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The behavior of an iterator is unspecified if the underlying collection is modified while the iteration is in progress in any way other than by calling this method.
It is possible that in Such behavior may be observed in both single and multithreaded programs. In single threaded programs, this usually happens when an element is removed when the iteration is in progress. In a multithreaded program, it is possible that one thread iterates over a collection while another concurrently modifies it. This can result in unspecified behavior. Many implementations throw a ConcurrentModificationException when such a condition is detected.
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