when Expressions

// when expression with an argument.
when (value) {
    pattern1 -> action or value
    pattern2, pattern3 -> action or value
    in range -> action or value
    is Type -> action or value
    else -> action or value
}

// when without an argument (write conditions directly).
when {
    condition1 -> action
    condition2 -> action
    else -> action
}

fun main() {
    val day = 3

    // Basic when expression.
    val dayName = when (day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3 -> "Wednesday"   // day is 3, so "Wednesday" is selected.
        4 -> "Thursday"
        5 -> "Friday"
        6, 7 -> "Weekend"  // Multiple values can be grouped together.
        else -> "Unknown"
    }
    println(dayName) // Prints "Wednesday".

    // Range check example.
    val score = 78
    val grade = when (score) {
        in 90..100 -> "A"
        in 70..89 -> "B"  // 78 falls in this range.
        in 60..69 -> "C"
        else -> "D"
    }
    println(grade) // Prints "B".

    // Type check and smart cast example.
    fun describe(obj: Any): String = when (obj) {
        is Int -> "Int: ${obj * 2}"         // obj is automatically used as Int.
        is String -> "String: ${obj.length} chars"
        is List<*> -> "List: ${obj.size} items"
        else -> "Other"
    }
    println(describe(42))         // Prints "Int: 84".
    println(describe("Kotlin"))   // Prints "String: 6 chars".

    // when without an argument (conditions written directly).
    val temperature = 30
    val comment = when {
        temperature >= 35 -> "Extremely hot day."
        temperature >= 30 -> "Very hot day." // This branch is selected.
        temperature >= 25 -> "Hot day."
        else -> "Comfortable temperature."
    }
    println(comment) // Prints "Very hot day.".
}

when is one of the most commonly used control flow constructs in Kotlin. When used as an expression, the else branch is required — unless all possible values are covered, such as with a sealed class or enum class, in which case the compiler allows it to be omitted. Combining when with a sealed class is especially powerful, as the compiler can detect missing branches at compile time.

For how to use when with sealed classes, see 'sealed class / sealed interface'. For details on ranges (in), see 'Ranges and for Loops'.