The ConcurrentModificationException occurs when an object is tried to be modified concurrently when it is not permissible. This exception usually comes when one is working with Java Collection classes. For Example - It is not permissible for a thread to modify a Collection when some other thread is iterating over it.

How is concurrent modification exception thrown?

If we invoke a sequence of methods on an object that violates its contract, then the object throws ConcurrentModificationException. For example: if while iterating over the collection, we directly try to modify that collection, then the given fail-fast iterator will throw this ConcurrentModificationException.

Which iterator can throw a concurrent modification exception?

Fail-Fast iterators immediately throw ConcurrentModificationException if there is structural modification of the collection. Structural modification means adding, removing any element from collection while a thread is iterating over that collection.