Kotlin when expression
last modified January 29, 2024
This article shows how to use when expressions in Kotlin. A when expression is a replacement of a switch statement known from C family of languages.
Kotlin is a statically-typed programming language that runs on the Java virtual machine.
Kotlin when matches its argument against all branches sequentially until some branch condition is satisfied. It can be used either as an expression or as a statement. If it is used as an expression, the value of the satisfied branch becomes the value of the overall expression.
Kotlin when simple example
The following is a simple example which uses when expression with days of week.
package com.zetcode
fun main() {
val dayOfWeek = 3
when (dayOfWeek) {
1 -> println("monday")
2 -> println("tuesday")
3 -> println("wednesday")
4 -> println("thursday")
5 -> println("friday")
6 -> println("saturday")
7 -> println("sunday")
else -> println("invalid day")
}
}
In the example, we print the day of week based on an integer value. Seven different brances are tested sequentially. If one of them is satisfied, the rest are skipped. If none of them is satisfied, the else branch is executed.
Kotlin when grouping values
It is possible to group values in branches with the comma operator.
package com.zetcode
fun main() {
val dayOfWeek = 5
when (dayOfWeek){
1,2,3,4,5 -> println("weekday")
6,7 -> println("weekend")
else -> println("wrong value")
}
}
In the example, we group values to form two branches: for weekdays and for weekend.
Kotlin when expression example
Kotlin when can be used as an expression. The value of the satisfied branch is returned.
package com.zetcode
import java.time.DayOfWeek
import java.time.LocalDate
fun main() {
val dayOfWeek: DayOfWeek = LocalDate.now().dayOfWeek
val msg:String = when (dayOfWeek) {
DayOfWeek.MONDAY -> "It is monday"
DayOfWeek.TUESDAY -> "It is tuesday"
DayOfWeek.WEDNESDAY -> "It is tuesday"
DayOfWeek.THURSDAY -> "It is tuesday"
DayOfWeek.FRIDAY -> "It is tuesday"
DayOfWeek.SATURDAY -> "It is tuesday"
DayOfWeek.SUNDAY -> "It is tuesday"
else -> "Invalid day of week"
}
println(msg)
}
We determine the current weekday. The string from the matched branch is assigned
to the msg variable, which is later printed to the console.
Kotlin when ranges
We can test ranges of values in when branches. (This can be done also with collections.)
package com.zetcode
fun main() {
val age = 40
when (age) {
in 0..14 -> println("children")
in 15..24 -> println("youth")
in 25..64 -> println("adults")
in 65..120 -> println("seniors")
in 120..130 -> println("unlikely age")
else -> println("wrong age value")
}
}
We have an age variable. We assign the value to an age group using Kotlin ranges.
Kotlin when enums
Kotlin when expression can be used with enums.
package com.zetcode
enum class Season {
SPRING, SUMMER, AUTUMN, WINTER
}
fun main() {
val season = Season.SUMMER
when (season) {
Season.SPRING -> println("It is spring")
Season.SUMMER -> println("It is summer")
Season.AUTUMN -> println("It is autumn")
Season.WINTER -> println("It is winter")
}
}
We have a Season enum. We pass the enum as the
argument of the when expression.
Kotlin when without argument
Kotlin when expression can be used without an argument.
package com.zetcode
import java.util.Random
fun main() {
val r:Int = Random().nextInt(10) - 5
when {
r < 0 -> println("negative value")
r == 0 -> println("zero")
r > 0 -> println("positive value")
}
}
In the example, we generate a random value. We test the value with the when expression without passing it as an argument in round brackets.
Kotlin when types
We can check for types with the is operator.
package com.zetcode
fun main() {
val data = listOf<Any>("sparrow", 12, true, intArrayOf(1, 2, 3), 12.1)
data.forEach { e ->
when (e) {
is String -> println("$e is a string")
is Int -> println("$e is an integer")
is Boolean -> println("$e is a boolean")
is IntArray-> println("[${e.joinToString()}] is an array of integers")
is Double -> println("$e is a double")
else -> println("$e is unknown")
}
}
}
In the example, we check for a type of a variable in a when expression.
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