I have recently downloaded IntelliJ IDEA (2018 version) and Java JDK 12. When I was configuring IntelliJ with JDK 12, the Project Language level is stuck at Java 9. Hence, I cannot use the new features in Java 12.
I have searched about this issue over the Internet, but this problem has not yet been identified, mostly because JDK 12 was released just last week (2019 March 19).