Why so red? IntelliJ seems to think every declaration/method cannot be found/resolved

Question

I just installed and re-installed IntelliJ. Every Java file is coming up RED. I checked the JDK; it is at 1.6.##. The maven clean install build worked just fine

Answer 1

Using IntelliJ 2019.3 and a Gradle project dependent on my own Gradle project, the only thing that fixed it for me was Reimport All Gradle Projects under View -> Tool Windows -> Gradle (and refresh button).

Before that I tried restarting IDE, I tried "Invalidate Caches and Restart", I tried Rebuild project, neither helped.