$ java -version
openjdk version "12.0.2" 2019-07-16
OpenJDK Runtime Environment (build 12.0.2+10)
OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode, sharing)

javac --module-path %PATH_TO_FX% --add-modules javafx.controls HelloFX.java

export PATH_TO_FX=path/to/javafx-sdk-14/lib

javac --module-path $PATH_TO_FX --add-modules javafx.controls HelloFX.java

set PATH_TO_FX="path\to\javafx-sdk-14\lib"

javac --module-path $PATH_TO_FX --add-modules javafx.controls,javafx.fxml HelloFX.java