A {@link Map} that further provides a <em>total ordering</em> on its keys. The map is ordered according to the {@linkplain Comparable natural ordering} of its keys, or by a {@link Comparator} typically provided at sorted map creation time. This order is reflected when iterating over the sorted map's collection views (returned by the {@code entrySet}, {@code keySet} and {@code values} methods). Several additional operations are provided to take advantage of the ordering. (This interface is the map analogue of {@link SortedSet}.)