Make sure you are using Java v20

java -version

openjdk version "20" 2023-03-21
OpenJDK Runtime Environment (build 20+36-2344)
OpenJDK 64-Bit Server VM (build 20+36-2344, mixed mode, sharing)

To run one of the class with Java 20 just run the following commands

mkdir out
DEMO=Concurrent
javac --enable-preview --release 20 --add-modules jdk.incubator.concurrent -d out/ src/ca/applin/demo/$DEMO.java
java --enable-preview --add-modules jdk.incubator.concurrent -cp out ca.applin.demo.$DEMO

Replace DEMO=Concurrent by DEMO=Pattern or other class in the ca.applin.demo package to run those instead. Or simply run ./demo.sh with the class name as argument, ie

./demo.sh Concurrent

• In Preferences > Build, Execution, Deployment > Java Compiler change:
  • in Additional command line parameters copy --enable-preview --add-modules jdk.incubator.concurrent
• In Project Structure > Project make sure:
  • SDK: your java 20 JDK
  • Langage level: X - Experimental features
• In your Run commands add
  • Edit configuration > Mpdify options > Add VM option and paste --enable-preview --add-modules jdk.incubator.concurrent