You can get a early access build for ARM hard float from http://jdk8.java.net/download.html. This bundle should be unpacked on the Pi. For example,
sudo tar zxvf jdk-8-ea-b97-linux-arm-vfp-hflt-03_jul_2013.tar.gz -C /opt
To check that the JDK is installed correctly, run:
This should show that you are running an EA build of JDK 8. If the VM won't even start, you might be running a hard-float VM on a soft-float system.
You can run these applications without any additional parameters. For example,
/opt/jdk1.8.0/bin/java -cp Stopwatch.jar stopwatch.MainScreen
Note that the default configuration of JavaFX on the Raspberry Pi does not use X11. Instead JavaFX works directly with the display framebuffer and input devices. So you should not have the X11 desktop running when starting JavaFX.