Fail Fast Vs Fail Safe Iterator In Java

Concurrent Modification
           When one or more thread is iterating over the collection, in between, one thread changes the structure of the collection (either adding the element to the collection or by deleting the element in the collection or by updating the value at particular position in the collection) is known as Concurrent Modification

Fail fast Iterator

          Fail fast iterator while iterating through the collection , instantly throws Concurrent Modification Exception if there is structural modification  of the collection . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly

Fail Safe Iterator makes copy of the internal data structure (object array) and iterates over the copied data structure.Any structural modification done to the iterator affects the copied data structure.  So , original data structure remains  structurally unchanged .Hence , no ConcurrentModificationException throws by the fail safe iterator.