Kotlin When

Basic when Syntax

A when block compares a subject value against a list of branches. As soon as a branch's value matches, its code runs and the rest of the branches are skipped — there is no fall-through, so you never need a break statement.

fun main() {
    val day = 3

    when (day) {
        1 -> println("Monday")
        2 -> println("Tuesday")
        3 -> println("Wednesday")
        else -> println("Some other day")
    }
}

Combining Branches

Multiple values can share a single branch by separating them with commas. This avoids repeating the same body for related cases, such as grouping weekend days together.

fun main() {
    val day = 6

    when (day) {
        1, 2, 3, 4, 5 -> println("Weekday")
        6, 7 -> println("Weekend")
        else -> println("Invalid day")
    }
}

when as an Expression

Just like if, when can be used as an expression that returns a value. In this form, the compiler requires the branches to be exhaustive — meaning every possible input must be covered, typically enforced with an else branch.

fun main() {
    val statusCode = 404

    val message = when (statusCode) {
        200 -> "OK"
        301 -> "Moved Permanently"
        404 -> "Not Found"
        500 -> "Server Error"
        else -> "Unknown Status"
    }

    println(message)
}

Ranges, Types, and Conditions in when

Kotlin's when is far more powerful than a typical switch. Branches can test ranges with in, check an object's type with is, or even omit the subject entirely and evaluate arbitrary Boolean conditions instead.

• Use in range to match a value falling within a range, such as 1..10.
• Use is Type to match based on runtime type, which also smart-casts the variable inside that branch.
• Omit the subject in when {} to write a chain of independent Boolean conditions, similar to else if.

fun describe(input: Any): String {
    return when (input) {
        in 1..9 -> "Single digit number"
        is String -> "A string of length ${input.length}"
        is Boolean -> "A boolean value"
        else -> "Something else"
    }
}

fun main() {
    println(describe(5))
    println(describe("Kotlin"))
    println(describe(true))
}