JavaFx is a comprehensive platform and set of APIs to develop desktop, web and even mobile applications. Although intended to replace the Swing GUI API, it can get significantly more complex, with FXML for XML based object specification; CSS styling; and special IDE tool chains.
Since JDK 8, it is supposedly included in the main install, however, on Debian I had to install it separately:
The Oracle documentation and tutorial is detailed and well written, so I will not go into further detail here. The following is a simple Hello World app.