How to use Scala Enumerations in Classes - scala

Question

I'm learning Scala and am trying to get a simple enum setup for a project. I've checked a few examples and none seem to fit, all of the examples in the Scala documentation and on StackOverflow are for enums inside objects, not classes. I'm getting an IDE warning that I don't understand. I'm coming from a beginner Java background into Scala, which may be the cause of my confusion.
Here's the code:
class Car(maxSpeed: Integer) {
// Enums
object CarType extends Enumeration {
type CarType = Value
val PERIPHERAL, COMPUTER, EMPTY = Value
}
import CarType._
// Fields
val repeated: CarType
}
When I mouse over the class name, I can see this Intellij warning:
Class 'Car' must either be declared abstract or implement abstract member 'typed: Car.this.CarType.CarType' in 'Car'
I'm not sure why it wants me to implement my variable, and the class is not intended to be abstract. I'd like to use Enums similarly to how they are used in Java.

Answer 1

Move the enumeration outside the class:
// Enums
object CarType extends Enumeration {
type CarType = Value
val PERIPHERAL, COMPUTER, EMPTY = Value
}
class Car(maxSpeed: Integer) {
import CarType._
// Fields
val repeated: CarType
}
Or move it to a companion object:
class Car(maxSpeed: Integer) {
import Car.CarType._
// Fields
val repeated: CarType
}
object Car {
object CarType extends Enumeration {
type CarType = Value
val PERIPHERAL, COMPUTER, EMPTY = Value
}
}
The problem is that things defined inside of a class are scoped to the the instance of that class (unlike some other languages).
That said, I recommend using algebraic data types instead of enums:
sealed trait CarType
object CarType {
case object Peripheral extends CarType // strange choice of names
case object Computer extends CarType
case object Empty extends CarType
}
case class Car(maxSpeed: Int, carType: CarType)
For more info about sealed traits see this SO q&a

Answer 2

What you're looking for is a Scala Case Class.
class Car(maxSpeed: Integer)
case class Minivan(maxSpeed: Integer) extends Car(maxSpeed: Integer)
case class Hodrod(maxSpeed: Integer) extends Car(maxSpeed: Integer)
case class Coupe(maxSpeed: Integer) extends Car(maxSpeed: Integer)
Enumerations as they exist in Java aren't really used in Scala. By having a construction like what I have above, you can leverage Scala's powerful pattern matching do something like this:
val unknownCar = getCar() // Some function that gets a car
unknownCar match {
case Minivan => println("Driving the kids")
case Hodrod => println("Tearing up the drag")
case Coupe => println("Riding low")
}
...while still allowing you to treat it as a Car.
Because they are case classes Scala has a lot of things that help you out.
Note the documentation for Enumeration:
Typically these values enumerate all possible forms something can take and provide a lightweight alternative to case classes.
You should really only use it if you aren't planning to use those values for anything else - but even there case classes will generally serve you better.