How do I add different inner classes to an ArrayList?

Question

So, I'm building a very basic program based on the Warhammer 40k tabletop game as an extra credit assignment for my Comp Sci class. I get to develop the specs and then I implement the specs. In this game, I have a class Army that has an ArrayList of class Units:

List<Units> unitsList = new ArrayList<Units>();

I've created the class Units with innerclasses that contain the different unit types. So, a code snippet from that would be:

public class Units 
{
   int strength, toughness, attacks, save, wounds;
   class TroopUnit
   {
      public static final int strength = 3;
      public static final int toughness = 3;
      public static final int attacks = 1;
      public static final int save = 5;

      public int wounds = 1;
   }

   class EliteTroopUnit
   {
      public static final int strength = 3;
      public static final int toughness = 3;
      public static final int attacks = 1;
      public static final int save = 4;

      public int wounds = 1;
   }

}

There are more unit types, but I think you can get the idea. I've tried adding these different innerclasses of Units inside the Units ArrayList:

public Army(int troopPoints, int eliteTroopPoints, int commandPoints,
     int commandTroopPoints, int fastAttackPoints, int heavyAttackPoints)
{      
   for (int i = 0; i < (troopPoints/TROOP_COST); i++)
      if (troopPoints % TROOP_COST == 0) 
      {
         if (points < MAX_POINTS) 
         {
            unitsList.add(new Units.TroopUnit());
            points += TROOP_COST;
            numUnits++;
         }
      }
   for (int i = 0; i < (eliteTroopPoints/ELITE_TROOP_COST); i++)
      if (eliteTroopPoints % ELITE_TROOP_COST == 0) 
      {
         if (points < MAX_POINTS) 
         {
            unitsList.add(new Units.EliteTroopUnit());
            points += ELITE_TROOP_COST;
            numUnits++;
         }
      }
}

However, I run into an issue, because I have to declare and instantiate the ArrayList with a specific type of innerclass in order to add an inner class to it. For example, List<Units.someUnitInnerClass> unitsList = new ArrayList<Units.someUnitInnerClass();

So, is there a workaround for this problem where I can add any of the innerclasses to the ArrayList? Am I going about this problem incorrectly? If so, what are other alternatives? Thank you!

EDIT: I originally had listed this as "How do I add different subclasses to an ArrayList, but that was a mistake (corrected by Rudi Kershaw).

Answer 1

In the example code you gave, TroopUnitand EliteTroopUnit are not subtypes of Unit. They are inner classes.

If you want them to be subclasses of Unit, they need to extend Unit. For example;

public class Unit {}
public class TroopUnit extends Unit {}

In the example above TroopUnit extends (and is therefore a subclass of) Unit. From a quick glance, it looks as though you could just add extends Unit to the end of your inner class declarations and it should do as you intend.

I hope this helps.

References & Further Reading:

• Oracle Java Inheritance Tutorial
• Oracle Nested Classes Tutorial

Answer 2

First of all, a subclass should extends a class. like this:

public class TroopUnit extends Units{}

Second of all, answer to your title question, which is irrelevant to your code problem, is that you should get a class above in hierarchy than your object which you are adding to list, and because in java Object is parent of all Classes then you can instantiate your List like this:

List<Object> unitsList = new ArrayList<Object>();