code: public ColumnMapperSingle<?> getMapperSingle(String field) {
          ColumnMapper columnMapper = getMapper(field);
          if (columnMapper != null && columnMapper instanceof ColumnMapperSingle<?>) {
              return (ColumnMapperSingle<?>) columnMapper;
          } else {
              return null;
          }
      }
docs: Returns the {@link com.stratio.cassandra.index.schema.mapping.ColumnMapperSingle} identified by the specified
  field name, or {@code null} if not found.
  
  @param field A field name.
  @return The {@link com.stratio.cassandra.index.schema.mapping.ColumnMapperSingle} identified by the specified
  field name, or {@code null} if not found.