1. You don't need to de-compile the java class file.
In the "Plugin developer's guide", at the end of "Creating the plug-in class" you have a sentences

The VolumePlugin plug-in class is now programmed, ...

with a link to the java file. You will see the differences between your and the original.
2. Continue carefully with the rest of the tutorial especially with the export part where I suggest to check the Export java source files and resources.
Here you can change the extension from jar to sh3p but it is not mandatory. The plugin should work even with jar extension.
Don't forget to Export the plugin BEFORE every test to see the effects of your modifications.
3. To verify what you've done, start SH3D from a terminal.
If something's wrong you will see some messages like The plugin [name_of_plugin] can't be loaded and the reason why.
4. Don't expect to succeed from the first attempt.
After you manage to pass this we could continue to help you.

PS. I set the eclipse workspace to use the jre1.8 provided by the SH3D.