You are on Debian 10 (buster), but the link you showed is for Debian 9 (stretch).

OpenJDK is on version 11 for Buster and OpenJDK 8 is not available.

https://packages.debian.org/buster/openjdk-11-jdk

If you need this, use a docker image based on Stretch.

UPDATE: installation without root privileges below

I advise you to not install packages manually on ubuntu system if there is already a (semi-official) repository able to solve your problem. Further, use Oracle JDK for development, just to avoid (very sporadic) compatibility issues (i've tried many years ago, it's surely better now).

Add the webupd8 repo to your system:

Copysudo add-apt-repository ppa:webupd8team/java
sudo apt-get update

Install your preferred version of jdk (versions from java-6 to java-9 available):

Copysudo apt-get install oracle-java8-installer

You can also install multiple version of jdk, mixing openjdk and oracle versions. Then you can use the command update-java-alternatives to switch between installed version:

Copy# list available jdk
update-java-alternatives --list

# use jdk7
sudo update-java-alternatives --set java-7-oracle

# use jdk8
sudo update-java-alternatives --set java-8-oracle

Requirements

If you get add-apt-repository: command not found be sure to have software-properties-common installed:

Copysudo apt-get install software-properties-common

If you're using an older version Ubuntu:

Copysudo apt-get install python-software-properties

JDK installation without root privileges

If you haven't administrator rights on your target machine your simplest bet is to use sdkman to install the zulu certified openjdk:

Copycurl -s "https://get.sdkman.io" | bash
source "$HOME/.sdkman/bin/sdkman-init.sh"
sdk install java

NOTE: sdkman allow to install also the official Oracle JDK, although it's not a the default option. View available versions with:

Copysdk ls java

Install the chosen version with:

Copysdk install java <version>

For example:

Copysdk install java 9.0.1-oracle

Glossary of commands

• sudo <command> [command_arguments]: execute a command with the superuser privilege.

• add-apt-repository <PPA_id>: Ubuntu (just like every Debian derivatives and generally speaking every Linux distribution) has a main repository of packages that handle things like package dependencies and updating. In Ubuntu is possible to extend the main repository using a PPA (Personal Package Archive) that usually contains packages not available in the system (just like oracle jdk) or updated versions of available ones (example: LibreOffice 5 in LTS is available only through this PPA).

• apt-get [install|update|upgrade|purge|...]: it's "the" command-line package handler used to manipulate the state of every repository on the system (installing / updating / upgrading can be viewed as an alteration of the repository current state).

In our case: with the command sudo add-apt-repository ppa:webupd8team/java we inform the system that the next repository update must retrieve packages information also from webupd8 repo.

With sudo apt-get update we actually update the system repository (all this operations requires superuser privileges, so we prepend sudo to the commands).

sudo apt-get install oracle-java8-installer

• update-java-alternatives (a specific java version of update-alternatives): in Ubuntu several packages provides the same functionality (browse the internet, compile mails, edit a text file or provides java/javac executables...). To allows the system to choose the user favourites tool given a specific task a mechanism using symlinks under /etc/alternatives/ is used. Try to update the jdk as indicated above (switch between java 7 and java 8) and view how change the output of this command:

  ls -l /etc/alternatives/java*

In our case: sudo update-java-alternatives --set java-8-oracle update symlinks under /etc/alternatives to point to java-8-oracle executables.

Extras:

• man <command>: start using man to read a really well written and detailed help on (almost) every shell command and its options (every command i mention in this little answer has a man page, try man update-java-alternatives).

• apt-cache search <search_key>: query the APT cache to search for a package related with the search_key provided (can be the package name or some word in package description).

• apt-cache show <package>: provides APT information for a specific package (package version, installed or not, description).

1. add the following line to /etc/apt/sources.list

   deb http://deb.debian.org/debian/ sid main

2. install openjdk8

   //Update the repositories<br/>
   $ sudo apt-get update<br/>
   $ sudo apt-get install -y openjdk-8-jdk
   

In fact, I think it is better to search for your question first before ask.

How to install openjdk-8-jdk on Debian 10 (Buster)?

You have two alternatives:

1. Manually download the .tar.gz file for Oracle Java SE Development Kit 8 from the official Oracle website, and compile it by following the instructions in How do I install a .tar.gz (or .tar.bz2) file?.

2. Install Java 7 from the default Debian jessie repositories.

   sudo apt-get install openjdk-7-jdk
   

The default Java JDK package is openjdk-8-jdk in Debian stretch and openjdk-11-jdk in buster.

Oracle is no longer maintaining OpenJDK 8 and OpenJDK 11. Red Hat is providing bug fixes and security patches for OpenJDK 8 and OpenJDK 11, taking over from Oracle. If you are using OpenJDK from the default Debian repositories, Oracle's license cancellation doesn't matter at all because Debian will continue to support their Java packages the same way they support every other package in their repositories.

OpenJDK8 is not included in Ubuntu 14.04 repositories.

You can install Openjdk8 to Ubuntu 14.04 from a PPA this way:

sudo add-apt-repository ppa:openjdk-r/ppa
sudo apt-get update
sudo apt-get install openjdk-8-jre

Copies of past Debian packages are available at http://snapshot.debian.org/

You can search there for openjdk-8. The last available version is 8u322-b06-1~deb9u1, logged at http://snapshot.debian.org/package/openjdk-8/8u322-b06-1%7Edeb9u1/ You can download the binary packages (.deb) for your architecture and install with sudo dpkg -i *.deb.

Whether it works or not depends on the library versions that it was built against. 8u312-b07-1 (and other older versions) is also available.

Alternatively you can access the Debian tracking page for the package at https://tracker.debian.org/pkg/openjdk-8. openjdk-8 is still listed there as available for unstable (as version 8u312-b07-1). If you click on the links for the binary packages, they will lead you to download links for the deb packages.

snapshot.debian.org is arguably simpler for ad-hoc downloading since it has the deb files in one place (the debian package repository finds them in http://ftp.us.debian.org/debian/pool/main/o/openjdk-8/ or its mirror sites)

snapshot.debian.org also suggests instructions for configuring sources.list. But if the package is not likely to be updated often then it might be simpler to just download the deb files manually.

This article you followed is outdated as the "PPA" it tries to add and use does not exist anymore. Worry not, I believe you are better off without Oracle's package anyway.

So, how do you get Java?
Simply install one of the so called openjdk packages.
You can get a list of them by using the command: apt-cache search openjdk and then using sudo apt-get install PACKAGENAME.
I believe you are looking for openjdk-8-jdk, which can be installed by using:
sudo apt-get install openjdk-8-jdk.

These packages work just as well as the Oracle one did/does.

Here is an online list of these packages:
https://packages.debian.org/search?keywords=openjdk&searchon=names&suite=stable&section=all

openjdk-8 is available in my default Jessie/stable install - just need to apt-get install it (openjdk-8-jdk and whatever else you want/need that it recommends or depends on)

Of course, it MAY depend on your /etc/apt/sources.list file including the contrib and non-free repositories

/etc/apt/sources.list should look something like

deb http://ftp.us.debian.org/debian/ jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
deb http://ftp.us.debian.org/debian/ jessie-updates main contrib non-free
deb http://ftp.us.debian.org/debian/ jessie-backports main contrib non-free

The answer over at SO is nicer:

wget -qO - https://adoptopenjdk.jfrog.io/adoptopenjdk/api/gpg/key/public | sudo apt-key add -

sudo add-apt-repository --yes https://adoptopenjdk.jfrog.io/adoptopenjdk/deb/

sudo apt-get update && sudo apt-get install adoptopenjdk-8-hotspot