Returns a {@link NavigableSet} view of the keys contained in this map.The set's iterator returns the keys in ascending order. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from the map, via the {@code Iterator.remove}, {@code Set.remove}, {@code removeAll}, {@code retainAll}, and {@code clear}operations. It does not support the {@code add} or {@code addAll}operations.
The view's iterators and spliterators are weakly consistent.
This method is equivalent to method {@code navigableKeySet}.
@return a navigable set view of the keys in this map