Answer 1

You might have some difficulties getting Java Applets working with today's operating systems and browsers, as described in the historical article here:
https://javatester.org/
I include that because it has simple discussions/examples of how Java/applets work in general (sufficient to start/keep researching).
Note that to see output from Java on SoloLearn, you need to write to the standard output stream and the process should terminate.