Installing OpenJDK for Mac and Linux

For Linux (for Ubuntu)

Download OpenJDK 21.0.7 from here
Create the /usr/java directory:
sudo mkdir /usr/java
Navigate to the directory where the archive was downloaded and move the archive to the new directory:
mv OpenJDK21U-jdk_x64_linux_hotspot_21.0.7_6.tar.gz /usr/java
Go to this directory:
cd /usr/java
Unpack archive:
sudo tar -xzvf OpenJDK21U-jdk_x64_linux_hotspot_21.0.7_6.tar.gz
Rename the archive: mv /usr/java/jdk-21.0.7+6 /usr/java/jdk-21.0.7
Set the environment variables, for this open the /etc/profile file and add the text mentioned below to the end of the file:

JAVA_HOME=/usr/java/jdk-21.0.7
PATH=$PATH:$HOME/bin:$JAVA_HOME/bin  
export JAVA_HOME  
export JRE_HOME  
export PATH

Download OpenJDK 21.0.7 from here
Navigate to the directory where the archive was downloaded and extract the file
tar -xzvf OpenJDK21U-jdk_aarch64_mac_hotspot_21.0.7_6.tar.gz
Install the JDK
sudo mv jdk-21.0.7+6 /Library/Java/JavaVirtualMachines/
Check java version:
java -version
In case you do not see openjdk version "21.0.7", then you need to switch the default to that version. This stackoverflow answer will guide you.