java retrieve a mac address

mac.java

/**
 * get local mac address as a string. useful for scheduling tasks to happen at the same time every day on a given machine, but at different times across all machines
 */
static String mac() {
    final ArrayList<NetworkInterface> ifaces;
    try {
        ifaces = Collections.list(NetworkInterface.getNetworkInterfaces());
        // sort the interfaces by name so we consistently pick the same interface
        Collections.sort(ifaces, new Comparator<NetworkInterface>() {
            @Override
            public int compare(final NetworkInterface o1, final NetworkInterface o2) {
                return o1.getName().compareTo(o2.getName());
            }
        });
    } catch (SocketException e) {
        throw new ContrastException("unable to find network interfaces", e);
    }
    String mac = null;
    for (NetworkInterface iface : ifaces) {
        final byte[] addr;
        try {
            addr = iface.getHardwareAddress();
        } catch (SocketException e) {
            continue;
        }
        if (addr == null || addr.length == 0) {
            continue;
        }
        // TODO:JAVA8 map + join
        final StringBuilder builder = new StringBuilder();
        for (final byte macByte : addr) {
            final String hex = String.format("%02X", macByte);
            builder.append(hex);
        }
        mac = builder.toString();
        break;
    }
    if (mac == null) {
        throw new ContrastException("unable to find network interface with a mac address");
    }
    return mac;
}