Swingx is not the same as swing, and it is not clear from what has been said if swingx is actually required, or if there is just some confusion. Some more detail of what was actually downloaded would help.
I am only an amateur as far as software goes, so there lies my misunderstanding. As Phemisters says, swingx is a demo application. AWT and SWING must be in jdk itself.
The java code, from https://blog.henrypoon.com/blog/2011/01/01/serial-communication-in-java-with-example-program/
compiles without warning on AWT and SWING. So, javac is finding them.