Java Null Pointer Exception - with arraycopy method.

Question

Hi all,
I have been trying to fix this for a while now, but I think I've been staring at the screen for too long to fix it!
Basically I am trying to 'crossover' some arrays (evolutionary technique) but I keep getting a null pointer exception.
Here is my customised class: class ChromosomeDetails {
     double[] chromosomePrices;
     int[] chromosomePoints;
     int[] chromosomePlayerIDs;
     int chromosomeFitness;
       ChromosomeDetails(double[] chromosomePrices, int[] chromosomePoints, int[] chromosomePlayerIDs, int chromosomeFitness) {
          this.chromosomePrices = chromosomePrices;
          this.chromosomePoints = chromosomePoints;
          this.chromosomePlayerIDs = chromosomePlayerIDs;
          this.chromosomeFitness = chromosomeFitness;     
       public ChromosomeDetails () {  
            chromosomePrices = null;
          chromosomePoints = null;
          chromosomePlayerIDs = null;
          chromosomeFitness = 0;
     public double[] getPrices() {
          return chromosomePrices;
     public int[] getPoints() {
          return chromosomePoints;
     public int[] getPlayerIDs() {
          return chromosomePlayerIDs;
     public int getFitness() {
          return chromosomeFitness;
}  In my main method I create 2 instances of this class like so:ChromosomeDetails parentChromosomeADetails = new ChromosomeDetails();          
ChromosomeDetails parentChromosomeBDetails = new ChromosomeDetails();...and 2 other instances that are left empty:ChromosomeDetails childChromosomeCDetails = new ChromosomeDetails();     
ChromosomeDetails childChromosomeDDetails = new ChromosomeDetails();     I then try to 'crossover' the values in the arrays like so: int randomCrossoverPoint = new Random().nextInt(parentChromosomeADetails.chromosomePoints.length-1);
System.arraycopy(parentChromosomeADetails.chromosomePrices, 0, childChromosomeCDetails.chromosomePrices, 0, randomCrossoverPoint);
System.arraycopy(parentChromosomeBDetails.chromosomePrices, randomCrossoverPoint, childChromosomeCDetails.chromosomePrices, randomCrossoverPoint, (childChromosomeCDetails.chromosomePrices.length-randomCrossoverPoint));
System.arraycopy(parentChromosomeADetails.chromosomePoints, 0, childChromosomeCDetails.chromosomePoints, 0, randomCrossoverPoint);
System.arraycopy(parentChromosomeBDetails.chromosomePoints, randomCrossoverPoint, childChromosomeCDetails.chromosomePoints, randomCrossoverPoint, (childChromosomeCDetails.chromosomePoints.length-randomCrossoverPoint));
System.arraycopy(parentChromosomeADetails.chromosomePlayerIDs, 0, childChromosomeCDetails.chromosomePlayerIDs, 0, randomCrossoverPoint);
System.arraycopy(parentChromosomeBDetails.chromosomePlayerIDs, randomCrossoverPoint, childChromosomeCDetails.chromosomePlayerIDs, randomCrossoverPoint, (childChromosomeCDetails.chromosomePlayerIDs.length-randomCrossoverPoint));
                    However it gives me this error, relating to the first arraycopy method: Exception in thread "main" java.lang.NullPointerException
     at java.lang.System.arraycopy(Native Method)Please note that in this program, parentChromosomeADetails and parentChromosomeBDetails already hold data when I try to use the arraycopy method, whereas the childChromosomeCDetails and childChromosomeDDetails objects are empty, waiting for the data from the arraycopy methods. Any help would be immensely appreciated.

Answer 1

it was learning very well yesterday, but that was before I introduced my customised class. I need my class as I need to be able to keep hold of all of the data, rather than just the chromosome points, so that I can see the exact strategy that the algorithm uses.
It is indeed the child chromosome that is null, and I suppose that it because I had to initialise the int[] chromosomePoints, double[] chromosomePrices and int[] chromosomePlayerIDs as null. What is the best solution to this? using loops to initialise all values in the arrays as 0 before the arraycopy?!