Defining Exceptions

You might have a need to create your own custom exceptions.

For example, suppose you are calling a Web service. You will get errors back from that Web service in a variety of ways, such as:

• Error HTTP result codes
• Error information encoded in the JSON (or whatever) payload sent back by the server

You might decide to convert those error conditions into exceptions.

As with Java, you can create subclasses of Exception to represent your custom exceptions:

class GoAway(message: String) : Exception(message)

fun lengthifier(nullAllowedButNotReally: String?): Int {
  if (nullAllowedButNotReally == null) throw GoAway("Please do not pass null to me!")

  return nullAllowedButNotReally!!.length
}

fun main() {
  val result = try {
    lengthifier(null)
  }
  catch (away: GoAway) {
    -2
  }
  catch (e: Exception) {
    -1
  }

  println(result)
}

Here, we create a custom GoAway exception and throw it rather than an Exception.

If you have a set of related exception types, a class hierarchy (using open, abstract, or sealed) may be useful:

open class MathBoom(message: String) : Exception(message)

class DivideByZeroBoom : MathBoom("Please do not divide by zero")

fun divider(numerator: Double, denominator: Double): Double {
  if (denominator == 0.0) throw DivideByZeroBoom()

  return numerator / denominator
}

val result = try {
  divider(1.0, 0.0).toString()
}
catch (e: MathBoom) {
  "whatever"
}

println(result)