Rectangle2D#getCenterY causes unrelated iterator to throw CME

Question

I'm currently working on a game that is multi-threaded, there is one thread running the updates for the game and one thread repainting the panel that the game is being played on. After editing some code that was responsible for an enemy firing projectiles at the player i noticed that i was getting a ConcurrentModificationException from the Iterator that was iterating through all the entities and updating them. I isolated the problem to this statement here:

getMyBounds2D().getCenterY();

Because when I change the the statement to:

getMyBounds2D().getY();

the Exception is no longer thrown from the updating Iterator. the method getMyBounds2D() just returns a Rectangle2D that is representative of the bounding box of the entity, is there anyone that could explain why changing the statement above causes a CME and how to fix this issue?

are you messing with the awt/swing objects from outside the EDT? — jtahlborn, Feb 04 '13 at 15:47
EventDispatchThread the thread that calls all listeners and does the painting of the gui — ratchet freak, Feb 04 '13 at 15:55
The iterator remembers the modification count of its collection from the start of the iteration. On each next, it will check if there was another modification and if so, fail with CME. Find the source of that change. — Ralf H, Feb 04 '13 at 15:58

score 1 · Answer 1 · answered Feb 04 '13 at 15:58

1

You cannot mess with awt/swing objects outside of the EDT (event dispatch thread). the various gui objects are meant to be handled in a single threaded manner, by this thread alone. if you have multiple threads handling these objects, then that will cause all kinds of problems, CME among others.

answered Feb 04 '13 at 15:58

jtahlborn

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How would this explain the fact that if i change the method call it doesn't throw the exception anymore? – Neil Locketz Feb 04 '13 at 16:02
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@NeilLocketz - is the exception consistent or sporadic? – jtahlborn Feb 04 '13 at 16:05
100% consistant every time i call getCenterY it throws it and every time i call getY it doesnt – Neil Locketz Feb 04 '13 at 16:25
@NeilLocketz - it isn't throwing from within that method though right? – jtahlborn Feb 04 '13 at 16:28
No from a completely unrelated method where all the updates are going on – Neil Locketz Feb 04 '13 at 16:30
@NeilLocketz - k, so you haven't included the actual _relevant_ code. i'm going to guess that changing that method call changes the logic so that you decide to muck with an collection which is currently being iterated. – jtahlborn Feb 04 '13 at 16:32
It is not changing any logic, it just finds the y value and the method executes the same regardless of its return – Neil Locketz Feb 04 '13 at 16:56
@NeilLocketz - since you are calling a different method _you are using a different Y value_, which is likely changing later logic. – jtahlborn Feb 04 '13 at 16:57
there is nothing that changing based on the y value – Neil Locketz Feb 04 '13 at 17:01
@NeilLocketz - at this point it's very hard to help, as you haven't shown any code relevant to the problem. i would suggest updating your question to show useful code. – jtahlborn Feb 04 '13 at 17:05

Rectangle2D#getCenterY causes unrelated iterator to throw CME

1 Answers1