Well, it all depends on how much you would like to complicate it. One simple way would be to get bytes from the String and convert them to a number, and then conver the number back to bytes and get the String, provided that you can use the same encoding for both operations all the time.
Anohter way could be encryption. See Java Almanac for an example.
I’ve looked at a lot of different solutions, and in my humble opinion Aspose is the way to go. Here’s the link: http://aspose.com