I am sharing an instance of a Vector in two places, one is populating the vector and another one is iterating it. two codes are synchronized. Very frequently I got in to a problem saying ConcurrentModificationException while iterating the code. Ofcourse, while iterating we may not update the collection. But will it lead to this kind of exception while the code is inside a synchronized. On cloning the collection, I cud come out of the problem. But this may not be a solution!