1. Remove all the Java related packages (Sun, Oracle, OpenJDK, IcedTea plugins, GIJ):

   dpkg-query -W -f='${binary:Package}\n' | grep -E -e '^(ia32-)?(sun|oracle)-java' -e '^openjdk-' -e '^icedtea' -e '^(default|gcj)-j(re|dk)' -e '^gcj-(.*)-j(re|dk)' -e '^java-common' | xargs sudo apt-get -y remove
   sudo apt-get -y autoremove
   

2. Purge config files (careful. This command removed libsgutils2-2 and virtualbox config files too):

   dpkg -l | grep ^rc | awk '{print($2)}' | xargs sudo apt-get -y purge
   

3. Remove Java config and cache directory:

   sudo bash -c 'ls -d /home/*/.java' | xargs sudo rm -rf
   

4. Remove manually installed JVMs:

   sudo rm -rf /usr/lib/jvm/*
   

5. Remove Java entries, if there is still any, from the alternatives:

   for g in ControlPanel java java_vm javaws jcontrol jexec keytool mozilla-javaplugin.so orbd pack200 policytool rmid rmiregistry servertool tnameserv unpack200 appletviewer apt extcheck HtmlConverter idlj jar jarsigner javac javadoc javah javap jconsole jdb jhat jinfo jmap jps jrunscript jsadebugd jstack jstat jstatd native2ascii rmic schemagen serialver wsgen wsimport xjc xulrunner-1.9-javaplugin.so; do sudo update-alternatives --remove-all $g; done
   

6. Search for possible remaining Java directories:

   sudo updatedb
   sudo locate -b '\pack200'
   

   If the command above produces any output like /path/to/jre1.6.0_34/bin/pack200 remove the directory that is parent of bin, like this: sudo rm -rf /path/to/jre1.6.0_34.

Try with this code in terminal to remove java 8:

sudo apt-get purge oracle-java8-installer

Then type:

javac -version

The output should be:

javac: command not found

and to install java 7 in Ubuntu I use this code in terminal:

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java7-installer

After that type the following to see if there is java installed:

java -version

The output should be:

java version "1.7.0_80"

You've got Oracle Java installed, not OpenJDK. If you want to uninstall it, you can do this:

sudo apt-get remove oracle-java7-*

In effect, you were telling apt to install something that was never installed on your machine.

Run:

sudo update-alternatives --remove-all java
sudo update-alternatives --remove-all javac
sudo update-alternatives --remove-all javaws
sudo rm -rf /usr/lib/jvm/jdk1.8.0

To completely remove jdk from your system, follow these below steps:

1. Type sudo apt-get autoremove default-jdk openjdk- (Don't hit Enter right now).
2. Now press tab button for 2 or 3 times, you will get list of packages starting with openjdk-.
3. Look for name like openjdk-11-jdk. You need to get java version, here is 11.
4. Now complete your command to sudo apt-get autoremove default-jdk openjdk-11-jdk. (Change 11 to your java version)
5. And Hit Enter button.

Now you have done removing java from your system.

The unwanted versions can be removed by name:

sudo apt remove openjdk-8-jdk
sudo apt remove openjdk-11-jdk

They are installed with different packages.

You can see all the openjdk packages available with this command:

sudo apt update
apt-cache search openjdk

You can see all openjdk packages in your system with:

dpkg -l | grep openjdk

Note: I am a RedHat user, not a Mint expert but this is pretty much the same on every distro. I just checked the package names on Mint for those commands to work.

This is an ubuntu question and not an android question, but you need to get a list of the packages that are responsible for java 9 and uninstall them.

apt list --installed | grep -i jdk

then call apt-get remove with the package for java 9

apt-get remove java9-sdk.xyz

You can also modify your configuration using the following:

sudo update-alternatives --config java

And outlined here:

https://www.digitalocean.com/community/tutorials/how-to-install-java-with-apt-get-on-debian-8#managing-java