CompactUuid.java

import java.nio.ByteBuffer;
import java.util.Base64;
import java.util.UUID;

class CompactUuid {

	public static void main(String[] args) {
		final UUID originalUuid = UUID.randomUUID();
		System.out.println("Original: " + originalUuid);

		final String encoded = encode(originalUuid);
		System.out.println("Encoded:  " + encoded);

		final UUID decoded = decode(encoded);
		System.out.println("Decoded:  " + decoded);
		
		assert originalUuid.equals(decoded);
	}

	private static String encode(UUID uuid) {

		final long mostSignificantBits = uuid.getMostSignificantBits();
		final long leastSignificantBits = uuid.getLeastSignificantBits();

		final ByteBuffer inputByteBuffer = ByteBuffer.allocate(Long.BYTES * 2);
		inputByteBuffer.putLong(mostSignificantBits);
		inputByteBuffer.putLong(leastSignificantBits);

		final byte[] bytes = inputByteBuffer.array();
		return Base64.getEncoder().encodeToString(bytes);
	}

	private static UUID decode(String base64String) {

		final byte[] bytes = Base64.getDecoder().decode(base64String);

		final ByteBuffer byteBuffer = ByteBuffer.allocate(Long.BYTES * 2);
		byteBuffer.put(bytes);
		byteBuffer.rewind();

		final long mostSignificantBits = byteBuffer.getLong();
		final long leastSignificantBits = byteBuffer.getLong();

		return new UUID(mostSignificantBits, leastSignificantBits);
	}

}