How to split an array into multiple threads Java

Question

So i have a MaxThread class and the run() method stores the maximum value from the vector passed, into maxValue class attribute. I want to split my initial array of 10 integers into 4 subarrays and each subarray to have a different thread. After i find the 4 maximum values from each one of the 4 subarrays, i want to create a new MaxThread and to display the maximum value from it. this code works but i cant think of a better way to do it as I'm sure what i did is silly( I'm new to this).

public class Main{

    public static void main(String[] args) {
        int step = 3;
        Integer[] vector = new Integer[10];
        readFromFile(vector);
        int i = 0;
        ArrayList<MaxThread> threads = new ArrayList<>();
        MaxThread t1 = null;
        MaxThread t2 = null;
        MaxThread t3 = null;
        MaxThread t4 = null;

            t1 = new MaxThread(vector, 0, 3);
            threads.add(t1);
            t1.start();

            t2 = new MaxThread(vector, 3, 6);
            threads.add(t2);
            t2.start();

            t3 = new MaxThread(vector, 6, 9);
            threads.add(t3);
            t3.start();

            t4 = new MaxThread(vector, 9, 10);
            threads.add(t4);
            t4.start();


        for (MaxThread thread : threads) {
            try {
                thread.join();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        Integer[] last = new Integer[4];
        last[0]=t1.getMaxVal();
        last[1]=t2.getMaxVal();
        last[2]=t3.getMaxVal();
        last[3]=t4.getMaxVal();

        MaxThread lastThread = new MaxThread(last, 0, last.length);
        lastThread.start();
        try {
            lastThread.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Valoarea maxima:" +lastThread.getMaxVal());
    }

Looks good to me. Besides the obvious flaw that you correctly decided to put your numbers into an array, but then go to have 4 distinct variables for your threads. To then be forced to handle them manually. — GhostCat
– GhostCat, Commented Jun 9, 2019 at 16:04
@GhostCat I'm not pleased with they way i divided the initial array. Is there a better way to do that? Also I knwo I need to find a better solution for using the 4 max values into a new thread. — Aurel Lungu
– Aurel Lungu, Commented Jun 9, 2019 at 16:17

Philippe Marschall · Accepted Answer · 2019-06-09 16:06:09Z

1

This is a good case for parallel streams. In general multiple threads are only worth it if you have at least a couple of tens of thousands of elements. In these cases it also pays to have a primitive array.

public static void main(String[] args) {
    int[] vector = // ...
    int max = Arrays.stream(vector).parallel().max().getAsInt();
}

answered Jun 9, 2019 at 16:06

Philippe Marschall

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4 Comments

markspace Over a year ago

Parallel stream is the way I would do it. Ad hoc frameworks like the OP's should be avoided. In a more general case, look into Java's Fork-Join framework.

GhostCat Over a year ago

I did a bit of benchmarking the other day (stackoverflow.com/questions/56488685/…). 2 million entries were not enough to get the parallel stream solution to be faster than the sequential stream :-o

markspace Over a year ago

That seems broken, frankly.

Philippe Marschall Over a year ago

@GhostCat I wrote a JMH microbenchmark that suggests they are about equal with 10'000 elements and parallel streams are faster with 100'000 elements github.com/marschall/jmh-experiments/blob/parallel-stream/src/…

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