Many problems as beginner + GUI + HelloWorld

I am trying to make a HelloWorld app with Swing (GTK3) in IntelliJIdea and I turning crazy.

I am looking for examples and reading, and reading again, and little problems always happens.
My snap is able to run (I learned experimetally how structure it), but is not “connecting with GUI” or something like this… and looks awful
*(Using UIManager.setLookAndFeel(info.getClassName()) inside try-catch)

Can anyone from snapcraft provide some updated templates? And to explain best practices 4 developers to choose between GTK3 vs QT, etc.¿
*Maybe it is me that I’m noob, but I wana believe that github repos are deprecated, so a little help please.