code: public <K, V> Map<K, V> unmodifiable(Map<K, V> input)
      {
          if (input == null) {
              return null;
          }
          return Collections.unmodifiableMap(input);
      }
docs: Returns an unmodifiable view of the specified map.
  
  @param <K> the type of keys maintained by this map
  @param <V> the type of mapped values
  @param input the map to wrap in an unmodifiable bridge
  @return an unmodifiable view of the map