Implement addition in abstract Scala class

Question

I have the below abstract class and its two subclasses:

abstract class BoundedNumber(val lowerBound: Double,
                             val upperBound: Double,
                             val value: Double) {
  require(value >= lowerBound && value <= upperBound)
}

final case class Percentage(override val value: Double)
  extends BoundedNumber(0, 100, value)

final case class Probability(override val value: Double)
  extends BoundedNumber(0, 1, value)

Can I somehow implement a "generic" addition in BoundedNumber even though it's abstract and cannot be instantiated?

abstract class BoundedNumber(val lowerBound: Double,
                             val upperBound: Double,
                             val value: Double) {
  def +(that: BoundedNumber): BoundedNumber = {
    require(this.getClass == that.getClass)
    // This of course won't compile:
    new BoundedNumber(lowerBound, upperBound, value + that.value)
  }
}

Or am I bound to (pun intended) implement addition in both subclasses thus duplicating code?

Answer 1

You cannot instantiate abstract class, However you can specify an abstract method in BoundedNumber that create new instance with required updated value.

  abstract class BoundedNumber(val lowerBound: Double,
                               val upperBound: Double,
                               val value: Double) {
    require(value >= lowerBound && value <= upperBound)

    def copy(value: Double): BoundedNumber

    def +(that: BoundedNumber): BoundedNumber = {
      require(this.getClass == that.getClass)
      that.copy(value + that.value)
    }
  }

  final case class Percentage(override val value: Double) extends BoundedNumber(0, 100, value) {
    override def copy(value: Double): BoundedNumber =  Percentage(value)
  }

  final case class Probability(override val value: Double) extends BoundedNumber(0, 1, value){
    override def copy(value: Double): BoundedNumber = Probability(value)
  }