Installing FreeBSD's JDK is typically straightforward:

cd /usr/ports/java/jdk
make
make install
make clean

The JDK does not require any dependencies to install. However, if you wish to run any graphics, you will need to have X installed and running.

Many applications in Java need Java Foundation Classes (Swing) to run. JFC can be installed in: /usr/ports/java/jfc

If you should have trouble, you can see the FreeBSD Handbook section on "installing applications" for help.