Explain the concept of 'when' expression in Kotlin programming language.

when in Kotlin is a versatile structural operator that replaces the traditional switch in other languages. It can be used as an expression (returning a value) or as a statement.

Main features:

• Value comparison: Matching specific values.

  fun describe(obj: Any): String =
      when (obj) {
          1           -> "One"
          "Hello"     -> "Greeting"
          is Long     -> "Long"
          !is String  -> "Not a string"
          else        -> "Unknown"
      }
  

• Type check (is/!is): Matching data types.

• Range or collection check (in/!in): Checking inclusion in a range or collection.

  fun checkRange(x: Int): String =
      when (x) {
          in 1..10   -> "Positive number in range"
          !in 10..20 -> "Not in the second range"
          else       -> "None of the above"
      }
  

• Without argument: Used as a more flexible replacement for a chain of if-else if-else. Conditions are evaluated sequentially.

  fun checkBoolean(x: Boolean, y: Boolean): String =
      when {
          x && y -> "Both true"
          !x && !y -> "Both false"
          else -> "Mixed"
      }
  

• Return value: If when is used as an expression, it must be exhaustive (cover all possible cases or have an else branch).

when enhances code readability and safety compared to traditional switch.