code: public static <K, V> BiMap<K, V> unmodifiableBiMap(BiMap<? extends K, ? extends V> bimap) {
      return new UnmodifiableBiMap<K, V>(bimap, null);
    }
docs: Returns an unmodifiable view of the specified bimap. This method allows
  modules to provide users with "read-only" access to internal bimaps. Query
  operations on the returned bimap "read through" to the specified bimap, and
  attempts to modify the returned map, whether direct or via its collection
  views, result in an {@code UnsupportedOperationException}.
  
  <p>The returned bimap will be serializable if the specified bimap is
  serializable.
  
  @param bimap the bimap for which an unmodifiable view is to be returned
  @return an unmodifiable view of the specified bimap