In what scenario does a ConcurrentModificationException occur?

• A. When using an iterator to modify items during traversal
• B. When a collection is accessed by multiple threads without synchronization
• C. When a null key is used in a HashMap
• D. When trying to add duplicate keys in a TreeMap

Answer: (A)

A ConcurrentModificationException occurs primarily when a collection is modified while it is being traversed by an iterator. This situation typically arises when one thread is iterating over a collection and another thread modifies the same collection simultaneously, or even when the same thread modifies the collection directly while it is still being iterated. Using an iterator to traverse a collection while directly modifying that collection—such as adding or removing elements—causes this exception to ensure safe iteration and to maintain data integrity in the collection.

The other scenarios described in the options do not lead to this specific exception. Accessing a collection from multiple threads without synchronization can lead to data inconsistency and other concurrency issues but does not inherently raise a ConcurrentModificationException. Utilizing a null key in a HashMap is permissible and does not trigger this exception. Lastly, attempting to add duplicate keys in a TreeMap would result in the second insertion simply overshadowing the first key, while also not being related to the iteration process that would trigger a ConcurrentModificationException. Thus, the key factor for this type of exception is the modification of a collection during iteration.