Question

I am writing a little game which has many circles moving on the screen.
I am managing the circles in two threads as following:

public void run() {
    int stepCount = 0;
    int dx;
    int dy;
    while (m_threadTrap){
        dx = 0;
        dy = 0;

        synchronized (m_circles) {
            for (Iterator<Circle> it = m_circles.iterator(); it.hasNext();){
                Circle c = it.next(); //Exception thrown here.
                if (c.getDirX() != 0)
                    if (stepCount % c.getDirX() == 0){
                        dx = 1;
                    }
                if (c.getDirY() != 0)
                    if (stepCount % c.getDirY() == 0){
                        dy = 1;                 
                    }

                c.move(dx, dy); 
            }
        }
        if (stepCount == 150000){
            stepCount = 0;
        }
        stepCount++;
    }
}

m_circles in an ArrayList of Circles.

And the following Thread:

public void run() {
    while (m_threadTrap){
        int topPosition;
        int bottomPosition;
        int leftPosition;
        int rightPosition;
        ArrayList<Circle> removedCircles = new ArrayList<Circle>();
        synchronized (m_circles.getCircles()) {
            for (Iterator<Circle> it = m_circles.getCircles().iterator(); it.hasNext();){
                Circle c = it.next();

                // Some calculation to evaluate which circles should be removed
                    removedCircles.add(c);
                }
            }
        }
        try{
            Thread.sleep(25);
        }
        catch (Exception e) { }

        m_circles.getCircles().removeAll(removedCircles);

        if (m_circles.getCircles().size() < 30)
            m_circles.addNewCircle();

        repaint(); 
    }
}

My problem is that I get ConcurrentModificationException at the line

Circle c = it.next();

in the first thread. At first I tried going over the ArrayList with a foreach loop and this gave me the same exception.
After researching a bit on this exception I saw two solutions:
1. Putting the part which accesses the collection in a synchronized block.
2. Using the Iterator object of the collection.
Neither of them solved it for me.

Answer 1

For a synchronized() {} block to be effective, all accesses to the protected objects must be wrapped in synchronized blocks. You probably forgot to wrap some access.

Another “gotcha” is that ConcurrentModificationException can also mean that it was concurrently modified in the same thread. For example, if you remove an element from a collection while traversing it, you may get this exception. (As an exception, you can safely remove elements via the iterator itself)