Get a Random Number in Kotlin

1. Introduction

This short tutorial will demonstrate how to generate a random number using Kotlin.

2. Random number using java.lang.Math

The easiest way to generate a random number in Kotlin is to use java.lang.Math. Below example will generate a random double number between 0 and 1.

@Test
fun whenRandomNumberWithJavaUtilMath_thenResultIsBetween0And1() {
    val randomNumber = Math.random()
    assertTrue { randomNumber >= 0 }
    assertTrue { randomNumber < 1 }
}

3. Random number using ThreadLocalRandom

We can also use java.util.concurrent.ThreadLocalRandom to generate a random double, integer or long value. Integer and long values generated this way can be both positive or negative.

ThreadLocalRandom is thread-safe and provides better performance in a multithreaded environment because it provides a separate Random object for every thread and thus reduces contention between threads:

@Test
fun whenRandomNumberWithJavaThreadLocalRandom_thenResultsInDefaultRanges() {
    val randomDouble = ThreadLocalRandom.current().nextDouble()
    val randomInteger = ThreadLocalRandom.current().nextInt()
    val randomLong = ThreadLocalRandom.current().nextLong()
    assertTrue { randomDouble >= 0 }
    assertTrue { randomDouble < 1 }
    assertTrue { randomInteger >= Integer.MIN_VALUE }
    assertTrue { randomInteger < Integer.MAX_VALUE }
    assertTrue { randomLong >= Long.MIN_VALUE }
    assertTrue { randomLong < Long.MAX_VALUE }
}

4. Random number using Kotlin.js

We can also generate a random double using the Math class from the kotlin.js library.

@Test
fun whenRandomNumberWithKotlinJSMath_thenResultIsBetween0And1() {
    val randomDouble = Math.random()
    assertTrue { randomDouble >=0 }
    assertTrue { randomDouble < 1 }
}

5. Random number in given range using pure Kotlin

Using pure Kotlin, we can create a list of numbers, shuffle it and then take the first element from that list:

@Test
fun whenRandomNumberWithKotlinNumberRange_thenResultInGivenRange() {
    val randomInteger = (1..12).shuffled().first()
    assertTrue { randomInteger >= 1 }
    assertTrue { randomInteger <= 12 }
}

6. Random number in given range using ThreadLocalRandom

ThreadLocalRandom presented in section 3 can also be used to generate a random number in a given range:

@Test
fun whenRandomNumberWithJavaThreadLocalRandom_thenResultsInGivenRanges() {
    val randomDouble = ThreadLocalRandom.current().nextDouble(1.0, 10.0)
    val randomInteger = ThreadLocalRandom.current().nextInt(1, 10)
    val randomLong = ThreadLocalRandom.current().nextLong(1, 10)
    assertTrue { randomDouble >= 1 }
    assertTrue { randomDouble < 10 }
    assertTrue { randomInteger >= 1 }
    assertTrue { randomInteger < 10 }
    assertTrue { randomLong >= 1 }
    assertTrue { randomLong < 10 }
}

7. Conclusion

In this article, we’ve learned a few ways to generate a random number in Kotlin.

As always, all code presented in this tutorial can be found over on GitHub.

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