*
     * <p>Prefer the equivalent method {@link #buildOrThrow()} to make it explicit that the method
     * will throw an exception if there are duplicate keys. The {@code build()} method will soon be
     * deprecated.
     *
     * @throws IllegalArgumentException if any two keys are equal according to the comparator (which
     *     might be the keys' natural order)
     */
    @Override