android studio error: cannot reslove symbol 'xxx'

Solution 1:

After putting the mouse on it, it will display "Cannot resolve symbol XXX" Click to complete the reference

Solution 2: (quote in full)

Restart Android Studio, re-sync gradle, Clean build

Solution 3: (quote in full)

Click "File" -> "Invalidate Caches/Restart" in the menu,

 

Then click "Invalidate Restart" in the dialog box to clear the cache and restart.