ParallelGroups.java

import com.google.common.collect.Lists;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class ParallelGroups {
    public static final int CONCURRENCY = 5;

    private static double[] compute(double[] a, double[] w) {
        return a;
    }

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        final ExecutorService executor = Executors.newCachedThreadPool();
        final List<double[]> input = new ArrayList<>();
        final double[] w = new double[0];
        for (List<double[]> group : Lists.partition(input, CONCURRENCY)) {
            List<Future<double[]>> futures = Lists.transform(group, elem -> executor.submit(() -> compute(elem, w)));
            for (Future<double[]> future : futures) {
                double[] result = future.get();
                for (int i = 0; i < result.length; i++) {
                    w[i] += result[i];
                }
            }
        }
    }
}