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In this guide, we will discuss When Expression in Kotlin. Consider a situation when you have large number of conditions to check. Though you can use if..else if expression to handle the situation, but Kotlin provides when expression to handle the situation in nicer way. Using when expression is far easy and more clean in comparison to writing many if…else if expressions. Kotlin when expression evaluates a section of code among many alternatives as explained in below example.

Kotlin when matches its argument against all branches sequentially until some branch condition is satisfied.

Kotlin when expression is similar to the switch statement in C, C++ and Java.

Example

fun main(args: Array<String>) {
   val day = 2

   val result = when (day) {
     1 -> "Monday"
     2 -> "Tuesday"
     3 -> "Wednesday"
     4 -> "Thursday"
     5 -> "Friday"
     6 -> "Saturday"
     7 -> "Sunday"
     else -> "Invalid day."
   }
   println(result)
}

When you run the above Kotlin program, it will generate the following output:

Tuesday

Kotlin when as Statement

Kotlin when can be used either as an expression or as a statement, simply like a switch statement in Java. If it is used as an expression, the value of the first matching branch becomes the value of the overall expression.

Example

Let’s write above example once again without using expression form:

fun main(args: Array<String>) {
   val day = 2

   when (day) {
     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.")
   }
}

When you run the above Kotlin program, it will generate the following output:

Tuesday

Combine when Conditions

We can combine multiple when conditions into a single condition.

Example

fun main(args: Array<String>) {
   val day = 2

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

When you run the above Kotlin program, it will generate the following output:

Weekday

Range in when Conditions

Kotlin ranges are created using double dots .. and we can use them while checking when condition with the help of in operator.

Example

fun main(args: Array<String>) {
   val day = 2

   when (day) {
     in 1..5 -> println("Weekday")
     else -> println("Weekend")
   }
}

When you run the above Kotlin program, it will generate the following output:

Weekday

Expression in when Conditions

Kotlin when can use arbitrary expressions instead of a constant as branch condition.

Example

fun main(args: Array<String>) {
   val x = 20
   val y = 10
   val z = 10
   
   when (x) {
      (y+z) -> print("y + z = x = $x")
      else -> print("Condition is not satisfied")
   }
}

When you run the above Kotlin program, it will generate the following output:

y + z = x = 20

Kotlin when with block of code

Kotlin when braches can be put as block of code enclosed within curly braces.

Example

fun main(args: Array<String>) {
   val day = 2

   when (day) {
     1 -> {
        println("First day of the week")
        println("Monday")
     }
     2 -> {
        println("Second day of the week")
        println("Tuesday")
     }
     3 -> {
        println("Third day of the week")
        println("Wednesday")
     }
     4 -> println("Thursday")
     5 -> println("Friday")
     6 -> println("Saturday")
     7 -> println("Sunday")
     else -> println("Invalid day.")
   }
}

When you run the above Kotlin program, it will generate the following output:

Second day of the week
Tuesday

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