Question

I’m programming a simple Snake game.
So, I have an update thread, executes an infinite while-loop, which updates all the GameObjects and repaints the JPanel.

// Run thread
public void run () {
    try {
        while (true) {
            this.update();
            this.getGamePanel().sceneShouldRepaint();

            Thread.sleep(this.getFps());
        }
    }
    catch (Exception e) {
        JOptionPane.showMessageDialog(null, e.getMessage(), "Error", JOptionPane.OK_CANCEL_OPTION);
    }
}

Now, the problem is that when I call myJFrame.repaint() it’s probably being executed in another thread. And when I paint, I obviously have to get the location of the objects, speed and so on…
I know the exception comes from paintComponent(), because it’s not being caught.
As you can see, the while loop is embedded in a try-catch.

What can I do about this?

Here’s the exception:

Exception in thread "AWT-EventQueue-0" java.util.ConcurrentModificationException
    at java.util.AbstractList$Itr.checkForComodification(AbstractList.java:372)
    at java.util.AbstractList$Itr.next(AbstractList.java:343)
    at ch.ilijatovilo.Scene.GameObjects.Creatures.Snake.Snake.render(Snake.java:52)
    at ch.ilijatovilo.Scene.SceneHandling.Scene.renderSceneObjects(Scene.java:57)
    at ch.ilijatovilo.Scene.SceneHandling.Scene.render(Scene.java:53)
    at ch.ilijatovilo.Scene.SceneHandling.GamePanel.paintComponent(GamePanel.java:53)
    at ch.ilijatovilo.Scene.SceneHandling.GamePanel.paint(GamePanel.java:30)
    at javax.swing.JComponent._paintImmediately(JComponent.java:5106)
    at javax.swing.JComponent.paintImmediately(JComponent.java:4890)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:812)
    at javax.swing.RepaintManager.paintDirtyRegions(RepaintManager.java:714)
    at javax.swing.RepaintManager.prePaintDirtyRegions(RepaintManager.java:694)
    at javax.swing.RepaintManager.access$700(RepaintManager.java:41)
    at javax.swing.RepaintManager$ProcessingRunnable.run(RepaintManager.java:1672)
    at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
    at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:702)
    at java.awt.EventQueue.access$400(EventQueue.java:82)
    at java.awt.EventQueue$2.run(EventQueue.java:663)
    at java.awt.EventQueue$2.run(EventQueue.java:661)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
    at java.awt.EventQueue.dispatchEvent(EventQueue.java:672)
    at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:296)
    at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:211)
    at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:201)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:196)
    at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:188)
    at java.awt.EventDispatchThread.run(EventDispatchThread.java:122)

Thanks for your help!

HINT

ConcurrentModificationException is being thrown if you try to access an instance at the same time from 2 different threads. That’s what I’ve heard about it at least.

More Code

// This is the render method in the Snake class at line 50.
// So 52 is the start of the for loop

public void render(Graphics g) {
    // We delegate the drawing to the body parts
    for (SnakeBodyPart sbp : this.getSnakeBodyParts()) {
        sbp.render(g);
    }

    this.getSnakeHead().render(g);
}

// And here the for loop from the Scene class
protected void renderSceneObjects (Graphics g) {
    for (SceneObject so : this.getSceneObjects()) {
        so.render(g);
    }
} 

Answer 1

Well CopyOnWriteArrayList is an answer. I will look after another solution when i have some time …

Another Solution in awt – Based Application should be

EventQueue.invokeLater(new Runnable() {...});