Companion class requires import of Companion objec

问题:

I am looking at Akka related typesafe activator code and the following construct intrigued me:

Companion object:

object MarkerActor {
  sealed trait MarkerMessage
  case object Stop extends MarkerMessage
   ..
  def objectMethod = print("hi from companion object")
}

Companion class: it imports the companion object methods:

class MarkerActor extends Actor with ActorLogging {
    import MarkerActor._   // Comment this line to compare w or w/o import available

    objectMethod  // just to see if 'visible' within companion class

    override def receive = {
      case Stop => {

So.. that is a bit surprising. Why is there not a "special relationship" between the companion class/object allowing the class to "see" the object methods automatically?

Update I was a bit skeptical on this, and so went ahead and commented out the "import MarkerActor._" This resulted in "Symbol not found: Stop" errors in the Companion Class. So .. the import really is required.

回答1:

Several years ago there was a discussion on whether to implicitly import all companion object members into their parent classes. The decision made at the time, which still makes sense today, was to require an explicit import, since it is easier to add an additional import than to remove an undesired one. Here is the full discussion.