code: public static <K,V> NavigableMap<K,V> unmodifiableNavigableMap(NavigableMap<K, ? extends V> m) {
          return new UnmodifiableNavigableMap<>(m);
      }
docs: Returns an unmodifiable view of the specified navigable map.  This method
  allows modules to provide users with "read-only" access to internal
  navigable maps.  Query operations on the returned navigable map "read
  through" to the specified navigable map.  Attempts to modify the returned
  navigable map, whether direct, via its collection views, or via its
  {@code subMap}, {@code headMap}, or {@code tailMap} views, result in
  an {@code UnsupportedOperationException}.<p>
  
  The returned navigable map will be serializable if the specified
  navigable map is serializable.
  
  @param <K> the class of the map keys
  @param <V> the class of the map values
  @param m the navigable map for which an unmodifiable view is to be
  returned
  @return an unmodifiable view of the specified navigable map
  @since 1.8