Java Data Parallelism

gistfile1.java

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.Iterator;
import java.util.List;
import java.util.ArrayList;

public class Par {
    int parallelism = Runtime.getRuntime().availableProcessors() + 1;
    ExecutorService executor = Executors.newCachedThreadPool();

    public Iterable<Integer> partition(final int total, final int chunkIndex,
            final int numChunks) {
        class Partition implements Iterator<Integer> {
            int chunkSize = total / numChunks;
            int start = chunkIndex * chunkSize;
            boolean lastChunk = chunkIndex + 1 == numChunks;
            int end = lastChunk ? total - 1 : (chunkIndex + 1) * chunkSize - 1;
            int current = start;

            @Override public boolean hasNext() {
                return current <= end;
            }

            @Override public Integer next() {
                return current++;
            }

            @Override public void remove() {
                throw new UnsupportedOperationException();
            }
        };

        return new Iterable<Integer>() {
            @Override public Iterator<Integer> iterator() {
                return new Partition();
            }
        };
    }

    public void run() {
        final List<Integer> list = new ArrayList<Integer>();
        final int numElements = 10;
        for (int i = 0; i < numElements; i++) {
            list.add(i);
        }

        final CountDownLatch finished = new CountDownLatch(parallelism);
        for (int i = 0; i < parallelism; i++) {
            final int i_ = i;
            executor.execute(new Runnable() {
                @Override public void run() {
                    for (int j : partition(numElements, i_, parallelism)) {
                        int elem = list.get(j);
                        list.set(j, elem * 2);
                    }
                    finished.countDown();
                }
            });
        }
        executor.shutdown();
        try {
            finished.await();
            System.out.println(list);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        new Par().run();
    }
}