How does an Iterator throw ConcurrentModificationException on add

Question

How does Iterator throw ConcurrentModificationException when we are adding some object after current node or removing some object after current node. Does Iterator maintain a copy or reference to the underlying collection?

Answer 1

The iterator maintains a reference to the underlying collection. If you add or remove an element, the iterator might be left at an impossible index, or the collection might change "out from underneath" the iterator.

Therefore, instead of letting the iterator get corrupted without letting you know, most collections do the courtesy of throwing a ConcurrentModificationException when you try to modify the collection while iterating, so you don't wind up with unpredictably corrupted iterators.

Answer 2

By contract, you are not allowed to modify the collection while iterating over it (except by using Iterator.remove() et al).

Instead of randomly failing when you do this, the collection is nice enough to keep track of how many times it's been modified, and throw ConcurrentModificationException when it detects concurrent modification.