return null;
      }
      V result = Maps.safeRemove(backingRowMap, key);
      maintainEmptyInvariant();
      return result;
    }

    @Override
    public void clear() {
      updateBackingRowMapField();
      if (backingRowMap != null) {
        backingRowMap.clear();
      }
      maintainEmptyInvariant();
    }

    @Override
    public int size() {
      updateBackingRowMapField();