3] Run a Java or JAR file. Now when you double click on the JAR file, it will automatically run like how EXE files execute. If there is a problem with version, the file should inform you.

Canonical has announced that OpenJDK 11 has become the default Java package on Ubuntu 18.04 LTS, replacing OpenJDK 10. The move comes days after Red Hat announced its stewardship of OpenJDK 11.

java - version Command 'java' not found, but can be installed with: sudo apt install default-jre. If the command indicates that Java is not found, proceed with the next steps. Update Ubuntu’s package ...