Download an appropriate JDK for your operating system. The latest JDK can be downloaded from the official OpenJDK website.
Once installed, use the java -version command from the command line to check the Java version.
Output for OpenJDK 12:
$ java -version
openjdk version "12.0.2" 2019-07-16
OpenJDK Runtime Environment (build 12.0.2+10)
OpenJDK 64-Bit Server VM (build 12.0.2+10, mixed mode, sharing)
Set the JAVA_HOME environment variable to the JDK installation directory. Follow this guide to set JAVA_HOME for your platform.
If your system has multiple versions of JDK installed, make sure the JAVA_HOME
environment variable points to correct JDK. JavaFX 11 needs at least JDK 11.