Installing Java JDK in Ubuntu Terminal

• Go to terminal mode in Ubuntu and enter:
• javac -version
• if this command gives you the following or similar message:
• The program 'javac' can be found in the following packages:
•     * default-jdk
•     * ecj
•     * gcj-5-jdk
•     * openjdk-8-jdk-headless
•     * gcj-4.8-jdk
•     * gcj-4.9-jdk
•     * openjdk-9-jdk-headless
• Try: sudo apt install
• it means that the JDK is NOT installed, and you need to run the following command:
• sudo apt install openjdk-9-jdk-headless
• then enter again: javac -version
• and you should get: javac 9-internal
• If you get the same above error, run again
• sudo apt install openjdk-9-jdk-headless
• Very rarely you need to install the JDK twice following the above instructions.