[sagher]

You can see from the output of $java - version that openjdk 8 is already installed in @sagher 's original post.

As far as I know there isn't a deb package for the oracle version; the only packaged version is an RPM. It can be built from source. https://docs.oracle.com/en/java/javase/1...D0B48C346A

Yes i just follow the link you given but command line not works. how can i help you to sort out actual issue? 
The destination Link is bellow:

Installation of the JDK on Linux Platforms (oracle.com)

but i unable to find the installing candidate of this

Code: (Select All)

jdk-15_linux-x64_bin.tar.gz