In short, you can't.  The best you can achieve is obfuscatation to make  
things more difficult, but there is nothing you can do to protect your  
code from a determined reverse engineer.

You may have heard of schemes for encrypting class files, but these do not  
work (see  
http://www.javaworld.com/javaworld/javaqa/2003-05/01-qa-0509-jcrypt.html).

For a good, free obfuscator, try Proguard  
(http://proguard.sourceforge.net).

Dan.

You can't.
If you provide the information needed in order to run the code, you provide
the information needed in order to reverse engineer it.

Alun Harford

Don't worry about encrypting code.  There's no really good reason for it.
For any sufficiently complex peice of code, it's easier for someone to
write it themselves than use a decompiler to reverse engineer it.  If
you've actually put enough effort into it to worry about someone stealing
it, either it's too complex for them to be able to want to, or you put
too much effort into the code.  ;-)

Don't worry about encrypting code.  There's no really good reason for it.
For any sufficiently complex peice of code, it's easier for someone to
write it themselves than use a decompiler to reverse engineer it.  If
you've actually put enough effort into it to worry about someone stealing
it, either it's too complex for them to be able to want to, or you put
too much effort into the code.  ;-)

Obfuscators should be a sufficient deturrent.

No, or otherwise how could the machine run it? The computer has to
know the key.

But you can make it more difficult to reverse engineer.  see
http://mindprod.com/jgloss/obfuscator.html