Javadoc
Returns a
Set view of the keys contained in this map. The set is backed by the map, so
changes to the map are reflected in the set, and vice-versa. If the map is modified while an
iteration over the set is in progress (except through the iterator's own remove
operation), the results of the iteration are undefined. The set supports element removal, which
removes the corresponding mapping from the map, via the Iterator.remove,
Set.remove, removeAll, retainAll, and clear operations. It
does not support the add or addAll operations.