code: public IIsotope getIsotope(String symbol, int massNumber) {
          int elem = Elements.ofString(symbol).number();
          List<IIsotope> isotopes = this.isotopes[elem];
          if (isotopes == null)
              return null;
          for (IIsotope isotope : isotopes) {
              if (isotope.getSymbol().equals(symbol) && isotope.getMassNumber() == massNumber) {
                  return clone(isotope);
              }
          }
          return null;
      }
docs: Get isotope based on element symbol and mass number.
  
  @param symbol the element symbol
  @param massNumber the mass number
  @return the corresponding isotope