code: public static double getTotalMassNumber(IMolecularFormula formula) {
          double mass = 0.0;
          for (IIsotope isotope : formula.isotopes()) {
              try {
                  IIsotope isotope2 = Isotopes.getInstance().getMajorIsotope(isotope.getSymbol());
                  if (isotope2 != null) {
                      mass += isotope2.getMassNumber() * formula.getIsotopeCount(isotope);
                  }
              } catch (IOException e) {
                  e.printStackTrace();
              }
          }
          return mass;
      }
docs: Get the summed mass number of all isotopes from an MolecularFormula. It
  assumes isotope masses to be preset, and returns 0.0 if not.
  
  @param  formula The IMolecularFormula to calculate
  @return         The summed nominal mass of all atoms in this MolecularFormula