Returns a
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
Iterator.remove,
Set.remove,
removeAll,
retainAll, and
clearoperations. It does not support the
add or
addAlloperations.
The view's
iterator is a "weakly consistent" iterator
that will never throw
java.util.ConcurrentModificationException,
and guarantees to traverse elements as they existed upon
construction of the iterator, and may (but is not guaranteed to)
reflect any modifications subsequent to construction.