In Kotlin, the when expression can be used as an alternative to the traditional switch statement found in languages like Java. The when expression is versatile, concise, and supports a wide range of matching conditions. Here’s how you can use it:

Examples of Using the when Expression

1. Using when as a simple switch:

   fun getResponse(code: Int): String {
      return when (code) {
          200 -> "OK"
          404 -> "Not Found"
          500 -> "Internal Server Error"
          else -> "Unknown Code"
      }
   }
   

2. Multiple matches for a single branch:
   You can check multiple values by separating them with a comma.

   fun isWeekend(day: String): Boolean {
      return when (day) {
          "Saturday", "Sunday" -> true
          else -> false
      }
   }
   

3. Range checking:
   You can check if a value falls within a certain range.

   fun categorizeAge(age: Int): String {
      return when (age) {
          in 0..12 -> "Child"
          in 13..19 -> "Teenager"
          in 20..59 -> "Adult"
          else -> "Senior"
      }
   }
   

4. Type checking:
   The is keyword can be used to check the type of a variable.

   fun describe(obj: Any): String {
      return when (obj) {
          is String -> "It's a string of length ${obj.length}"
          is Int -> "It's an integer"
          else -> "Unknown type"
      }
   }
   

5. Checking conditions (arbitrary boolean expressions):
   You can use boolean expressions inside the when expression.

   fun checkNumber(num: Int): String {
      return when {
          num < 0 -> "Negative Number"
          num == 0 -> "Zero"
          num > 0 -> "Positive Number"
          else -> "Unknown"
      }
   }
   

6. when without an argument:
   You can omit the argument from when if you just want to match conditions.

   fun getResult(value: Int): String {
      return when {
          value % 2 == 0 -> "Even"
          value % 2 != 0 -> "Odd"
          else -> "Unknown"
      }
   }
   

when as an Expression vs Statement

In Kotlin, when is an expression, which means it can return a value:

val message = when (val age = 25) {
    in 0..12 -> "You're a child"
    in 13..19 -> "You're a teenager"
    else -> "You're an adult"
}
println(message)

But you can also use it as a statement if you don’t need the result:

fun printMessage(code: Int) {
    when (code) {
        200 -> println("Request was successful")
        404 -> println("Resource not found")
        else -> println("Unexpected response code")
    }
}

Summary

The when expression in Kotlin allows for cleaner and more expressive code compared to a traditional switch statement. It supports:

• Multiple case matching.
• Type checking.
• Arbitrary conditions.
• Use as a statement or an expression.

This flexibility makes it a powerful tool for decision-making in Kotlin!