Question

EDIT: I really appreciate everyone’s input. I gained something from all the responses and learned a good deal about OOD.

I am making a simple virtual tabletop war game. To represent units on the battlefield I have the following simple class hierarchy: An abstract class Unit, and two derived classes, Troop and Vehicle.

I have another class that has a hashtable for all the units in the game. The hashtable values are of Unit type, so I can reference them in O(1) time.

For the most part, this is fine, but sometimes the caller NEEDS to know if something is a troop or a vehicle to call specific methods from those derived classes. To accommodate for this, I’ve created two get methods that will enforce the types:

  public Troop getTroop(String uniqueID) {
    Unit potentialTroop = get(uniqueID);
    if(potentialTroop instanceof Vehicle) {
      throw new InternalError();
    }
    return (Troop) potentialTroop;
  }

  public Vehicle getVehicle(String uniqueID) {
    Unit potentialVehicle = get(uniqueID);
    if(potentialVehicle instanceof Troop) {
      throw new InternalError();
    }
    return (Vehicle) potentialVehicle;
  }

(Note the class for which this belongs merely extends Hashtable, so the get method being used here is the Java’s hashtable’s get method.)

So I think this is poor OOD design because if I ever further extend unit I’m going to have to add more checks and more #get methods to this hashtable.

Am I correct in saying this? Does anyone have alternative OOD suggestions if this is the case?

Answer 1

Here’s a simple way to do this, not necessarily the best, but it meets these requirements:

1. Be able to dynamically obtain a specialized type from your Unit collection
2. Be able to add additional Unit types later on, without having to add a bunch of handler methods.

The solution uses a ‘template’ class to perform matching:

@SuppressWarnings("unchecked")
public <T extends Unit> T getSpecializedUnitType(Class<T> unitTypeClass, String uniqueID) {
    Unit potentialTroop = units.get(uniqueID);
    if(potentialTroop == null) return null;

    return potentialTroop.getClass().equals(unitTypeClass) ?
        (T) potentialTroop : null;
}

I made the assumption that you are going to correct your code, to not extend from Map, but rather to encapsulate it.