mediumConceptCodingEntryMidSenior
How do you remove elements from a collection while iterating it, and what makes an iterator fail-fast?
Structural modification through the collection during a for-each invalidates the iterator, which detects it by comparing a modification counter and throws ConcurrentModificationException. Remove through the iterator, or use removeIf, or pick a collection whose iterator is weakly consistent instead.
5 min readUpdated 2026-08-15
Answer
Short answer
In Java, remove elements while iterating by using Iterator.remove(), Collection.removeIf(...), or a collection whose iterator is designed for concurrent changes. Removing through the collection inside a for-each loop changes the collection's structure behind the iterator, so fail-fast iterators usually throw ConcurrentModificationException. That exception is a bug detector, not a correctness guarantee.
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