What is 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, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Fail-fast iterator can throw ConcurrentModificationException in two scenarios :
Single Threaded Environment : After the creation of the iterator , structure is modified at any time by any method other than iterator’s own remove method.
Multiple Threaded Environment: If one thread is modifying the structure of the collection while other thread is iterating over it.
According to Oracle docs , the fail-fast behavior of an iterator cannot be guarantee, fail-fast behavior of iterators should be used only to detect bugs.
How Fail Fast Iterator come to know that the internal structure is modified ?
Iterator read internal data structure (object array) directly . The internal data structure(i.e object array) should not be modified while iterating through the collection. To ensure this it maintains an internal flag “mods” .Iterator checks the “mods” flag whenever it gets the next value (using hasNext() method and next() method). Value of mods flag changes whenever there is an structural modification. Thus indicating iterator to throw ConcurrentModificationException.
Fail Safe Iterator :
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.
Two issues associated with Fail Safe Iterator are :
1. Overhead of maintaining the copied data structure i.e memory.
2. Fail safe iterator does not guarantee that the data being read is the data currently in the original data structure.
Fail safe iterator is ordinarily too costly, but may be more efficient than alternatives when traversal operations vastly outnumber mutations, and is useful when you cannot or don’t want to synchronize traversals, yet need to preclude interference among concurrent threads.
The “snapshot” style iterator method uses a reference to the state of the array at the point that the iterator was created. This array never changes during the lifetime of the iterator, so interference is impossible and the iterator is guaranteed not to throw ConcurrentModificationException.
The iterator will not reflect additions, removals, or changes to the list since the iterator was created. Element-changing operations on iterators themselves (remove(), set(), and add()) are not supported. These methods throw UnsupportedOperationException.
eg. HashMap has a fail fast iterator while ConcurrentHashMap uses a fail slow iterator.
For detail description and examples please refer to the post: http://javahungry.blogspot.com/2014/04/fail-fast-iterator-vs-fail-safe-iterator-difference-with-example-in-java.html