Install Java 9 on Ubuntu

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java9-installer

if you want to set Oracle Java 9 as default

sudo apt-get install oracle-java9-set-default

If you don’t want to make Oracle Java 9 default

sudo apt-get install --no-install-recommends oracle-java9-installer

References
http://www.webupd8.org/2015/02/install-oracle-java-9-in-ubuntu-linux.html