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Java collection series - Fail-Fast vs Fail-Safe iterators

Fail-Fast iterators

  • works on the original collection
  • doesn't allow modifications to the collection by any other means except the methods provided by the iterator, otherwise throws ConcurrentModificationException
  • has methods like add() and remove() for ListIterator and remove() for Iterator to modify the collection
  • thread safety is an issue if the collection gets modified externally
Fail-Safe iterators
  • works on a snapshot of the collection
  • doesn't allow collection modification operations like add() and remove() and as a result doesn't throw ConcurrentModificationException
  • if any collection modification operation is performed, UnsupportedOperationException is thrown
  • thread safety is not an issue

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