Switching Java versions by command in Ubuntu

At first install multiple versions of Java. 

$ sudo apt update
$ sudo apt install openjdk-8-jdk
$ sudo apt install openjdk-11-jdk

Then you can switch java versions as follows:

$ update-java-alternatives --list
java-1.11.0-openjdk-amd64      1111       /usr/lib/jvm/java-1.11.0-openjdk-amd64
java-1.8.0-openjdk-amd64       1081       /usr/lib/jvm/java-1.8.0-openjdk-amd64
$ sudo update-java-alternatives --set /usr/lib/jvm/java-1.11.0-openjdk-amd64
$ javac -version
javac 11.0.9.1
$ java -version
openjdk version "11.0.9.1" 2020-11-04

If you set JAVA_HOME, maybe you have to change this too.