code: public DBObject get() {
          for (final String key : _query.keySet()) {
              if (_query.get(key) instanceof NullObject) {
                  throw new QueryBuilderException("No operand for key:" + key);
              }
          }
          return _query;
      }
docs: Creates a {@code DBObject} query to be used for the driver's find operations
  
  @return {@code this}
  @throws RuntimeException if a key does not have a matching operand