Java n threads updates value

Question

I'm trying to run n threads at the same time. Every thread should sum different array and update the global value.

Unfortunately the global value is updated incorrectly.

I don't want to use thread.join().

This is my code so far:

public class myClass {
private static class Values {
    private static double sum;

    public synchronized static void add(double dd) {
        sum += dd;
    };
    public synchronized double get() {
        return sum;
    }
}


public static double CreateThreads(double[] array) {
    final Values values = new Values();

    ...
    ...
    ...


    Thread[] threads = new Thread[nOP];

    for (int i = 0; i<threads.length; i++) {


        threads[i] = new Thread(new Runnable() {

            public void run() {

                    Values.add(sum(tab));

            }

        });
        threads[i].start();

    }

    return values.get();
}

public static void main(String[] args) throws IOException {
    double[] arr = createArray(4);
    double sumLogg = CreateThreads(arr);

    System.out.println("\n\nSum: " + sumLogg);      
}

Any ideas?

Answer 1

If you dont want to use Thread.join you can use CountDountLatch:

    CountDownLatch cdl = new CountDownLatch(nOP);
    Thread[] threads = new Thread[nOP];
    for (int i = 0; i<threads.length; i++) {
        threads[i] = new Thread(new Runnable() {
            public void run() {
                values.add(sum(tab));
                cdl.countDown();
            }
        });
        threads[i].start();
    }
    cdl.await();

In this case you dont need to use additional synchronization, CountDownLatch is a synchronzier (see java.util.concurrent package description) and according to its javadoc "Until the count reaches zero, actions in a thread prior to calling countDown() happen-before actions following a successful return from a corresponding await() in another thread."