code: public WorkbookReader turnToSheet(int index) {
      checkState(!isClosed, WORKBOOK_CLOSED);
      sheet = workbook.getSheetAt(index);
      setHeader();
      return this;
    }
docs: Turns this {@link WorkbookReader} to certain sheet. Sheet names can be
  found by {@link #getAllSheetNames}.
  
  @param index
  of a sheet
  @return this {@link WorkbookReader}