UUID

UUID → Byte

public byte[] getIdAsByte(UUID uuid)
{
    ByteBuffer bb = ByteBuffer.wrap(new byte[16]);
    bb.putLong(uuid.getMostSignificantBits());
    bb.putLong(uuid.getLeastSignificantBits());
    return bb.array();
}

Byte → UUID

ByteBuffer b = ByteBuffer.wrap(byteArray);
UUID uuid = new UUID(b.getLong(), b.getLong());