Hi,
to change a Jar file you can change its extension from ".jar" to ".zip".
This enables you to use the zip tool of your choice to unzip, change and re-zip the archive.
Finally, don't forget to rename the zip to jar again.
To get the changed class file into a jar, you'ld have to compile the java source into a binary class file.
Things get complicated, if the jar file is protected by a Manifest entry.
Good luck!
A1880