What is the content of the $CLASSPATH variable in El Capitan?

I installed OS X El Capitan and, along with it, Oracle's Java 8 SDK. I've already verified that, using java -version, java is correctly installed. I need to add some user classes to the file path, but, when I use echo $CLASSPATH, I cannot see the actual path being used by OS X. How can I know where should I install my user classes?