Yes, you can decompile your hello.class file to hello.java.
In fact you can decompile any .class file...
For details of decompilation process, go to :
http://en.wikipedia.org/wiki/Decompiler
Many decompilation tools are available in market...
However for your ease, you may try any one of these.
Installing and usage instructions are written there.
DJ :
http://members.fortunecity.com/neshkov/dj.html
CAVAJ :
http://www.bysoft.se/sureshot/cavaj/
JAD :
http://www.kpdus.com/jad.html
MOCHA :
http://www.brouhaha.com/~eric/software/mocha/
I personally feels JAD is best....
I have tried JAD and DJ.