Here, I have written steps during changing default java from JDK version 11 to JDK version 8. All installed JDKs are located and can be found at: /usr/lib/jvm
Install new JDKs by running below command, in my case OpenJDK 8
sudo apt-get update
sudo apt-get install openjdk-8-jdk
Confirm current java version
java -version
All installed java version can be found by running below command
update-java-alternatives -l
Configure which java to use by default by running below
sudo update-alternatives --config java
Remove unnecessary JDK files if needed, in my case JDK 11
sudo apt-get remove openjdk-11-jdk
sudo apt-get autoremove
sudo rm -r /usr/lib/jvm/java-11-openjdk-amd64
Make sure default-java link points to expected JDK else change it manually.
cd /usr/lib/jvm
sudo ln -sfn java-1.8.0-openjdk-amd64 default-java
Again , confirm current java version changes
java -version
Output will be something like below
openjdk version "1.8.0_275" OpenJDK Runtime Environment (build 1.8.0_275-8u275-b01-0ubuntu1~18.04-b01) OpenJDK 64-Bit Server VM (build 25.275-b01, mixed mode)
Optional (advanced): See all program links related to java
update-alternatives --get-selections | grep java