Returns a
Set view of the keys contained in this map; with care
the keys may be iterated over
without auto-boxing. 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 this map, via the
Iterator.remove, Set.remove, removeAll,
retainAll, and clear operations. It does not support
the add or addAll operations.
The view's iterator is a "weakly consistent" iterator that
will never throw
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.