When ConcurrentModificationException occurs ?

ConcurrentModificationException is a unchecked (runtime) exception that may be thrown :

 1. When concurrent modification of an object detected while such modification is not permissible

 Example. One thread to modify a Collection while another thread is iterating over it)

2. If a thread modifies a collection directly while it is iterating over the collection with a Fail-fast iterator

Note : ConcurrentModificationException should be used only to detect bugs.