Hexadecimal is a custom Charset that I wrote. I don't have it lying around anymore, but I can rewrite it for you if you give me a few hours.
User is also a custom class. Looking back at the example I wrote, it needs at least email and firstName fields, but you can add anything else to it that you need for your application. I notice that the RegisterServlet that I wrote doesn't take the user's first name, so you might still need to fix that.