Thread: Cannot access class attributes

Hi everyone,

This problem is starting to drive me insane. I'm sure it must be something really simple, but I just can't seem to find out where the problem is. I initialize a population from the main() function, containing a collection of individuals with:

Population assemblage = new Population(1);

I then try to access the attributes of one of those Individuals to test if the program works with:

// DEBUG!!!!
System.out.println("positionX: " + assemblage.robots[0].getPositionX());
// DEBUG!!!!

But it does not work, the compiler throws a null pointer exception. Any help anyone can provide will be gratefully accepted.


Cheers,
Paul

public class Main {
 
  public static void main(String[] args) {
 
  Population assemblage = new Population(1);
 
    // DEBUG!!!!
    System.out.println("positionX: " + assemblage.robots[0].getPositionX());
    // DEBUG!!!!
 
  }
 
}

public class Population {
 
  // attributes
  Individual[] robots;
 
  // constructor
  Population(int populationSize) {
    // create a population of random individuals
    this.robots = new Individual[populationSize];
  }
 
}

public class Individual {
 
  // attributes
  int[] chromosome; // robots chromosome bit string array
  float fitness; // a measure of the fitness of this individual
  int positionX; // x coordinate of the robot within the Donutland
  int positionY; // y coordinate of the robot within the Donutland
  int velocityX; // value of the robots velocity for the x axis
  int velocityY; // value of the robots velocity for the y axis
 
 
  // constructors
  Individual() {
    // the chromosome bit string is generated randomly
    this.chromosome = generateRandomChromosome(10); // USING LENGTH 10 FOR NOW!!
 
    // now fire function which decodes chromosome bit string into attributes
    decodeChromosomeString();
  }
 
  Individual(int[] chromosome) {
    // the chromosome bit string is passed in parameters
    this.chromosome = chromosome;
 
    // now fire function which decodes chromosome bit string into attributes
    decodeChromosomeString();
  }
 
 
  // private functions
  private void decodeChromosomeString() {
    // fill attributes with decoded values from the chromosome bit string
    // WRITE CODE TO GO HERE!!!!!
 
    // DEBUG!!!!
    this.positionX = 20;
    this.positionY = 20;
    this.velocityX = 1;
    this.velocityY = 1;
    // DEBUG!!!!
 
  }
 
  private void encodeChromosomeString() {
    // write a chromosome bit string from instance attributes
    // WRITE CODE TO GO HERE!!!!!
 
  }
 
  private int[] generateRandomChromosome(int length) {
    // make a temporary array to work with of the required length
    int[] tempChromosome;
    tempChromosome = new int[length];
 
    // setup a for loop in order to populate the bit string
    for(int i=0; i==length; i++) {
      tempChromosome[i] = (int)(Math.random()*2);
    }
 
    // now return the chromosome to the calling code
    return tempChromosome;
  }
 
 
  // public functions
  public int compareTo(Individual robot) {
    if (this.fitness == ((Individual) robot).fitness)
      return 0;
    else if ((this.fitness) > ((Individual) robot).fitness)
      return 1;
    else
      return -1;
  }
 
  public int compare(Individual robotA, Individual robotB) {
    if (((Individual) robotA).fitness == ((Individual) robotB).fitness)
      return 0;
    else if (((Individual) robotA).fitness > ((Individual) robotB).fitness)
      return 1;
    else
      return -1;
  }
 
  public int getPositionX() { return this.positionX; }
  public int getPositionY() { return this.positionY; }
  public int getVelocityX() { return this.velocityX; }
  public int getVelocityY() { return this.velocityY; }
  public int[] getChromosome() { return this.chromosome; }
 
  public void setPositionX(int positionX) { this.positionX = positionX; }
  public void setPositionY(int positionY) { this.positionY = positionY; }
  public void setVelocityX(int velocityX) { this.velocityX = velocityX; }
  public void setVelocityY(int velocityY) { this.velocityY = velocityY; }
  public void setChromosome(int[] chromosome) { this.chromosome = chromosome; }
 
}