what is the main difference between Java FX and Java Swing?

Answer 1

Swing since its advent can create UI component using standard Java component classes whereas Java FX initially uses a declarative language called JavaFX script. Swing has a UI component library, and act as a legacy whereas JavaFX has several components built over Swing.
read more here:
https://www.quora.com/What-is-the-difference-between-Java-Swing-and-JavaFX

Answer 2

https://www.dummies.com/programming/java/10-differences-between-javafx-and-swing/

You can also have look into the oracle tutorial:
https://docs.oracle.com/javafx/2/get_started/jfxpub-get_started.htm

Answer 3

FX is about a decade newer and provide richer graphics.