code: public static void setColumnNumber(Configuration conf, int columnNum) {
      assert columnNum > 0;
      conf.setInt(RCFile.COLUMN_NUMBER_CONF_STR, columnNum);
    }
docs: set number of columns into the given configuration.
  
  @param conf
  configuration instance which need to set the column number
  @param columnNum
  column number for RCFile's Writer