UUID v3 with namespace in Java (examples)

UUIDv3WithNamespace1.java

package com.example;

import java.nio.ByteBuffer;

import java.nio.charset.StandardCharsets;
import java.util.UUID;

public class UUIDv3WithNamespace1 {

	/**
	 * Returns a name-based UUID v3 using a namespace.
	 * 
	 * @param namespace a UUID
	 * @param name      a string
	 * @return a UUID
	 */
	public static UUID getNameBased(UUID namespace, String name) {

		// 1. Get the NAMESPACE bytes
		final byte[] nameBytes = name.getBytes(StandardCharsets.UTF_8);

		// 2. Concatenate NAMESPACE and NAME bytes
		ByteBuffer buffer = ByteBuffer.allocate(nameBytes.length + 16);
		buffer.putLong(namespace.getMostSignificantBits());
		buffer.putLong(namespace.getLeastSignificantBits());
		buffer.put(nameBytes);

		// 3. Generate a name-based UUID
		return UUID.nameUUIDFromBytes(buffer.array());
	}

	public static void main(String[] args) {
		UUID namespace = UUID.fromString("11111111-2222-3333-4444-555555555555");
		String name = "this is a test";
		UUID uuid = getNameBased(namespace, name);
		System.out.println(uuid); // prints: 04334e59-716d-3078-9722-63e2ec20d4ec
	}
}

UUIDv3WithNamespace2.java

package com.example;

import java.nio.charset.StandardCharsets;
import java.util.UUID;

public class UUIDv3WithNamespace2 {

	/**
	 * Returns a name-based UUID v3 using a namespace.
	 * 
	 * @param namespace a UUID
	 * @param name      a string
	 * @return a UUID
	 */
	public static UUID getNameBased(UUID namespace, String name) {

		// 1. Get NAMESPACE and NAME bytes
		final byte[] msb = toBytes(namespace.getMostSignificantBits());
		final byte[] lsb = toBytes(namespace.getLeastSignificantBits());
		final byte[] nam = name.getBytes(StandardCharsets.UTF_8);

		// 2. Concatenate NAMESPACE and NAME bytes
		final byte[] bytes = new byte[16 + nam.length];
		System.arraycopy(msb, 0, bytes, 0, 8);
		System.arraycopy(lsb, 0, bytes, 8, 8);
		System.arraycopy(nam, 0, bytes, 16, nam.length);

		// 3. Generate a name-based UUID
		return UUID.nameUUIDFromBytes(bytes);
	}

	private static byte[] toBytes(final long number) {
		return new byte[] { (byte) (number >>> 56), (byte) (number >>> 48), (byte) (number >>> 40),
				(byte) (number >>> 32), (byte) (number >>> 24), (byte) (number >>> 16), (byte) (number >>> 8),
				(byte) (number) };
	}

	public static void main(String[] args) {
		UUID namespace = UUID.fromString("11111111-2222-3333-4444-555555555555");
		String name = "this is a test";
		UUID uuid = getNameBased(namespace, name);
		System.out.println(uuid); // prints: 04334e59-716d-3078-9722-63e2ec20d4ec
	}
}

The first example uses ByteBuffer to concatenate namespace and name. The second one uses System.arraycopy(). This is the only difference.