Setting up Oracle Java 9 on Ubuntu
Since Oracles own instructions (https://docs.oracle.com/javase/9/install/installation-jdk-and-jre-linux-platforms.htm) sucks big time, I can recommend this one instead:
https://www.digitalocean.com/community/tutorials/how-to-install-java-with-apt-get-on-ubuntu-16-04
Simple and easy to follow. And it WORKS!