Make run always launch the last-run application
Annoyingly, the "run" button (the green arrow) tries to run the current class if possible. That can be a pain if you're trying to test an application, but some of your classes have mains in them -- you'll often launch the wrong thing.
To get the preferred behavior back -- that is, to re-run whatever was explicitly launched last -- go to Run/Debug...Launching and select the "Always launch previously launched application" under the Launch Operation section.