To clarify the question; are you looking for the ciphertext to be from
the alphanumeric set?  Or you need to use an alphanumeric key?  The
example demonstrates the latter; I suspect you're really looking for the
former.

To convert a arbitrary binary ciphertext to a printable character set,
you would want to apply an encoding scheme.  Typically base64 is used
(although there are a few characters outsize [a-zA-Z0-9]).  You can also
encode the output in hexadecimal (zero through nine, and 'a' through 'f').

There's further explanation and example code for both at Roedy's site:

http://mindprod.com/jgloss/base64.html
http://mindprod.com/jgloss/hex.html

That process is called "armouring".  Often all you have to do is
request it with a boolean on some encryption method.  If binary byte[]
output is all that is provided, you can use base64, the most common
way of making binary printable.  See
http://mindprod.com/jgloss/base64.html

--
Canadian Mind Products, Roedy Green.
Coaching, problem solving, economical contract programming.
See http://mindprod.com/jgloss/jgloss.html for The Java Glossary.