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Kotlin When

Kotlin when

Instead of writing many if..else expressions, you can use the when expression, which is much easier to read.

It is used to select one of many code blocks to be executed:


Use the weekday number to calculate the weekday name:

val day = 4

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

// Outputs "Thursday" (day 4)
Try it Yourself »

The when expression is similar to the switch statement in Java.

This is how it works:

  • The when variable (day) is evaluated once
  • The value of the day variable is compared with the values of each "branch"
  • Each branch starts with a value, followed by an arrow (->) and a result
  • If there is a match, the associated block of code is executed
  • else is used to specify some code to run if there is no match
  • In the example above, the value of day is 4, meaning "Thursday" will be printed