Can't seem to get my plugins working on other servers, Please Help!

I recently created a plugin and can't get it to work on a friend's server. I referenced spigot-1.8.3.jar which is what he used. When I transfer the pluginname.jar file to his plugins folder do I have to re-reference it or how does it work? Assuming I created the plugin with spigot-1.8.3.jar from my computer files and used up-to-date docs, etc and there's no errors on my local server, i need help on how to load it up on my friend's server. Please help

Error:
UnsupporterClassVersionError

 

I've had this error too. Be sure you're compiling your Java Plugin in Java Version 7, and not Java 8. Otherwise, for whatever reason, it will run on a local copy, but not on an official server. Maybe that will help.

 

Yay It worked! So it's a version incompatibility error. Oh, it's probably because BuildTools was only running on Java 7. That's why most servers have Java 7. I see. Java really needs to give understandable names to its errors. Thanks! It was really helpful