Kotlin when Expression

Kotlin, when expression is a conditional expression which returns the value. Kotlin, when expression is replacement of switch statement. Kotlin, when expression works as a switch statement of other language (Java, C++, C).

Using when as an Expression

Let's see a simple example of when expression.

fun main(args: Array<String>){
    var number = 4
    var numberProvided = when(number) {
        1 -> "One"
        2 -> "Two"
        3 -> "Three"
        4 -> "Four"
        5 -> "Five"
        else -> "invalid number"
    }
    println("You provide $numberProvided")
}

Output:

You provide Four

Using when Without Expression

It is not mandatory to use when as an expression, it can be used as normally as it used in other language.

For Example

fun main(args: Array<String>){

    var number = 4
    when(number) {
        1 -> println("One")
        2 -> println("Two")
        3 -> println("Three")
        4 -> println("Four")
        5 -> println("Five")
        else -> println("invalid number")
    }

}

Output:

Four

Multiple Statement of when Using Braces

We can use multiple statement enclosed within block of condition.

For Example

fun main(args: Array<String>){
    var number = 1
    when(number) {
        1 -> {
            println("Monday")
            println("First day of the week")
        }
        7 -> println("Sunday")
        else -> println("Other days")
    }
}

Output:

Monday
First day of the week

Multiple branches of when

We can use multiple branches of condition separated with a comma. It is used, when we need to run a same logic for multiple choices.

fun main(args: Array<String>){
    var number = 8
    when(number) {
        3, 4, 5, 6 ->
            println("It is summer season")
        7, 8, 9 ->
            println("It is rainy season")
        10, 11 ->
            println("It is autumn season")
        12, 1, 2 ->
            println("It is winter season")
        else -> println("invalid input")
    }
}

Output:

It is rainy season

Using when in the range

The when expression also check the ranges of input provided in when condition. A range is created using .. (double dot) operator. The in operator is used to check if a value belongs to a range.

For Example:

fun main(args: Array<String>){
    var number = 7
    when(number) {
        in 1..5 -> println("Input is provided in the range 1 to 5")
        in 6..10 -> println("Input is provided in the range 6 to 10")
        else -> println("none of the above")
    }
}

Output: