A Brief Introduction…
It all started when a co-worker asked for help when trying to update an IP Address of a java API, however, he only had the JAR file available. With another co-worker, we dug up the internet and tried to find a way to do this, and we finally did. I thought of sharing what we acquire for any future help.
In order to do any of the operations mentioned in the title, you will need to have java and JDK installed, a desired jar you want to modify, and basic knowledge of commands for the terminal. In order to get the .java file from the JAR, I would recommend a program called JD-GUI. You will just need to open the jar file, and look for a desired file and save it.