Introduction#

Sometimes, one might need to assign a random value to a variable. Random numbers within a specific range of type integer, float, double, long, boolean can be generated in Java.

There are three methods to generate random numbers in Java.

1. By using - Math.random#

This Math.random() gives a random double from 0.0 (inclusive) to 1.0 (exclusive).

For generating random numbers within a range using Math.random(), see the example below:

public class Randomizer {
    /**
    * @param min - minimum range value
    * @param max - maximum range value
    * @return random integer between min (inclusive) to max (inclusive) with uniform probability
    */
    public static int next(int min, int max) {
        if (min >= max) {
         throw new IllegalArgumentException("max must be greater than min");
        }
        return (int)(Math.random() * ((max - min) + 1)) + min;
    }
}

2. By using - java.util.Random#

For generating random numbers within a range using java.util.Random, see the example below:

public class Randomizer {

    /**
    * @param min - minimum range value
    * @param max - maximum range value
    * @return random integer between min (inclusive) to max (inclusive) with uniform probability
    */
    public static int next(int min, int max) {
        if (min >= max) {
         throw new IllegalArgumentException("max must be greater than min");
        }
        Random r = new Random();
        return r.nextInt((max - min) + 1) + min;
    }
}

The Random.nextInt(n) is more efficient than Math.random() * n, read this post.

In addition, Math.random() is thread safe by itself but if you want to generate numbers using Random class then ThreadLocalRandom is more preferable which thread safe.

3. [Java 8] By using - Random.ints#

For generating random numbers within a range using Random.ints, see the example below:

public class Randomizer {

    /**
    * @param min - minimum range value
    * @param max - maximum range value
    * @return random integer between min (inclusive) to max (inclusive) with uniform probability
    */
    public static int next(int min, int max) {
        if (min >= max) {
         throw new IllegalArgumentException("max must be greater than min");
        }
        Random r = new Random();
        return r.ints(min, (max + 1)).limit(1).findFirst().getAsInt();
    }
}

Note: To generates random integers in a range between 15 (inclusive) and 20 (exclusive), with stream size of 3.

new Random().ints(3, 15, 20).forEach(System.out::println);