GNU/Linux Lubuntu 20.04 Installing Java Guide
How to Install Oracle Java SE on Lubuntu 20.04 Focal LTS GNU/Linux – Step by step Tutorial.
And the Oracle Java SE Lubuntu Focal Release includes:
- Java Runtime Environment to Run Java Software
- Java Development Kit to Develop Java Software
Moreover, the Java OpenJDK (Open Java Development Kit) is the Free and Open Source Implementation of the Java Platform, Standard Edition (Java SE).
Where the Oracle JRE/JDK is based on the OpenJDK Source Code. In Addition, it contains Closed-Source Components.

GNU/Linux Lubuntu 20.04 Java SE Installation Guide