How to create a jar file:

jar cvf jar-file input-file(s)

For instance: go to the directory containing the .java and .class files, and type

jar cvf groupxx.jar *

Or if you only want to send the .java files:

jar cvf groupxx.jar *.java

Now you can send the file groupxx.jar.

How to unpack a jar file:

jar xvf jar-file

For instance:

jar xvf groupxx.jar