java.util.ConcurrentModificationException in Android animation

Question

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()) {                          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.

Answer 1

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); 

Answer 2

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);         }     } }