iteratorremove

Iteratorremove is a term used to describe a pattern in which elements are removed from a collection during iteration using the iterator itself. This approach enables modifying a collection while traversing it without disrupting the iteration flow in languages that support it.

In Java, the interface java.util.Iterator provides a remove method that deletes from the underlying collection the last element returned by next(). The remove operation may be called only once per call to next(), and invoking it otherwise results in an IllegalStateException. Using the iterator’s remove keeps the iteration in a consistent state and helps avoid concurrency-related errors such as ConcurrentModificationException, which can occur when a collection is modified outside of the iterator.

Some languages and collection types offer explicit iterator-based removal, while others do not. In those that do not, modifying the collection during iteration can lead to runtime errors or unpredictable behavior. Alternatives include collecting items to remove in a separate pass and applying the removals afterward, or constructing a new collection that excludes the unwanted elements.

Examples across languages vary. In Java, typical usage is to call next() to fetch an element, test a condition, and call remove() when the condition is met. In Python, iterators do not provide a built-in remove, so methods include building a new list with only desired elements or filtering with a comprehension. In C++, erasing elements often uses the erase method in combination with iterators, taking care to preserve a valid iterator after removal.

Designers consider iterator-remove a safe pattern when supported, but it is not universal. If removal is not supported, alternative strategies must be employed to avoid invalidating iteration.