java.util.ConcurrentModificationException in Android animation

There is something I miss with the notion of Synchronizing code in Android.

Scenario

There are always 3 items drawn on the screen. Each image is stored in a ArrayList (lstGraphics). For this purpose I use a SurfaceView. Once the user taps on a image, the image get's market to be removed and a new one will be added.

Code samples:

AnimationHideThread

...
    @Override
        public void run() {
            Canvas c;
            while (run) {
                c = null;
                try {
                    c = panel.getHolder().lockCanvas(null);
                      synchronized (panel.getHolder()) {
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                        panel.updatePhysics();
                        panel.manageAnimations();
                        panel.onDraw(c);

                    }
                } finally {
                    if (c != null) {
                        panel.getHolder().unlockCanvasAndPost(c);
                    }
                }
            }
        }    
...

So as you can seem first I updatePhysics(). This means I calculate direction where each image will move to. In here I will also remove clicked images from my list. After that I check if I need to add a new Item in my list in manageAnimations() and then the final step draw the whole thing.

public class Panel extends SurfaceView implements SurfaceHolder.Callback {
....
 public void manageAnimations()
    {
          synchronized (this.getHolder()) {
            ...
        while (lstGraphics.size()<3) {
                lstGraphics.add(createRandomGraphic());
                }
        }
          }
    }

 @Override
    public boolean onTouchEvent(MotionEvent event) {
         synchronized (getHolder()) {
            if (event.getAction() == MotionEvent.ACTION_DOWN) {
                 //... check if a image has been clicked and then set its property
                        graphic.setTouched(true);

                 }
            }

            return true;
         }
    }

 public void updatePhysics() {
       synchronized (getHolder()) {

     for (Graphic graphic : lstGraphics) {
           //.... Do some checks
     if (graphic.isTouched())
      {
        lstGraphics.remove(graphic);
      }
     }
  }
 }

 @Override
    public void onDraw(Canvas canvas) {
         /// draw the backgrounds and each element from lstGraphics
}

public class Graphic {

        private Bitmap bitmap;
            private boolean touched;
            private Coordinates initialCoordinates; 
....
}

The error I get is:

> 03-01 10:01:53.365: ERROR/AndroidRuntime(454): Uncaught handler: thread Thread-12 exiting due to uncaught exception 
> 03-01 10:01:53.365: ERROR/AndroidRuntime(454): java.util.ConcurrentModificationException
> 03-01 10:01:53.365: ERROR/AndroidRuntime(454): at java.util.AbstractList$SimpleListIterator.next(AbstractList.java:66)
> 03-01 10:01:53.365: ERROR/AndroidRuntime(454): at com.test.customcontrols.Panel.updatePhysics(Panel.java:290)
> 03-01 10:01:53.365: ERROR/AndroidRuntime(454): at com.test.customcontrols.AnimationHideThread.run(AnimationHideThread.java:41)

Any help is greatly appreciated. Thank you.

---

Your problem is in your physics method, where you add the graphic and the list

public void updatePhysics() {
    synchronized (getHolder()) {
        for (Graphic graphic : lstGraphics) {
        //.... Do some checks
        if (graphic.isTouched()) {
            lstGraphics.remove(graphic); //your problem
        }
    }
}

the combination of for(Graphic graphic : lstGraphics) and lst.Graphics.remove(graphic); causes the ConcurrentModificationException because you are running over your list and concurrently try to modify it.

So far I know two solutions:

1. Use an Iterator instead if one is available (never coded for Android so far).

   while (iter.hasNext) {
       if (physicsCondition) iter.remove();
   }
   

2. use a second list to store the elements to remove and remove them afterwards

   List<GraphicsItem> toRemove = new ....
   for (Graphic graphic : lstGraphics) {
       if (physicsCondition) {
           toRemove.add(graphic);
       }
   }
   lstGraphics.removeAll(toRemove);
   

---

As @idefix said, you can easily get ConcurrentModificationException in single-threaded context like this:

public static void main(String[] args) {
    List<String> list = new ArrayList<String>(Arrays.asList("AAA", "BBB"));
    for (String s : list) {
        if ("BBB".equals(s)) {
            list.remove(s);
        }
    }
}

---

You can use CopyOnWriteArrayList like below:

    List<String> myList = new CopyOnWriteArrayList<String>();

    myList.add("1");
    myList.add("2");
    myList.add("3");
    myList.add("4");
    myList.add("5");

    Iterator<String> it = myList.iterator();
    while(it.hasNext()){
        String value = it.next();
        System.out.println("List Value:"+value);
        if(value.equals("3")){
            myList.remove("4");
            myList.add("6");
            myList.add("7");
        }
    }

---

This is my method using @idefix second solution:

private List<TYPE> getFilteredData(List<TYPE> data){                
    List<TYPE> toRemove = new ArrayList<TYPE>(data.size());     
    synchronized(data){
        for(TYPE f : data){
            if([CONDITION]){                        
                toRemove.add(f);
                Log.w(TAG, "Element removed: "+ f);                 
            }
        }
    }                   
    data.removeAll(toRemove);
    return data;        
}

Thanks @idefix +1