Why can an enhanced for-each loop throw ConcurrentModificationException, and how do you remove elements correctly?

for-each desugars to an Iterator, and most collections track a modCount; structurally modifying the collection (add/remove) directly while iterating makes the iterator's expected count diverge, so the next call throws ConcurrentModificationException fail-fast.

To remove during iteration, use the explicit Iterator.remove(), or Collection.removeIf(predicate) which handles it internally. Note this is a best-effort safety check, not a thread-safety guarantee.