@SuppressWarnings("ConstantConditions")
  private ImmutableBiMap<K,V> toImmutableBiMap() {
    final ImmutableBiMap.Builder<K,V> ret = ImmutableBiMap.builder();

    for (final MapEntry<K, V> entry : entries) {
      ret.put(entry.key, entry.value);
    }

    return ret.build();
  }
@ SuppressWarnings ( "ConstantConditions" ) private ImmutableBiMap < K , V > toImmutableBiMap ( ) { final ImmutableBiMap . Builder < K , V > ret = ImmutableBiMap . builder ( ) ; for ( final MapEntry < K , V > entry : entries ) { ret . put ( entry . key , entry . value ) ; } return ret . build ( ) ; }
if any element is null