Many of the Collectors defined in java.util.stream.Collectors, that accumulate, group, etc, elements into a Collection, and do not accept a Collection supplier, come with a caveat on the Collection they create, along the lines of, "There are no guarantees on the type, mutability, serializability, or thread-safety of the ... returned".

This caveat should be applied to all Collectors that create such Collections. This appears to be a minor oversight in the original spec.