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;
    }
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 ; }
Get isotope based on element symbol and mass number.

@param symbol the element symbol
@param massNumber the mass number
@return the corresponding isotope