Thread: java concurrency problem

Hi ! I want to make a simple math operations on a vector(array) using two cores of my CPU. The program doesn't work correctly. Please explain me how to solve my problem.

public class MyRunnable implements Runnable {
 
    private int startIndex;
    private int endIndex;
    private float[] tab;
 
    public MyRunnable(int startIndex, int endIndex, float[] tab)
    {
        this.startIndex = startIndex;
        this.endIndex = endIndex;
        this.tab = tab;
    }
 
    @Override
    public void run() 
    {
        for(int i = startIndex; i < endIndex; i++)
        {
            tab[i] = i * 2;
        }
 
        System.out.println("Finished");
    }
}

public class Test {
 
    public static void main(String[] args) {
 
        int size = 10;
        int n_threads = 2;
        float tab[] = new float[size];
 
        for(int i = 0; i < size; i++)
        {
            tab[i] = i;
        }
 
        for(int i = 0; i < size; i++)
        {
            System.out.println(tab[i]);
        }
 
        Runnable r1 = new MyRunnable(0, size / n_threads, tab );
        Runnable r2 = new MyRunnable(size / n_threads, size, tab );
 
        Thread t1 = new Thread(r1);
        Thread t2 = new Thread(r2);
 
        t1.start();
        t2.start();
 
        for(int i = 0; i < size; i++)
        {
            System.out.println(tab[i]);
        }
   }
 
}

doesn't work what??

What does your code attempt to do?

Ok, I added println to the run:

public void run() 
    {
        System.out.println(Thread.currentThread());
 
        for(int i = startIndex; i < endIndex; i++)
        {
            tab[i] = i * 2;
        }
 
        System.out.println("Finished");
    }

With join() calls the program prints: 0.0, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, Thread[Thread-0,5,main], Finished, Thread[Thread-1,5,main], Finished, 0.0, 2.0, 4.0, 6.0, 8.0, 10.0, 12.0, 14.0, 16.0, 18.0 and that make sense.