Training - kotlin: enum, sealed classes
Published on 17 May 2022
sealed classes are not enumerations(keyword:enum).
Coming from Java, you might be tempted to overload your enumeration to provide features: // Java code
enum PizzaOrderStatus {
ORDER_RECEIVED,
PIZZA_BEING_MADE,
OUT_FOR_DELIVERY,
COMPLETED;
public PizzaOrderStatus nextStatus() {
switch (this) {
case ORDER_RECEIVED: return PIZZA_BEING_MADE;
case PIZZA_BEING_MADE: return OUT_FOR_DELIVERY;
case OUT_FOR_DELIVERY: return COMPLETED;
case COMPLETED:return COMPLETED;
}
}
}Instead, you can use a sealed class:
sealed class PizzaOrderStatus(protected val orderId: Int) {
abstract fun nextStatus() : PizzaOrderStatus
class OrderReceived(orderId: Int) : PizzaOrderStatus(orderId) {
override fun nextStatus(): PizzaOrderStatus {
return PizzaBeingMade(orderId)
}
}
class PizzaBeingMade(orderId: Int) : PizzaOrderStatus(orderId) {
override fun nextStatus(): PizzaOrderStatus {
return OutForDelivery(orderId)
}
}
class OutForDelivery(orderId: Int) : PizzaOrderStatus(orderId) {
override fun nextStatus(): PizzaOrderStatus {
return Completed(orderId)
}
}
class Completed(orderId: Int) : PizzaOrderStatus(orderId) {
override fun nextStatus(): PizzaOrderStatus {
return this
}
}
}The advantage of this approach is that we can now pass data with the status:
var status: PizzaOrderStatus = OrderReceived(123)
while (status !is Completed) {
status = when (status) {
is OrderReceived -> status.nextStatus()
is PizzaBeingMade -> status.nextStatus()
is OutForDelivery -> status.nextStatus()
is Completed -> status
}
}In general, sealed classes are good if you want to have data associated with a State.