I've not used Eclipse for many years (prefer IJ Idea) but I would start by taking a look at one of our sample apps. This is a good basic one to get going with:
arcgis-runtime-samples-java/map/display-map at master · Esri/arcgis-runtime-samples-java · GitHub
You will see the structure of the app contains a launcher class which is needed for JavaFX 11 apps to work in a runnable jar.
If you use gradle, then run the jar task which will build you a runnable jar. In the project above this will appear in the build/libs directory.
Remember that when you deploy, you will need the native libraries next to the jar.
This doc explains the structure above: Deploy your app—ArcGIS Runtime SDK for Java | ArcGIS for Developers
Depending on how your machine is set up you can double click on the jar or just do a 'java -jar' from the command line.
Does this help?