I've been working on a a few projects that use the JDK 9 with IntelliJ. My latest started getting an error like this:

view plain print about
1java package java.time does not exists
2java cannot find symbol
3symbol: class LocalDate

The LocalDate class was added to the JDK 8, so this error makes sense if you are building using an earlier version of JDK. But, I should not be seeing it when using JDK 9.

Googling on similar errors seems to bring me back to the same thing I already knew. You're using the wrong version of the JDK, be sure to upgrade to JDK 8 or later.

But, I am already using JDK 9, so should not be having this problem.

I spent an hour pouring over IntelliJ settings before stumbling onto this screen:

The Java Compiler target bytecode version of my project was set to 1.7 instead of 9. I already fixed that in the screenshot above. It made my errors go away immediately.

The most confusing thing is that I didn't have errors on any of the other projects.