FizzBuzzGenerator.kt

//
// at domain layer
//
class FizzBuzzNumber(private val number: Int) {

    fun expression(): String {
        val isMul3 = number % 3 == 0
        val isMul5 = number % 5 == 0

        return when {
            isMul3 && isMul5 -> "fizzbuzz"
            isMul3 -> "fizz"
            isMul5 -> "buzz"
            else -> number.toString()
        }
    }
}

//
// at application layer
//
class FizzBuzz {
    private var current = 0

    fun next(): String {
        current++
        return FizzBuzzNumber(current).expression()
    }
}