Question on "inner Objects" (as opposed to "inner Classes")

I'm defining a Document class, and two characteristics of a Document instance are that it should have exactly one Cursor, and exactly one Root (which is basically a range of the document that encompasses the entire document.) In my current coding, my handling those cases by declaring Cursor and Root as inner Objects, not classes. However, I still have a fair bit of coding to do before I can really try anything that will test if this works or not, so I thought I would post the relevant code to see if anyone can tell me if I might be shooting myself in the foot.
I've edited out most of the actual "action" code, what's left are (commented) classes/objects that should, I hope, be easy to understand.
/** A data structure containing all of the content of a document. */class Document { private val contents = new ArrayBuffer[Node]
/** A Node is a single entry in the document; a character, image, or some other "non-divisible" piece of information. */ class Node { def text = "" def length = 0 }
/** A single character in a document. */ class Char(override val text: String) extends Node { assert(text.length == 1) override def length = text.length }
/**A Marker indicates a location in the document, but does not possess any content itself. * Markers are used as a base classes for other classes that also indicate a location in * the document but add additional functionality */ abstract class Marker extends Node
/**A Position is a location in a document that can be used in various ways. Positions can be * created, inserted, moved, or used as the basis for operations such as inserting content. */ class Position extends Marker { /**Insert a string either before or after this Position. * @param str The string to be inserted. * @param before True (the default) if the string is to be inserted before the * Position, false if the string is to be inserted after the Position. The default * cause corresponds to typing and expecting the Position (cursor) to end at the end * of the typed text. */ def insert(str: String, before: Boolean = true) { } }
/** There is exactly one Cursor associated with each Document object Cursor extends Position
/**A Fragment denotes a structured piece of the document. It possesses Start and End markers * that indicate where in the Document the Fragment content starts and ends. */ class Fragment { val start = new Start val end = new End
/**A Start is a Marker that indicates a location in the document where some fragment * starts. */ class Start extends Marker /**A End is a Marker that indicates a location in the document where some fragment * end. */ class End extends Marker }
/**A Root is a special Fragment. There is exactly one Root per Document, and its Start marker is at * the start of the Document's contents, while its End marker is at the end of the Document's contents. */ object Root extends Fragment}
As you can see, the only top-level class is Document. Feel free to point out other mistakes I might have made, but my big questions right now are, can I create Cursor and Root in this manner, and then (presumably) refer to them as Cursor and Root to invoke methods on them?
Thanks,Ken