A constructor with a parameter in Scala

Question

I need only one instance of a class, so I have to use an object instead of a class. I also need to set some initial value chosen by a client, so I need to a constructor for an object, something like this:

object Object1(val initValue: Int){
  //.....
}

I can't use this exact code in Scala. How do I deal with that then?

Answer 1

You have a couple of choices:

1. Make it a class, have the client construct it, give the value in the parameter
   Pro: Preserves immutability
   Con: Having only a single instance might be hard to manage

2. Add a variable for the param to the object, add a setter.
   Pro: You still have a singleton
   Con: There is mutable state now

3. Implement a multiton
   Pro: Gives you (apparent) immutability and singleton (per param)
   Con: More code to implement

You could implement a multiton like this in scala:

class Object1 private (val initValue: Int) {
  // ...
}

object Object1 {
  val insts = mutable.Map.empty[Int, Object1]

  def apply(initV: Int) =
    insts.getOrElseUpdate(initV, new Object1(initV))
}

UPDATE You could also turn this into a "singleton with parameter":

object Object1 {
  var inst: Option[(Int, Object1)] = None

  def apply(initV: Int) = inst match {
    case Some((`initV`, i)) => i
    case Some(_) =>
      sys.error("Object1 already instantiated with different param")
    case None =>
      val i = new Object1(initV)
      inst = Some((initV, i))
      i
  }
}

Answer 2

The object isn't created until you reference it, so you could do something like the following:

object Test1 extends App {
  var x = Console.readLine
  println(Object1.initVal)
}

object Object1 {
  val initVal:String = Test1.x
}