Careeroza

Random Numbers

Math.random and Random

Math.random() returns a double from 0.0 (inclusive) to 1.0 (exclusive). The java.util.Random class gives more control. A seed makes the sequence repeatable, which is useful in tests and games replays.

double r = Math.random();                 // e.g. 0.7302...
int dice = (int) (Math.random() * 6) + 1; // 1 to 6

Random with a Seed

Example

With the same seed, Random always produces the same numbers, so this output is always identical.

import java.util.Random;

public class Main {
    public static void main(String[] args) {
        Random random = new Random(42);
        for (int i = 0; i < 5; i++) {
            int dice = random.nextInt(6) + 1;      // 1 to 6
            System.out.print(dice + " ");
        }
        System.out.println();
        int between = random.nextInt(10, 21);      // 10 to 20 (Java 17+)
        System.out.println("Between 10 and 20: " + between);
    }
}

Output

3 4 1 3 1 
Between 10 and 20: 18

Note: For passwords, OTPs and tokens use java.security.SecureRandom, which is unpredictable. Random and Math.random() are not secure.

Exercise: Write an expression that gives a random integer from 1 to 100 using Math.random().

Show answer
int n = (int) (Math.random() * 100) + 1;