I don't think you can edit an external resource like gridworld.jar. You are going to need to create 4 new packages:
- actor
- grid
- gui
- world
Then copy the class files from gridworld.jar packages into your corresponding ones. Now you can edit the files. Make sure you include any miscellaneous files that might also be in the gridworld.jar packages, their important.
With your own packages you no longer need the gridworld.jar external resource, and there is a difference in your import statements. Instead of
import gridworld.actor.Actor;
Do this:
import actor.Actor;
Note: You will need to change all of the import statements in the new package classes to reference the other new packages. I believe that there were also some errors, not project breaking, but I just added suppressors.