"Cannot resolve symbol" in Intellij IDEA

Question

I am unable to use libraries from one "module" in another. I am seeing red class names and getting "Cannot resolve symbol ..." errors all over the place:

The other modules are referenced in File | Project Structure all the other module's Paths are correct and include the *.classes I'm trying to use:

I have also tried a File | Invalidate Caches but am still having the same problems.

What do I need to do to fix this?

Answer 1

Listed below are steps which may fix the problem:

1.Invalidate and refresh the IntelliJ's caches

File Menu -> Invalidate Caches / Restart -> Invalidate and restart

2.Project SDK selection

Check project settings from File Menu -> Project Structure Ensure an SDK is selected for the Project SDK. If is selected, click the 'New' button, select JDK and then select the correct path to the JDK e.g. /opt/java/jdk1.8 or c:\Java\Jdk1.8

3.Corrupt JDK

There may be a problem with your JDK installation, re-install JDK

4.Re-import the project

Close the project if it is open in IntelliJ Delete the .idea folder in the project source code folder. Start IntelliJ and Import the project.

You can choose any of them which will work for you.