JavaFX is no longer part of the jdk. You need to install it separately. The following provides some instructions on how to install JavaFX to run the examples available at GUICodeFX.zip. JavaFX documentation can be found at https://openjfx.io/.
--module-path C:\Users\Nelson\Desktop\javafx-sdk-16\lib --add-modules=javafx.controls
When you cut and paste, you might only see the end of the above information in Eclipse. Now select "Run" to run the example. Important:in a Mac, uncheck the option "Use the -XstartOnFirstThread argument when launching with SWT", otherwise the program will not run. If you try to run an example and you see the error message "Graphics Device initialization failed" you need to update some library (this is specific to your computer/environment). A google search could provide some leads.
--module-path /Users/nelson/Desktop/javafx-sdk-16/lib --add-modules=javafx.controls