Odd ConcurrentModificationException

[Knute Johnson]

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.

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();
}
}

Mouse3 Down
Exception in thread "AWT-EventQueue-0"
java.util.ConcurrentModificationException

at java.util.ArrayList$Itr.checkForComodification(Unknown Source)
at java.util.ArrayList$Itr.next(Unknown Source)
at
com.knutejohnson.stationcasinos.backgroundgenerator.Yard$1.mousePress
ed(Yard.java:47)
at java.awt.Component.processMouseEvent(Unknown Source)
at javax.swing.JComponent.processMouseEvent(Unknown Source)
at java.awt.Component.processEvent(Unknown Source)
at java.awt.Container.processEvent(Unknown Source)
at java.awt.Component.dispatchEventImpl(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown
Source)
at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source)
at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source)
at java.awt.Container.dispatchEventImpl(Unknown Source)
at java.awt.Window.dispatchEventImpl(Unknown Source)
at java.awt.Component.dispatchEvent(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$200(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at
java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Sour
ce)
at
java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Sour
ce)
at java.awt.EventQueue$4.run(Unknown Source)
at java.awt.EventQueue$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at
java.security.ProtectionDomain$1.doIntersectionPrivilege(Unknown Sour
ce)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown
Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown
Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

 

[Stefan Ram]

I'm getting an odd ConcurrentModificationException in a MouseListener
when I try to remove an element from an ArrayList. The really strange ....
for (DisplayBox box : boxes) { ....
boxes.remove(box);

This is an FAQ. Try using the iterator directly and then use Iterator#remove.

 

[Eric Sosman]

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.

 

[Knute Johnson]

This is an FAQ. Try using the iterator directly and then use Iterator#remove.

Stefan:

I don't know why my previous reply didn't show up but you are correct, I
am seriously confused. But your reply did allow me to correct my problem.

Thanks,