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.