Re: Odd ConcurrentModificationException

Eric Sosman <esosman@comcast-dot-net.invalid>
Tue, 10 Dec 2013 14:22:12 -0500
On 12/10/2013 12:41 PM, Knute Johnson wrote:

I'm getting an odd ConcurrentModificationException in a MouseListener
when I try to remove an element from an ArrayList. The really strange
thing about it is that the exception occurs every other time I call
remove(). The ArrayList is only accessed on the EDT and I tried
wrapping it in Collections.synchronizedList but that made no difference.

The ArrayList variable is boxes and the type is DisplayBox, an extended
JComponent. The error shows the line number of the for instruction but
actually fails on the remove(box) call.

     What's your evidence for the "actually fails" location, given
that the JVM says otherwise?

Any ideas on where to start looking?



     public void mousePressed(MouseEvent me) {
         if (me.getButton() == MouseEvent.BUTTON1) {
             System.out.println("Mouse1 Down");
             mouseDown = true;
             startX = me.getX();
             startY = me.getY();
         } else if (me.getButton() == MouseEvent.BUTTON3) {
             System.out.println("Mouse3 Down");
             for (DisplayBox box : boxes) {
                 if (box.getBounds().contains(me.getX(),me.getY())) {

     The "for-each" construct (for an Iterable as opposed to an
array) expands more or less like

    for (Thing thing : somethingIterableOfThing) {
        // do stuff


    for (Iterator<Thing> it = somethingIterableOfThing.iterator();
         it.hasNext(); ) {
        Thing thing =;
        // do stuff

If "do stuff" modifies "somethingIterableOfThing", *that's* the
ConcurrentModificationException, and the Iterator throws up next
time you use it.

Eric Sosman

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