About fail-fast nature of Iterator

The iterators returned by this class's iterator and listIterator methods are fail-fast. If the list is  structurally modified at any time after the iterator is created, in any way except through the Iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.  Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

         Example:

                                  List<String> staff = new ArrayList<String>();
                                     staff.add("Ajay");
                                     staff.add("Kumar");
                                     staff.add("Singh");
                                     staff.add("sunny");


                          ListIterator listItr = staff.listIterator();  //get the ListIterator

                       // after getting iterator if we change(add or remove) here by ArrayList.add of ArrayList.remove then  it will throw a ConcurrentModificationException except ListIterator.add or ListIterotr.remove.

                         
                         staff.add("Ajay Kumar");      // will throw a ConcurrentModificationException
                         
                         listItr.add("Ajay Kumar");   // OK


But there may be a exceptional situation where more than one thread of Iterator is trying to modify a collection in unsynchronized  block or method then it will throw ConcurrentModificationException.

                      
                          ListIterator listItr = staff.listIterator();  //get the ListIterator
                          
                                       Iterator itr = staff.iterator();        //get the Iterator
                         
                        listItr.add("Ajay Kumar");   // throw ConcurrentModificationException