Install Java 11 on Kubuntu

sudo apt update

sudo apt install openjdk-11-jdk

java -version

Version

sudo update-alternatives --config java

Set JAVA_HOME

echo $JAVA_HOME

export JAVA_HOME=$(readlink -f /usr/bin/java | sed "s:/bin/java::")

Then, you can set the JAVA_HOME variable in your shell's configuration file (e.g., .bashrc or .zshrc). For example: 
   export JAVA_HOME=/usr/lib/jvm/java-11-openjdk-amd64
   export PATH=$PATH:$JAVA_HOME/bin
After that, source the configuration file:
   source ~/.bashrc  # or your shell's configuration file