code: public double getExactMass(Integer atomicNumber, Integer massNumber) {
          if (atomicNumber == null || massNumber == null)
              return 0;
          for (IIsotope isotope : this.isotopes[atomicNumber]) {
              if (isotope.getMassNumber().equals(massNumber))
                  return isotope.getExactMass();
          }
          return 0;
      }
docs: Get the exact mass of a specified isotope for an atom.
  @param atomicNumber atomic number
  @param massNumber the mass number
  @return the exact mass