Kotlin’s when expression is the replacement of switch statement from other languages like C, C++, and Java. It is concise and more powerful than switch statements.

Kotlin - When Expression Examples

Using when as an Expression

Let's see a simple example of when expression:

package net.javaguides.kotlin

fun main(args: Array < String > ) {

    // Using when as an Expression
    var month = 8;
    var monthString = when(month) {
        1 - > "January"
        2 - >
            "February"
        3 - >
            "March"
        4 - >
            "April"
        5 - >
            "May"
        6 - >
            "June"
        7 - >
            "July"
        8 - >
            "August"
        9 - >
            "September"
        10 - >
            "October"
        11 - >
            "November"
        12 - >
            "December"
        else - > "Invalid Month"
    }

    println(monthString);
}

Output:

August

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 languages. For example:

package net.javaguides.kotlin

fun main(args: Array < String > ) {

    // Using when Without Expression
    var dayOfWeek = 4

    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")
    }
}

Output:

Thursday

Multiple Statement of when Using Braces

We can use multiple statements enclosed within a block of condition. For example:

package net.javaguides.kotlin

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")
    }
}