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.

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.