How to avoid 'unchecked cast'?

1

Can you guys help me to fix the following code in Kotlin? Any suggestion is really welcome.

Thanks in advance.

// EVENT -----------------------------------------------
interface Event {
}
class NewTicketEvent(val title: String) : Event
class NewCommentEvent(val comment: String) : Event
// -----------------------------------------------------

// FORMATTER FOR EVENTS --------------------------------

interface Formatter<T: Event> {

  val eventType: Class<out T>

  fun format(event: T): String

}

class FormatterForNewTicketEvent : Formatter<NewTicketEvent> {

  override val eventType: Class<out NewTicketEvent>

  get() = NewTicketEvent::class.java


  override fun format(event: NewTicketEvent): String {

  return “New Ticket Event: ${event.title}”

  }

}

class FormatterForNewCommentEvent : Formatter<NewCommentEvent> {

  override val eventType: Class<out NewCommentEvent>

  get() = NewCommentEvent::class.java


  override fun format(event: NewCommentEvent): String {

  return “Ticket Commented Event: ${event.comment}”

  }

}

// -----------------------------------------------------


// REGISTRY FOR FORMATTERS -----------------------------

class FormatterRegistry(

  initialFormatters: List<Formatter<out Event>> = listOf(

                FormatterForNewTicketEvent(),

                FormatterForNewCommentEvent()

  )

) {

  var formatters = initialFormatters.toMapBy { it.eventType }


  fun <T: Event> getByClass(eventType: Class<out T>): Formatter<T> {

  return formatters[eventType] as Formatter<T>? // <-------- How to avoid this casting?

                ?: throw IllegalArgumentException(“Invalid event type: ‘$eventType’”);

  }

}

// -----------------------------------------------------


// SOME TESTS ------------------------------------------

fun main(args: Array<String>) {

  val formatterRegistry = FormatterRegistry()


  var formatter: Formatter<Event> = formatterRegistry.getByClass(NewTicketEvent::class.java)

  println(formatter.format(NewTicketEvent(“a new ticket!”)))


  formatter = formatterRegistry.getByClass(NewCommentEvent::class.java)

  println(formatter.format(NewCommentEvent(“a new comment!”)))

}

// -----------------------------------------------------