When Expression

When you have multiple specific values to check against, long chains of else if statements can become hard to read. Kotlin provides the when expression as a cleaner alternative for matching a value against multiple possibilities.

fun main() {
    val day = 3
    
    val dayName = when (day) {
        1 -> "Monday"
        2 -> "Tuesday"
        3 -> "Wednesday"
        4 -> "Thursday"
        5 -> "Friday"
        else -> "Weekend"
    }
    
    println(dayName)  // Prints: Wednesday
}

The when expression takes a value and compares it against each branch. When a match is found, the corresponding result is returned. The else branch handles any value that doesn't match the listed cases.

You can also match multiple values in a single branch by separating them with commas:

fun main() {
    val day = 6
    
    val type = when (day) {
        1, 2, 3, 4, 5 -> "Weekday"
        6, 7 -> "Weekend"
        else -> "Invalid"
    }
    
    println(type)  // Prints: Weekend
}

A value-producing when must be exhaustive: every possible input must have a result. These integer examples need else, but a Boolean when with both true and false branches does not.