Fail fast iterator : This iterator throws "ConcurrentModificationException" when a thread iterating a collection realizes that the same collection is modified by another thread while under operation. Iterator identifies change in collection is by looking at it's modification count. Ex: HashMap
Fail safe iterator : If we are using fails safe iterator, the above scenario is handled and java doesn't throw the ConcurrentModificationException. To handle this scenario, iterator makes a clone of the collection and works on it. Ex: ConcurrentHashMap
