once installed, you'll need to install Xorg (X11) for gui programs
Not sure how much freebsd you might know... The package manager is pkg, in the same way that apt would work, so to install xorg use this command:
pkg install xorg 'it will run for a while installing many packages but that should install everything
or
pkg xorg-minimal 'if you're short on space on the sdcard
The first time you run pkg it will update and get the lists and then move on to installing whatever pkg you wanted
as far as java is concerned, you can find which ones are available, using this command
there a number of openjdk available in the ports tree, to search run this command
if you need access to the GPIO check this link:
Raspberry Pi is based on Broadcom BCM2835 chip. We can find description of Broadcom BCM2835 Peripherals here. There are 54 general-purpose I/O (GPIO) lines split into two banks. Here is instruction…
I think you'll find a number of advantages over Rasbian, the biggest one being speed and security
feel free to shoot me a PM if you have any questions... I am actually in the process creating my own image that has everything ready to go for B4J but I haven't had a lot of time recently. I don't think it will take me much longer to finish
Larry