Java Program to Generate a Random Number

Random numbers are used in games, simulations, test data and many other programs. Java offers three common ways to generate them:

• the java.util.Random class,
• the Math.random() method,
• the java.util.concurrent.ThreadLocalRandom class.

Because the numbers are random, the output on this page will be different every time you run the programs.

1. Random Numbers Using the Random Class

Create a Random object and call methods such as nextInt(), nextDouble() or nextBoolean(). nextInt(bound) returns a number from 0 up to, but not including, bound, so nextInt(100) + 1 gives a number from 1 to 100.

import java.util.Random;

public class RandomNumbers {
    public static void main(String[] args) {
        Random random = new Random();

        System.out.println("5 random numbers from 1 to 100:");
        for (int i = 0; i < 5; i++) {
            System.out.println(random.nextInt(100) + 1);
        }

        System.out.println("Random double: " + random.nextDouble());
        System.out.println("Random boolean: " + random.nextBoolean());
    }
}

5 random numbers from 1 to 100:
45
1
67
67
19
Random double: 0.5555215832659274
Random boolean: true

2. Random Numbers Using Math.random()

Math.random() returns a double greater than or equal to 0.0 and less than 1.0. To get a whole number in the range min to max, multiply by the size of the range, cast to int and add min.

public class MathRandom {
    public static void main(String[] args) {
        double value = Math.random();
        System.out.println("Random value: " + value);

        int min = 1, max = 6;
        int dice = (int) (Math.random() * (max - min + 1)) + min;
        System.out.println("Dice roll: " + dice);
    }
}

Random value: 0.31767054428967223
Dice roll: 3

3. Random Numbers Using ThreadLocalRandom

ThreadLocalRandom was added in Java 7. You do not create it with new; call ThreadLocalRandom.current() instead. Its nextInt(origin, bound) method takes a range directly, and it is the best choice in programs that use several threads.

import java.util.concurrent.ThreadLocalRandom;

public class ThreadLocalRandomDemo {
    public static void main(String[] args) {
        ThreadLocalRandom random = ThreadLocalRandom.current();

        int number = random.nextInt(10, 21);          // 10 to 20
        double price = random.nextDouble(5.0, 50.0);  // 5.0 to just under 50.0
        long id = random.nextLong(1_000, 10_000);     // 1000 to 9999

        System.out.println("Random int (10-20): " + number);
        System.out.printf("Random price: %.2f%n", price);
        System.out.println("Random id: " + id);
    }
}

Random int (10-20): 10
Random price: 11.14
Random id: 8196

4. Summary

• new Random().nextInt(bound) returns a number from 0 to bound - 1.
• Math.random() returns a double from 0.0 (inclusive) to 1.0 (exclusive); scale and cast it for a range.
• ThreadLocalRandom.current().nextInt(origin, bound) takes a range directly and suits multi-threaded code.
• For passwords or security tokens, use java.security.SecureRandom instead.

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