code: public static <K,V> SortedMap<K,V> unmodifiableSortedMap(SortedMap<K, ? extends V> m) {
          return new UnmodifiableSortedMap<>(m);
      }
docs: Returns an unmodifiable view of the specified sorted map.  This method
  allows modules to provide users with "read-only" access to internal
  sorted maps.  Query operations on the returned sorted map "read through"
  to the specified sorted map.  Attempts to modify the returned
  sorted map, whether direct, via its collection views, or via its
  <tt>subMap</tt>, <tt>headMap</tt>, or <tt>tailMap</tt> views, result in
  an <tt>UnsupportedOperationException</tt>.<p>
  
  The returned sorted map will be serializable if the specified sorted map
  is serializable.
  
  @param <K> the class of the map keys
  @param <V> the class of the map values
  @param m the sorted map for which an unmodifiable view is to be
  returned.
  @return an unmodifiable view of the specified sorted map.