Kotlin: when expression

‘when’ is equivalent to switch statement in Java.

Syntax

when(expression){

         1 -> {//Perform Something}

         2 -> {//Perform Something}

         else -> {//Perform something}

}

‘when’ evaluates the expression  against all the possible cases sequentially. If none of the case is satisfied, then else block will execute.

WhenDemo.kt

fun main(args: Array<String>) {
 
 var a : Int = 5

 when(a){
  12 -> println("Value 12 is checked")
  5 -> println("Value 5 is checked")
  3 -> println("Value 3 is checked")
  else -> println("No value is checked")
 }
}

Output

Value 5 is checked

You can combine multiple branch statements in when expression.

WhenDemo.kt

fun main(args: Array<String>) {
 
 var a : Int = 5

 when(a){
  2, 3, 5, 7 -> println("Prime number")
  4, 6, 8 -> println("Even number other than 2")
  else -> println("No value is checked")
 }
}

Output

Prime number

You can also use arbitrary expressions as branch conditions. For example, I used 2+3 as branch condition.

WhenDemo.kt

fun main(args: Array<String>) {
 
 var a : Int = 5

 when(a){
  2+3 -> println("Checked value 5")
  4, 6, 8 -> println("Even number other than 2")
  else -> println("No value is checked")
 }
}

Output

Checked value 5

You can also check the ranges (or) not in ranges in branch condition.

WhenDemo.kt

fun main(args: Array<String>) {
 
 var a : Int = 5

 when(a){
  in 1..5, in 11..15 -> println("a is in between 1 to 5 (or) 11 to 15")
  in 6..10 -> println("a is in range of 6 to 10")
  else -> println("x is not in range")
 }
}

Output

a is in between 1 to 5 (or) 11 to 15

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