Modeling complex tree structure

Question

After asking it in stackoverflow : polymorphism-and-tree-structre I came to a conclusion that I am not being specific and my question is not about any specific technology so I will try to rewrite it. in the stackoverflow question I wrote about a grandparent/parent/child classes in order for my question to be concise, this time I will be specific:
My real domain is of army simulator/game
the domain models are : Corps -> Division -> Brigade which form sort of a tree structure a Brigade parent must be a Division, a Division parent must be a Corps
a unit without a parent (except corps is not allowed)

furthermore the game is divided into players from three roles : corps general, divisional general and brigade general, each player need to select his managed unit according to his role - corps general selects a corps etc..

The ui for managing units looks like a tree in which you add, delete and edit the units:

• coprs
  • division
    • birgade

In my previous question the answer I got was to model it as

asbtact class Unit {  
   List<Unit> children;
   Unit parent;
}

which means I don't treat each unit type any differently However, it doesn't fit quite well to my game, since it seems awkward to determine the unit type by state(doesn't have parent == corps for example), it allows for situations where a brigades parent is a corps which is not possible. and I don't want a corps general to be able to select a brigade to manage just because somehow it wasn't assigned a parent and thus the game thinks its a corps. to be more presice, I don't want to determine the units type by their location in the three, but rather the opposite, determine their location by their type.

---

So my question is how do you think is the best way to model it in classes?
I thought of a class of each type

interface Unit {
  Unit getParent();
  List<Unit>  getChildren()

}

   class Brigade implements Unit
   class Divison implements Unit
   class Corps implements Unit

Answer 1

Based on your description, the simplest model (in terms of pure structure) would be something like:

class Corps {
    final List<Division> children;
}

class Division {
    final Corps parent;
    final List<Brigade> children;
}

class Brigade {
    final Division parent;
}