Why can't I access private class methods in the class's companion object in Scala?

Question

I'm working on a homework assignment for my object oriented design class, and I'm running into trouble with Scala's companion objects. I've read in a few places that companion objects are supposed to have access to their companion class's private methods, but I can't seem to get it to work. (Just as a note, the meat of the assignment had to do with implementing a binary search tree, so I'm not just asking for answers...)

I have an object that is supposed to create an instance of my private class, BstAtlas (Bst is also defined in the Atlas object, took it out for clarity):

object Atlas {                                             
  def focusRoom(newRoom:Room,a:Atlas):Atlas = a.helpFocusRoom(newRoom);

  abstract class Atlas {
    ...
    protected def helpFocusRoom(n:Room):Atlas;
    ...
  }

  private class BstAtlas(bst:Bst) extends Atlas {
    ...
    protected def helpFocusRoom(newRoom:Room):Atlas = ...
       // uses some of bst's methods
    ...
  }
}

But when I go to compile, I get the following error:

Question23.scala:15: error: method helpFocusRoom cannot be accessed in Atlas.Atlas a.helpFocusRoom(newRoom);

The function helpFocusRoom needs to be hidden, but I don't know how to hide it and still have access to it inside of the companion object.

Can anyone tell me what I'm doing wrong here?

Answer 1

The problem is that classes and companion objects can't be nested like that. To define a companion object, you need to define the class outside of the object's body but in the same file.

Answer 2

Companion objects should be next to their real object, not containing it:

object Example {
  class C(val i: Int = C.DefaultI) { }
  object C { protected val DefaultI = 5 }
}

scala> (new Example.C).i
res0: Int = 5

scala> Example.C.DefaultI
<console>:11: error: value DefaultI cannot be accessed in object Example.C
   Example.C.DefaultI

Alternatively, you can alter the scope of the protected keyword to include the enclosing object:

object Example {
  def value = (new D).hidden
  class D(val i: Int = 5) {
    protected[Example] def hidden = i*i
  }
}

scala> Example.value
res1: Int = 25

but here you ought not name the outer object the same thing as the inner class or you'll have trouble referring to it from within the class.