code: public int position(String fieldName) {
      if (fieldPositions.containsKey(fieldName)) {
        return fieldPositions.get(fieldName);
      } else {
        throw new DatasetException("Cannot recover " + fieldName + " from key");
      }
    }
docs: Resolves the storage position of a field in keys with this KeySchema.
  
  @param fieldName
  The source field name.
  @return
  The position of the field in keys with this schema.
  @throws DatasetException
  If the field cannot be recovered from keys with this schema.