Why are iterators called "fail-safe" and "fail-fast" even though Iterator.remove() doesn't throw any ConcurrentModificationException while iterating through a Collection (for example, HashMap) using Iterator ?
Using Iterator.remove() doesn't throw ConcurrentModificationException, but if something else (in another thread) modifies the Collection which backs the Iterator during iteration, then it will.
From the Java API docs for java.util.ArrayList; bold emphasis added by me:
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.
The next sentence of the same documentation then explains exactly what is meant by fail-fast in this case; again, bold emphasis added by me:
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.
In other words, the simple and safe thing to do, if another thread modifies the collection, is to cause a deliberate, explicit failure in the form of ConcurrentModificationException, rather than attempting to work out a non-failing, ie fail-safe, logic (such as having the modification show up via the Iterator depending upon whether that element has already been iterated over) which could be complex and error-prone.