E: Package 'oracle-java8-installer' has no installation candidate

I was facing the same issue and resolved with this:\

sudo apt install openjdk-8-jdk

I just went through the same process and I fixed the problem by manually installing java 8 on my ubuntu 18.04 machine. Hopefully my answer will help you.

1. Download the latest JAVA 8 SE development kit from here: https://www.oracle.com/technetwork/java/javase/downloads/java-archive-javase8-2177648.html
2. Open your terminal.
3. Type $mkdir /opt/jdk.
4. Untar Java in your new folder $tar -zxf jdk-8u5-linux-x64.tar.gz -C /opt/jdk.
5. Set oracle JDK as the default JVM by running those two instructions (or something around that depending on your configuration):

Note: Each bullet point is one single command

• update-alternatives --install /usr/bin/java java /opt/jdk/jdk1.8.0_<YourVersion>/bin/java 100
• update-alternatives --install /usr/bin/javac javac /opt/jdk/jdk1.8.0_<YourVersion>/bin/javac 100

You can check the result by running java -version !

Before I started, I went through those steps as well in order to remove completely java from my system: https://askubuntu.com/questions/84483/how-to-completely-uninstall-java#185250

The source I used for the manual installation of JAVA was this article: https://www.digitalocean.com/community/tutorials/how-to-manually-install-oracle-java-on-a-debian-or-ubuntu-vps

Add the following line to /etc/apt/sources.list:

deb https://debian.opennms.org/ stable main

Install GPG key of the repository:

wget -O - http://debian.opennms.org/OPENNMS-GPG-KEY | sudo apt-key add -

Update the package index:

sudo apt-get update

Install oracle-java8-installer deb package:

sudo apt-get install oracle-java8-installer

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