Enums with when · Enum Classes in Kotlin | Kotlin

Enums with when

Exhaustive when

When when is used as an expression on an enum, it must cover every constant (or have else). If you later add a new constant, the compiler shows an error everywhere it is not handled - a big safety win.

Example

enum class TrafficLight { RED, YELLOW, GREEN }

fun action(light: TrafficLight): String = when (light) {
    TrafficLight.RED -> "Stop"
    TrafficLight.YELLOW -> "Get ready"
    TrafficLight.GREEN -> "Go"
    // no else needed: all cases are covered
}

fun main() {
    TrafficLight.entries.forEach { println("$it -> ${action(it)}") }
}

Output

RED -> Stop
YELLOW -> Get ready
GREEN -> Go

Tip: Avoid else in a when over an enum or sealed type. Without else, the compiler reminds you to handle new values.

Exercise

Exercise: Create enum class Day with the 7 days and a function isWeekend() written with when. Print the weekend days.

Show answer

enum class Day {
    MON, TUE, WED, THU, FRI, SAT, SUN;

    fun isWeekend() = when (this) {
        SAT, SUN -> true
        else -> false
    }
}

fun main() {
    println(Day.entries.filter { it.isWeekend() }) // [SAT, SUN]
}