code: private static int getMonomerCountFromMonomerNotation(MonomerNotation monomerNotation) {
      int multiply;
      try {
        multiply = Integer.parseInt(monomerNotation.getCount());
        if (multiply < 1) {
          multiply = 1;
        }
      } catch (NumberFormatException e) {
        multiply = 1;
      }
  
      if (monomerNotation instanceof MonomerNotationGroup) {
        return 1 * multiply;
      }
      if (monomerNotation instanceof MonomerNotationList) {
        int count = 0;
        for (MonomerNotation unit : ((MonomerNotationList) monomerNotation).getListofMonomerUnits()) {
          count += getMonomerCountFromMonomerNotation(unit);
        }
        return count * multiply;
      }
  
      if (monomerNotation instanceof MonomerNotationUnitRNA) {
        int count = 0;
        for (MonomerNotationUnit unit : ((MonomerNotationUnitRNA) monomerNotation).getContents()) {
          count += getMonomerCountFromMonomerNotation(unit);
        }
        return count * multiply;
      }
      return 1 * multiply;
    }
docs: method to get the number of all existing monomers from one MonomerNotation
  
  @param monomerNotation MonomerNotation
  @return number of monomers in the given MonomerNotation