code: public IIsotope[] getIsotopes(int elem) {
          if (isotopes[elem] == null)
              return EMPTY_ISOTOPE_ARRAY;
          List<IIsotope> list = new ArrayList<>();
          for (IIsotope isotope : isotopes[elem]) {
              list.add(clone(isotope));
          }
          return list.toArray(new IIsotope[0]);
      }
docs: Gets an array of all isotopes known to the IsotopeFactory for the given
  element symbol.
  
  @param elem atomic number
  @return An array of isotopes that matches the given element symbol