How do you troubleshoot a ConcurrentModificationException when using CollectionUtils.filter()?

CollectionUtils.filter(collection, predicate) mutates the collection in place: it walks the collection's own Iterator and calls Iterator.remove() on every element that fails the predicate. A ConcurrentModificationException (CME) shows up when something else disturbs that same collection while filter() is mid-iteration.

// risky: filtering while also iterating the same list elsewhere
for (String s : names) {
    if (s.isEmpty()) {
        CollectionUtils.filter(names, StringUtils::isNotBlank); // CME
    }
}

// safer: snapshot first, or use select() instead of filter()
List<String> snapshot = new ArrayList<>(names);
Collection<String> kept = CollectionUtils.select(snapshot, StringUtils::isNotBlank);

Common causes are: calling filter() from inside a for-each loop over the same reference, calling it concurrently from another thread without synchronization, or passing a fixed-size/immutable view whose iterator doesn't support remove() at all (which throws UnsupportedOperationException rather than CME, but is diagnosed the same way - check what kind of collection was actually passed in).

The fix is usually one of two things: copy the input into a fresh, independent collection before filtering if the original reference must stay untouched during the operation, or switch to CollectionUtils.select()/selectRejected(), which build and return a brand-new filtered collection instead of mutating the source - sidestepping the in-place removal that causes the CME in the first place.