Thread Pool "Null Pointer" exception

Question

Hello,
I have an embarrassingly parallel algorithm. The parallel part is solved by 2 objects called shifters which implement the Runnable interface. However, for reasons I can't figure out, sometimes my code will randomly output this error.
Exception in thread "pool-1-thread-1" java.lang.NullPointerException
at cvtpmshiftlabnormalized.Shifter.run(Shifter.java:63)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
Exception in thread "pool-1-thread-2" java.lang.NullPointerException
at cvtpmshiftlabnormalized.Shifter.run(Shifter.java:63)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:619)
My code concerning the parallelization is as follows:
        shifter_1 = new Shifter(list_1 , clusterTree);
        shifter_2 = new Shifter(list_2 , clusterTree);
        ExecutorService executor = Executors.newFixedThreadPool(5);
        executor.execute(shifter_1);
        executor.execute(shifter_2);
        executor.shutdown();Any help would be greatly appreciated, seeing how my computer throws this error for the same data set whenever it feels like it.
Sincerely,
Chem E

Answer 1

Here is the run() method of the shifter. It's pretty big, but I don't know what you would want to look at , and what is garbage.. Thanks for helping me.
public void run()
        ArrayList<Cluster> neighbors = new ArrayList();
        ArrayList<Cluster> removeUs  = new ArrayList();
        double resultantVector[] = new double[3];
        double dotProduct;
        double gaussian;
        double gaussianWeight;
        while(workingClusters.size() > 0)
            workingClusters.removeAll(removeUs);
            removeUs.clear();
            System.out.println("Working Clusters: " + workingClusters.size());
            for(Cluster cluster : workingClusters)
                neighbors.clear();
                neighbors = clusterTree.returnNeighbors(cluster.values, returnRange);
                clusterTree.remove(cluster);
                resultantVector[0] = 0;
                resultantVector[1] = 0;
                resultantVector[2] = 0;
                gaussianWeight = 0;
                for(Cluster nCluster : neighbors)
                     dotProduct = dotProduct( cluster.values, nCluster.values);
                     gaussian = Math.exp(-dotProduct / Math.pow(returnRange , 2));
                     resultantVector[0] += gaussian * nCluster.values[0] * nCluster.weight;
                     resultantVector[1] += gaussian * nCluster.values[1] * nCluster.weight;
                     resultantVector[2] += gaussian * nCluster.values[2] * nCluster.weight;
                     gaussianWeight += gaussian * nCluster.weight;
                if(neighbors.size() > 0)
                    resultantVector[0] /= gaussianWeight;
                    resultantVector[1] /= gaussianWeight;
                    resultantVector[2] /= gaussianWeight;
                    if(dotProduct(resultantVector , cluster.values) < 0.01)
                        removeUs.add(cluster);
                    cluster.values[0] = resultantVector[0];
                    cluster.values[1] = resultantVector[1];
                    cluster.values[2] = resultantVector[2];
                clusterTree.put(cluster);
    }