Check out example codes for "how to set java_home in ubuntu". It will help you in understanding the concepts better.

Code Example 1

#Update repo
sudo apt update

# Install java 8 from ubuntu repo
sudo apt install openjdk-8-jdk -y

ls /usr/lib/jvm/java-8-openjdk-amd64/
 export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64
 echo $JAVA_HOME
 #Add JAVA bin directory to the PATH variable
 export PATH=$PATH:$JAVA_HOME/bin
 echo $PATH
 java -version

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