I have tested the Java Swing support on repl.it but it seems that it is not working properly, and I don´t know the reason.
The question is that the code compiles flawlessly but as a result you obtain a frame (see the image) in which is difficult to see your GUI and also is not interactive.
¿What's happening here? ¿Any clue?
I think this is a problem with all graphics repls currently. It will probably be fixed soon.