Rolling dice in Kotlin

A little more than 2 years ago, I wrote a post on how you could create a Die rolling API in Scala. As I’m more and more interested in Kotlin, let’s do that in Kotlin.

At the root of the hierarchy lies the Rollable interface:

interface Rollable<T> {
    fun roll(): T

The base class is the Die:

open class Die(val sides: Int): Rollable<Int> {

    init {
        val random = new SecureRandom()

    override fun roll() = random.nextInt(sides)

Now let’s create some objects:

object d2: Die(2)
object d3: Die(3)
object d4: Die(4)
object d6: Die(6)
object d10: Die(10)
object d12: Die(12)
object d20: Die(20)

Finally, in order to make code using Die instances testable, let’s change the class to inject the Random instead:

open class Die(val sides: Int, private val random: Random = SecureRandom()): Rollable<Int> {
    override fun roll() = random.nextInt(sides)

Note that the random property is private, so that only the class itself can use it - there won’t even be a getter.

The coolest thing about that I that I hacked the above code in 15 minutes in the plane. I love Kotlin :-)

Nicolas Fränkel

Nicolas Fränkel

Nicolas Fränkel is a Developer Advocate with 15+ years experience consulting for many different customers, in a wide range of contexts (such as telecoms, banking, insurances, large retail and public sector). Usually working on Java/Java EE and Spring technologies, but with focused interests like Rich Internet Applications, Testing, CI/CD and DevOps. Currently working for Exoscale. Also double as a teacher in universities and higher education schools, a trainer and triples as a book author.

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