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See this thread and this one for details.
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or Docker in Action in the Cloud/Virtualization forum!
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java.util.ConcurrentModificationException

 
Rob Brew
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Hi all. Trying to manipulate an ArrayList and i get this error, what do i do?

 
Rob Spoor
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When you iterate over most collections (either using an explicit Iterator or by using the for-each loop), any method call that modifies the collection will throw this exception. There are only two solutions:
1) use an explicit iterator and use the iterator's remove() method.
2) for lists only, loop using indexing. Be aware that when you delete an element, the indexing will also change. It's usually safest to loop backwards:
 
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