class Utils {
    /**
     * A {@link DateTimeFormatter} like {@link DateTimeFormatter#ISO_INSTANT} with
     * the exception that it always appends exactly three fractional digits (nano
     * seconds).
     *
     * <p>
     * This is required in order to guarantee natural sorting, which enables us to
     * use <code>&lt;=
     * </code> comparision in queries.
     *
     * <pre>
     * 2018-12-07T12:30:37.000Z
     * 2018-12-07T12:30:37.810Z
     * 2018-12-07T12:30:37.819Z
     * 2018-12-07T12:30:37.820Z
     * </pre>
     *
     * <p>
     * When using variable fractional digit count as done in
     * {@link DateTimeFormatter#ISO_INSTANT ISO_INSTANT} and
     * {@link DateTimeFormatter#ISO_OFFSET_DATE_TIME ISO_OFFSET_DATE_TIME} the
     * following sorting occurs:
     *
     * <pre>
     * 2018-12-07T12:30:37.819Z
     * 2018-12-07T12:30:37.81Z
     * 2018-12-07T12:30:37.820Z
     * 2018-12-07T12:30:37Z
     * </pre>
     *
     * @see <a href="https://stackoverflow.com/a/5098252">natural sorting of ISO
     *      8601 time format</a>
     */
    private static final DateTimeFormatter formatter = new DateTimeFormatterBuilder()
            .parseCaseInsensitive()
            .appendInstant(3)
            .toFormatter();

    private static final String hostname = initHostname();

    private Utils() {}

    public static String getHostname() {
        return hostname;
    }

    public static String toIsoString(Instant instant) {
        OffsetDateTime utc = instant.atOffset(ZoneOffset.UTC);
        return formatter.format(utc);
    }

    private static String initHostname() {<FILL_FUNCTION_BODY>}
}

        try {
            return InetAddress.getLocalHost().getHostName();
        } catch (UnknownHostException e) {
            return "unknown";
        }
    
<no_super_class>