Javadoc
Returns a reverse order
NavigableSet view of the keys contained in this map.
The set's iterator returns the keys in descending order.
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.