code: public double getMajorIsotopeMass(int elem) {
          if (this.majorIsotope[elem] != null)
              return this.majorIsotope[elem].getExactMass();
          IIsotope major = getMajorIsotope(elem);
          return major != null ? major.getExactMass() : 0;
      }
docs: Get the mass of the most abundant (major) isotope, if there is no
  major isotopes 0 is returned.
  
  @param elem the atomic number
  @return the major isotope mass