Java UUID generation example

JavaUuidGenerator.java

import java.net.NetworkInterface;
import java.net.SocketException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Enumeration;
import java.util.UUID;

public class JavaUuidGenerator {

    public static void main(String[] args) {
        System.out.println("Version 1 UUID: " + generateVersion1UUID());
        System.out.println("Version 4 UUID: " + generateVersion4UUID());
        System.out.println("Nil UUID: " + generateNilUUID());
    }

    public static UUID generateVersion1UUID() {
        try {
            byte[] mac = getMACAddress();
            long timestamp = (System.currentTimeMillis() * 10000) + 0x01B21DD213814000L;
            timestamp &= ~(0xF000L);
            timestamp |= 0x1000L;
            byte[] uuidBytes = new byte[16];
            System.arraycopy(mac, 0, uuidBytes, 10, 6);
            for (int i = 0; i < 8; i++) {
                uuidBytes[15 - i] = (byte) (timestamp >>> (i * 8));
            }
            MessageDigest md = MessageDigest.getInstance("MD5");
            byte[] hash = md.digest(uuidBytes);
            hash[6] &= 0x0F;
            hash[6] |= 0x10;
            hash[8] &= 0x3F;
            hash[8] |= 0x80;
            long mostSigBits = 0;
            long leastSigBits = 0;
            for (int i = 0; i < 8; i++) {
                mostSigBits = (mostSigBits << 8) | (hash[i] & 0xFF);
                leastSigBits = (leastSigBits << 8) | (hash[i + 8] & 0xFF);
            }
            return new UUID(mostSigBits, leastSigBits);
        } catch (SocketException | NoSuchAlgorithmException e) {
            throw new RuntimeException(e);
        }
    }

    public static UUID generateVersion4UUID() {
        return UUID.randomUUID();
    }

    public static UUID generateNilUUID() {
        return new UUID(0L, 0L);
    }

    private static byte[] getMACAddress() throws SocketException {
        Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
        while (interfaces.hasMoreElements()) {
            NetworkInterface networkInterface = interfaces.nextElement();
            if (!networkInterface.isLoopback() && networkInterface.getHardwareAddress() != null) {
                return networkInterface.getHardwareAddress();
            }
        }
        throw new RuntimeException("MAC address not found");
    }

}