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Thread: Cannot access class attributes

  1. #1
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    Default 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; }
     
    }
    Last edited by Cyburg; August 29th, 2010 at 06:22 AM.


  2. #2
    Junior Member
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    Default Re: Cannot access class attributes

    I had stupidly neglected to create array members, that is why the function return null pointer error.

    Cheers,
    Paul

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