I think Javadocs for Collections.synchronizedXxx() should mention not only "traversing it via Iterator, Spliterator or Stream" as something that must be synchronized externally, but also cases when a synchronized collection is an argument to a bulk collection method, including: - new ArrayList(syncList), new HashSet(syncSet), etc. - List.copyOf(syncList) - otherColl.addAll(syncList), otherColl.removeAll(syncList), otherMap.putAll(syncMap).
Though this is logically inferable, this is far from being obvious and apparent.
