NullPointerException

[Mate de Vita]

This is what I have now:

Main:

public class Main {    
    public static void main (String[] args) {
        Unit unit = UnitList.getInstance().legionnaire;
    }
}

Unit:

public class Unit {
    String title;
    Tribe tribe;
}

UnitList:

public final class UnitList {
    private static final UnitList instance = new UnitList();

    public final Unit legionnaire;
    public final Unit phalanx;
    public final Unit[][] units;
    public final static int ROMAN = 0, GAUL = 1;

    private UnitList() {
        legionnaire = new Legionnaire();
        phalanx = new Phalanx();
        units = new Unit[2][1]
        units[ROMAN][0] = legionnaire;
        units[GAUL][1] = phalanx;
    }

    public static getInstance() {
        return instance;
    }
}

Legionnaire:

public final class Legionnaire extends Unit {
        public Legionnaire() {
        title = "Legionnaire";
        tribe = TribeList.getInstance().roman;
    }
}

Phalanx:

public final class Phalanx extends Unit {
    public Phalanx() {
        title = "Phalanx";
        tribe = TribeList.getInstance().gaul;
    }
}

Tribe:

public class Tribe {
    Unit[] tribeUnits = new Unit[1];
}

Roman:

public class Roman extends Tribe {
    public Roman() {
        for (int i = 0; i < tribeUnits.length; i++) tribeUnits[i] = UnitList.getInstance().units[UnitList.ROMAN][i];
    }
}

Gaul:

public class Gaul extends Tribe {
    public Gaul() {
        title = "Gaul";
        for (int i = 0; i < tribeUnits.length; i++) tribeUnits[i] = UnitList.getInstance().units[UnitList.GAUL][i];
    }
}

TribeList:

public class TribeList {
     private static final TribeList instance = new TribeList();

    public final Tribe roman, gaul;

    private UnitList() {
        roman = new Roman();
        gaul = new Gaul();
    }

    public static getInstance() {
        return instance;
    }
}

Building:

public class Building {
    Unit[] trainedUnits = new Unit[1];
    Tribe tribe;
    Building requiredBuilding;
}

BuildingList:

public class BuildingList {
     private static final BuildingList instance = new BuildingList();

    public final Building academy, mainBuilding;

    private BuildingList() {
        academy = new Academy();
        mainBuilding = new MainBuilding();
    }

    public static getInstance() {
        return instance;
    }
}

Academy:

public class Academy {
    public Academy() {
        trainedUnits[0] = UnitList.getInstance().legionnaire;
        tribe = TribeList.getInstance().roman;
        requiredBuilding = BuildingList.getInstance().mainBuilding;
    }
}

MainBuilding:

public class MainBuilding {
    public MainBuilding() {
        trainedUnits[0] = UnitList.getInstance().phalanx;
        tribe = TribeList.getInstance().gaul;
    }
}

And I'm getting the same NPE that I was getting with static List classes:

Exception in thread "main" java.lang.ExceptionInInitializerError	at troopInfo.Legionnaire.<init>(Legionnaire.java:6)
	at troopInfo.UnitList.<init>(UnitList.java:14)
	at troopInfo.UnitList.<clinit>(UnitList.java:4)
	at troopInfo.UnitSelection.refreshChoices(UnitSelection.java:19)
	at troopInfo.UnitSelection.<init>(UnitSelection.java:11)
	at troopInfo.TroopPanel.<clinit>(TroopPanel.java:8)
	at main.MyTabbedPane.<init>(MyTabbedPane.java:8)
	at main.MainWindow.<init>(MainWindow.java:18)
	at main.MainWindow.main(MainWindow.java:25)
Caused by: java.lang.NullPointerException
	at tribeInfo.Roman.<init>(Roman.java:7)
	at tribeInfo.TribeList.<init>(TribeList.java:9)
	at tribeInfo.TribeList.<clinit>(TribeList.java:4)
	... 9 more

[Tolls]

THere's no need to have the ROMAN units stored in the Roman instance.
Indeed, any of the tribe units.
You can get them anytime by doing:

UnitList.getInstance().units[UnitList.ROMAN];

And if you aren't storing them, then you won't hit the problem of trying to use them in the constructor before they are populated.

Essentially, you need to plan the model such that Something is the owner of a Thing.
Other classes will then request this data from the owner of the Thing as they need it, rather than storing their own copies of the Thing.

[Mate de Vita]

OK, I removed that now, but now I'm getting a NPE in the Academy class:

Exception in thread "main" java.lang.ExceptionInInitializerError
    at tribeInfo.Roman.<init>(Roman.java:7)
    at tribeInfo.TribeList.<init>(TribeList.java:9)
    at tribeInfo.TribeList.<clinit>(TribeList.java:4)
    at troopInfo.Legionnaire.<init>(Legionnaire.java:6)
    at troopInfo.UnitList.<init>(UnitList.java:14)
    at troopInfo.UnitList.<clinit>(UnitList.java:4)
    at troopInfo.UnitSelection.refreshChoices(UnitSelection.java:19)
    at troopInfo.UnitSelection.<init>(UnitSelection.java:11)
    at troopInfo.TroopPanel.<clinit>(TroopPanel.java:8)
    at main.MyTabbedPane.<init>(MyTabbedPane.java:8)
    at main.MainWindow.<init>(MainWindow.java:18)
    at main.MainWindow.main(MainWindow.java:25)
Caused by: java.lang.NullPointerException
    at buildingInfo.Academy.<init>(Academy.java:6)
    at buildingInfo.BuildingList.<init>(BuildingList.java:9)
    at buildingInfo.BuildingList.<clinit>(BuildingList.java:4)
    ... 12 more

BuildingList's constructor calls Academy and Academy calls BuildingList.getInstance().mainBuilding, which I guess hasn't been initiated yet. But the problem persists even if I move the mainBuilding line above the academy line in the constructor (in which case it should be initiated before academy, no?):

public class BuildingList {
    private static final BuildingList instance = new BuildingList();

    public final Building academy, mainBuilding;

    private BuildingList() {
        mainBuilding = new MainBuilding();
        academy = new Academy();
    }

    public static getInstance() {
        return instance;
    }
}

[Tolls]

Why does Academy need access to the MainBuilding during contruction?

[Mate de Vita]

Well, mainBuilding is the requirement for academy, so the requiredBuilding field is set to mainBuilding in Academy's constructor.
mainBuilding is in turn initiated as new MainBuilding() .

[Tolls]

I think you might need to sit down and try and detach your dependencies somehow.
You may need a two-pass construction in this case.
First pass to actually make the objects, second to assign the requirements.

I've had to do similar stuff before when mucking about with these sorts of strategy game ideas.

[Mate de Vita]

Is it possible create a reference to an object without actually constructing that object? Cause that would solve my problems, I believe.

[Tolls]

Nope.
I would suggest:

public class Something {
    public Something() {
        // Basic constructor, possibly with some data set up here
    }

    public void loadData() {
        // Do the rest in here, that depends on other classes.
    }
}

public class GameLoader {
    public GameLoader() {
        // NOthing in here
    }

    pubic void loadGame() {
        // Create everything in here then call the load methods
        Something s = new Something();
        ... etc...
        s.loadData();
    }
}

I've called it load, because I tend to load stuff from file, but the concept is the same.
The advantage of having a GameLoader is you can then control the order of things, without relying purely on constructors.

[Mate de Vita]

Interesting.. I'll try something like this when I get home.

[Mate de Vita]

Wait, so in the GameLoader, the code can't be in the constructor? I was going to simply write the code that you put in loadGame into UnitList's constructor.

Something like this:

public final class UnitList {
    private static final UnitList instance = new UnitList();

    public final Unit legionnaire;
    public final Unit phalanx;
    public final Unit[][] units;
    public final static int ROMAN = 0, GAUL = 1;

    private UnitList() {
        legionnaire = new Legionnaire();
        phalanx = new Phalanx();

        units = new Unit[2][1];
        units[ROMAN][0] = legionnaire;
        units[GAUL][0] = phalanx;
          
        for (int i = 0; i < units.length; i++)
            for (int j = 0; j < units[i].length; j++) units[i][j].loadData();
    }

    public static getInstance() {
        return instance;
    }
}

Entity:

public abstract class Entity {
    protected String title;
    
    public String toString() { return title; }
    public void loadData();
}

Unit:

public abstract class Unit extends Entity {
    protected Tribe tribe;
}

Legionnaire:

public final class Legionnaire extends Unit {
   public Legionnaire() {
       title = "Legionnaire";
   }

    public void loadData() {
        tribe = TribeList.getInstance().roman;
    }
}

Phalanx:

public final class Phalanx extends Unit {
   public Phalanx() {
       title = "Phalanx";
   }

    public void loadData() {
        tribe = TribeList.getInstance().gaul;
    }
}

And something similar for buildings and tribes.

[Tolls]

So now the UnitList creates a skeleton set of Units, which are then populated, except you might still clash since the TribeList class also needs to be constructed.
It looks like you should be OK, though, since none of the Units use TribeList until the loadData is called, and by that point UnitList has a complete set of (incomplete, but existing) Units.

[Mate de Vita]

All right, I managed to successfully run the program now without any NPEs or similar problems. I have a few problems with my Swing GUI, but I'll open a new thread for those, since it's a completely different subject.
Thanks for all the help, really appreciate it.

[Tolls]

No problem.