NullPointerException - but why?

Question

I have a line of code throwing a NullPointerException:
nearestPrey = preyCreature[0];Now, I know that the Prey object in preyCreature[0] exists because it's a Thread and is running already by this point! I've tried using index 1 to check for a slip up when instantiating, but it's not that.
Any ideas?
Cheers,
Charles

Answer 1

The array gets initilised in this bit of code:
    Prey[] preyCreature = new Prey[maxPrey];..
    for (int i = 0; i < noOfPrey; i++){
      preyCreature[i] = new Prey(i, env, initLocation, preyChange, gordon);
    preyCreature.initPrey(noOfGenerations, f, gridWidth,
gridHeight, preyCreature, noOfPrey, maxPrey, preyGroupSize,
noOfPredTeams, predtm, maxPreySpeed, preyAcceleration,
preyDeceleration, easeOfPreyTurning, preySightRange, birthChance);
System.out.println("Starting prey...");
for (int i = 0; i < noOfPrey; i++)
preyCreature[i].start();
The line of code mentioned in my first post is in this method:
  public void findNearestPrey()
    preyChange.P(); // avoid prey being killed while searching for it
    nearestPreyDead = false;
    nearestPrey = preyCreature[0];
    preyDistance = f.distanceTo(predX, predY,
     preyCreature[0].getX(), preyCreature[0].getY());
    preySpeed = preyCreature[0].getSpeed();
    preyEnergy = preyCreature[0].getEnergy();
    angleToPrey = f.angleTo(predX, predY, preyCreature[0].getX(),
    preyCreature[0].getY(), predDir);
    for (int i = 1; i < noOfPrey; i++)
      if (f.distanceTo(predX, predY, preyCreature.getX(),
preyCreature[i].getY()) < preyDistance) {
nearestPrey = preyCreature[i];
preyDistance = f.distanceTo(predX, predY,
preyCreature[i].getX(), preyCreature[i].getY());
preySpeed = preyCreature[i].getSpeed();
preyEnergy = preyCreature[i].getEnergy();
angleToPrey = f.angleTo(predX, predY, preyCreature[i].getX(),
preyCreature[i].getY(), predDir);
preyChange.V();
This method is in a different object that is also a Thread (started immediately after the Prey Threads). Do you need anything else?
Charles