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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Odd ConcurrentModificationException
Date: Tue, 10 Dec 2013 14:22:12 -0500
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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?
>
> Thanks,
>
> knute...
>
>
>      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())) {
>                      remove(box);
>                      boxes.remove(box);
>                  }
>              }
>              repaint();
>          }
>      }

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

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

becomes

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

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

-- 
Eric Sosman
esosman@comcast-dot-net.invalid