Question

For example (maybe a bit clumsy from a real life view, but just to illustrate):

“User” is a case class containing user name and id. Id can be never set manually, and a User class instance with no id set has no sense.

A UserBase class maintains users base and has a “getUser (name : String) : User” method returning a consistent User instance.

No one other than a UserBase object can know (well, someone can, but really shouldn’t rely on this knowledge) a user’s id, so constructing a User instance manually makes no sense (and can cause errors in future if someone accidentally hardcodes this and forgets). Moreover, having an orphan User instance not tracked by a UserBase is also undesired.

So the task is to make calling UserBase.getUser the only way to get a User instance.

Can this be implemented in Scala?

Answer 1

You have to put the classes in the same package or make them part of the same class or object. Then:

object O {
  class C private[O] (val x: Int) { }
  object D { def apply(i:Int) = new C(i) }
  def getC(i:Int) = new C(i)
}

scala> O.D(5)
res0: O.C = O$C@5fa6fb3e

scala> new O.C(5)
<console>:10: error: constructor C cannot be accessed in object $iw
       new O.C(5)

scala> O.getC(5)
res1: O.C = O$C@127208e4