Writing a battlefield simulation; except I'm the one fighting here ;-)

Question

Hi, all:
I'm having a problem getting my Soldier class to find instances of each other (NPEs) and change aspects of each other. This is an agent-based model in which I generate multiple Soldier objects. The soldiers win or lose based on their strength as compared to the other soldier they are fighting in any given step (round of the battle). Their strength is increased if they have other soldiers of their own team (meaning their own color, green, blue, whatever) as their neighbor. If they see a neighbor of the opposing color team in their Moore neighborhood (like a tic-tac-toe board where the soldier is in the middle square and "sees" the eight squares around him), they fight him, and kill him if their strength is greater, and die if their strength is lesser.
Here's the problem: have I screwed up the communication between soldiers? Why is there an NPE when I'm asking about the other soldier's color? The NPE error is, obviously, because one is pointing at a null object. However, I included that debug message about the other soldier's color and ID to see if it was finding a color and ID, and it does (see the stack trace below), so where am I going wrong?
public Soldier () {
          ID = nextID++;
          x = 0;
          y = 0;
          myColor = Color.magenta;
          myBorder = Color.black;
          strength = Normal.staticNextDouble(50.00, 23.75);
          if (strength >= 100 ){
               strength = 100;
          if (strength <= 0 ) {
               strength = 0;
     public void step () {
          getMyNeighbor();
          move();
     public void getMyNeighbor () {
          neighborList = new Vector();
          soldierNeighbors = new Vector();
          neighborList = world.getMooreNeighbors(x, y, false);
          int random = Random.uniform.nextIntFromTo(0, 99);
          for (int i = 0; i < neighborList.size(); i++){
               Object o = neighborList.get (i);
               if (o instanceof Soldier) {
                    Soldier otherSoldier = (Soldier)o;
                    otherSoldier.getID();
                    otherSoldier.getMyColor();
                    System.out.println ("The other soldier's color is " +
                              otherSoldier.getMyColor() + " and his ID is " + otherSoldier.getID() );
                    otherSoldier.getStrength();
               if (otherSoldier.getMyColor() == this.getMyColor()){//NPE here
                    strength = strength + 1;
               if (otherSoldier.getMyColor()!= this.getMyColor()){
                    fight();
     public void fight () {
          if (strength > otherSoldierStrength){
               otherSoldier.setMyColor(Color.red);//another problem here
          if (strength < otherSoldierStrength){
                        Model.soldierList.remove(this);
     public void draw (SimGraphics g) {
          g.drawFastRoundRect(myColor);
          g.drawRectBorder(new BasicStroke(2), myBorder);
     public double getStrength() { return strength; }
     public void setStrength( double i ) { strength = i; }
     public Color getMyColor() { return myColor; }
     public void setMyColor( Color i ) { myColor = i; }
     public Color getMyBorder() { return myBorder; }
     public void setMyBorder( Color i ) { myBorder = i; }
     public static void setModel( Model m ) { model = m; }
     public static void setWorld( Object2DGrid world ) {     Soldier.world = world;}Here's the stack trace:
Model.main() beginning...
Model.setup() beginning...
Model.setup() finished.
Model.main() finished...
Model.begin() beginning...
Model.buildModel() beginning.
creating people now
Model.buildModel() finished.
Model.buildDisplay() beginning.
Model.buildDisplay() finished.
Model.buildSchedule() beginning
Model.buildSchedule() finished
Model.begin() finished...
Exception in thread "Thread-4" java.lang.NullPointerException
     at tyranny.Soldier.getMyNeighbor(Soldier.java:71)
     at tyranny.Soldier.step(Soldier.java:52)
     at tyranny.Model.step(Model.java:145)
     at BAB_SYNTH_1.execute(Unknown Source)
     at uchicago.src.sim.engine.ScheduleGroup.execute(Unknown Source)
     at uchicago.src.sim.engine.RandomScheduleGroup.execute(Unknown Source)
     at uchicago.src.sim.engine.Schedule.execute(Unknown Source)
     at uchicago.src.sim.engine.BaseController$1.run(Unknown Source)
     at java.lang.Thread.run(Unknown Source)
==> Model step 1.0
The other soldier's color is java.awt.Color[r=0,g=255,b=0] and his ID is 924

Answer 1

CeciNEstPasUnProgrammeur wrote:
if (o instanceof Soldier) {
Soldier otherSoldier = (Soldier)o;
if (otherSoldier.getMyColor() == this.getMyColor()){//NPE hereThis really surprises you? Where do you set "otherSoldier"? You're assigning something to some local variable earlier, but that's out of scope, and that variable just coincidentally has the same name as your attribute.
Just use
this.otherSoldier = (Soldier)o;
Set otherSoldier? I'm not setting it anywhere, I'm retrieving otherSoldier from a list of previously generated instances of Soldier from within a given instance of Soldier. This soldier is fighting the otherSoldier, so I'm confused as to how telling this soldier that it is also the otherSoldier by saying this.otherSoldier (if I'm understanding you correctly), isn't conflating two different objects. The otherSoldier is not this soldier. It's distinctly possible that I'm misunderstanding you, but I dont' want THIS instance of Soldier to think that it is ALSO otherSoldier, which I believe it would do if I told it this.otherSoldier. I'm sure I'm wrong, please tell me how.