Fail fast and fail safe iterator

Fail Fast :
When we iterate over a collection, if during iteration, we modify the collection , then the iteration halts and we get "ConcurrentModificationException" (e.g Hashtable)

Fail Safe :
During iteration, a separate copy of the colleciton object is created and iteration occurs on that. So if we modify it during the iteration process, it wont throw an exception.(e.g HashMap)

Best Explanation (with example) : http://www.certpal.com/blogs/2009/09/iterators-fail-fast-vs-fail-safe/