Java 'java.util.ConcurrentModificationException' when removing from a List or Map while iterating (for-each loop): how do I fix it?

Cause: Standard collections such as ArrayList and HashMap have fail-fast iterators. If the collection's structure is changed directly (add or remove) while an iterator is in use, the next iterator step throws ConcurrentModificationException. The Javadoc notes this does not require a second thread: modifying a list inside a for-each loop over it in a single thread is enough, because the enhanced for loop uses an iterator behind the scenes. The behavior is best-effort and only meant to detect bugs, so some bad code might not throw; don't rely on the exception for correctness. Fix (best first): 1. To remove matching elements, call removeIf on the collection, for example list.removeIf(x -> x.isExpired()). The default Collection.removeIf walks the iterator and removes through it. Maps have entrySet().removeIf(...), keySet().removeIf(...) and values().removeIf(...). 2. Or use the iterator explicitly and call Iterator.remove(), the only safe way to remove during iteration with a plain loop. 3. To add or replace elements, collect changes in a separate list and apply them after the loop, or build a new collection with streams and assign it. 4. If several threads share the collection, using the same fixes is not enough: use a concurrent collection (ConcurrentHashMap, CopyOnWriteArrayList) or guard all access with a lock. Synchronized wrappers still need external locking while iterating. 5. Do not 'fix' it by catching the exception. Verify: The loop runs to completion with the expected remaining elements, tests pass repeatedly, and under concurrent use a stress test does not produce the exception. Sources: https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/ConcurrentModificationException.html https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Collection.html