Android Studio “cannot resolve symbol” but project compiles and works

Question

The project compiles fine. However, Android studio 3.0.1 shows reference to one particular library in red and displays \"cannot resolve symbol\". Even single stepping into

Answer 1

None of the answers worked for me

Fixed it by:

1. Removing & adding all java files via Project window on the left side of Android Studio

2. Commenting/un-commenting & syncing libraries in the gradle file in small groups.

And cleaning all caches before that (as part of other fixes here).