Generating Random Numbers in a Range in Java

1. Overview

In this tutorial, we'll explore different ways of generating random numbers within a range.

2. Generating Random Numbers in a Range

2.1. Math.random

Math.random gives a random double value that is greater than or equal to 0.0 and less than 1.0.

Let's use the Math.random method to generate a random number in a given range [min, max):

public int getRandomNumber(int min, int max) {
    return (int) ((Math.random() * (max - min)) + min);
}

Why does that work? Let's look at what happens when Math.random returns 0.0, which is the lowest possible output:

0.0 * (max - min) + min => min

So, the lowest number we can get is min.

Since 1.0 is the exclusive upper bound of Math.random, this is what we get:

1.0 * (max - min) + min => max - min + min => max

Therefore, the exclusive upper bound of our method's return is max.

In the next section, we'll see this same pattern repeated with Random#nextInt.

2.2. java.util.Random.nextInt

We can also use an instance of java.util.Random to do the same.

Let's make use of the java.util.Random.nextInt method to get a random number:

public int getRandomNumberUsingNextInt(int min, int max) {
    Random random = new Random();
    return random.nextInt(max - min) + min;
}

The min parameter (the origin) is inclusive, whereas the upper bound max is exclusive.

2.3. java.util.Random.ints

The java.util.Random.ints method returns an IntStream of random integers.

So, we can utilize the java.util.Random.ints method and return a random number:

public int getRandomNumberUsingInts(int min, int max) {
    Random random = new Random();
    return random.ints(min, max)
      .findFirst()
      .getAsInt();
}

Here as well, the specified origin min is inclusive, and max is exclusive.

3. Conclusion

In this article, we saw alternative ways of generating random numbers within a range.

Code snippets, as always, can be found over on GitHub.

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