random Integers: 536953314
Random Integers: 25905330
Random Doubles: 0.7504989954390163
Random Doubles: 0.7658597196204409
Random Booleans: false
Random Booleans: true

4）java.Security.SecureRandom

SecureRandom默认的算法是SHA1PRNG.此类提供加密强随机数生成器 ----cryptographically secure PRNG (CSPRNG)。

SecureRandom random1 = new SecureRandom();
SecureRandom random2 = SecureRandom.getInstance("SHA1PRNG");

for (int i = 0; i < 5; i++) {System.out.println(random1.nextInt() + " != " + random2.nextInt());
}

704046703 != 2117229935
60819811 != 107252259
425075610 != -295395347
682299589 != -1637998900
-1147654329 != 1418666937

性能比较：

SecureRandom比Random要慢1个数量级.

    @Testpublic void performanceCompare(){final long start = System.currentTimeMillis();randomInt(new Random());System.out.println("cost:" + (System.currentTimeMillis() - start) + "ms");final long start2 = System.currentTimeMillis();randomInt(new SecureRandom());System.out.println("cost:" + (System.currentTimeMillis() - start2) + "ms");}public void randomInt(Random r){final HashMap<Integer, Integer> map = new HashMap<>();for (int i = 0; i < 100000; i++) {final int val = r.nextInt(10);final Integer count = map.get(val);if(count == null){map.put(val, 0);}else{map.put(val, count + 1);}try {Thread.sleep(5);} catch (InterruptedException e) {e.printStackTrace();}}map.forEach((k,v)-> System.out.println(k+"-->"+v));}

0–>9968
1–>10008
2–>10003
3–>9862
4–>9975
5–>10044
6–>10247
7–>9925
8–>10001
9–>9957
cost:602079ms
0–>10150
1–>10001
2–>9960
3–>9789
4–>9973
5–>9946
6–>9958
7–>10100
8–>10024
9–>10089
cost:5024065ms

参考：
Generating random numbers in Java: https://www.geeksforgeeks.org/generating-random-numbers-in-java/

Java 生成随机数的 5 种方式，你知道几种:https://segmentfault.com/a/1190000038405773

官方文档： https://docs.oracle.com/javase/8/docs/api/java/util/Random.html
https://docs.oracle.com/javase/8/docs/api/java/security/SecureRandom.html

维基百科
https://en.wikipedia.org/wiki/Pseudorandom_number_generator
https://en.wikipedia.org/wiki/Linear_congruential_generator