Installing OpenJDK for Mac and Linux

For Linux (for Ubuntu)

1. Download OpenJDK 15.0.1 from here

2. Create the /usr/java directory:
   sudo mkdir /usr/java

3. Navigate to the directory where the archive was downloaded and move the archive to the new directory:
   mv openjdk-15.0.1_linux-x64_bin.tar.gz /usr/java

4. Go to this directory:
   cd /usr/java

5. Unpack archive:
   sudo tar -xzvf openjdk-15.0.1_linux-x64_bin.tar.gz

6. 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-15.0.1
PATH=$PATH:$HOME/bin:$JAVA_HOME/bin  
export JAVA_HOME  
export JRE_HOME  
export PATH

7. Check Java version:
   java -version

For Mac

1. Download OpenJDK 15.0.1 from here
2. Navigate to the directory where the archive was downloaded and extract the file
   tar -xzvf openjdk-15.0.1_osx-x64_bin.tar.gz
3. Install the JDK
   sudo mv jdk-15.0.1.jdk /Library/Java/JavaVirtualMachines/
4. Check java version:
   java -version
5. In case you do not see openjdk version "15.0.1", then you need to switch the default to that version. This stackoverflow answer will guide you.