After extensive effort in running one single java file, I eventually reached the step of compiling a file. But then upon reading the steps of pi4j's guide http://pi4j.com/install.html, i realized that i must compile with the class file from pi4j's library. But it does not exist.
I do not understand what I am doing wrong, as I am folowing the exact steps from the link above and yet I cannot ever seem to attain those folders of /opt/pi4j.
I also tried making them myself before installing the package, and that did not work