Open a command line interface and enter the following (Ubuntu):
$ sudo add-apt-repository ppa:webupd8team/java $ sudo apt-get update $ sudo apt-get install oracle-java8-installer $ sudo apt-get install oracle-java8-set-default |
For other *nix variants you may have to add other repos or manually download and install Java.
*Do not use OpenJDK because some component you might need down to road e.g. Djatoka will only work with Oracle Java