* {@link #create(Object...)}.
     */
    protected abstract List<Integer> create(Integer[] elements);

    @Override
    public Integer[] createArray(int length) {
      return new Integer[length];
    }

    /** Returns the original element list, unchanged. */
    @Override
    public List<Integer> order(List<Integer> insertionOrder) {

     * {@link #create(Object...)}.
     */
    protected abstract List<Double> create(Double[] elements);

    @Override
    public Double[] createArray(int length) {
      return new Double[length];
    }

    /** Returns the original element list, unchanged. */
    @Override
    public List<Double> order(List<Double> insertionOrder) {