A modern programming language that makes developers happier.

kotlinlang

Good morning everyone!
I'd like to know how to add an icon to the GUI, in other words the window that's opened after running the program. (I have no experience whit JavaFX).

if you mean the taskbar icon in windows or doc icon in Mac - once you package the application, you can add an icon.

There are many third-party tools
Maybe try this Oracle article as a start:
<https://docs.oracle.com/javafx/2/deployment/self-contained-packaging.htm>