IDEA error class does not exist

In the process of using IDEA to develop, there is a problem like this: the
project starts after compilation without any problems, but the IDEA editor reports errors in this type of reference and compilation errors. In
summary, the problem is IDEA's own project cache problem.
Solution:
Open the editor , Click File-"Invalidate Caches/Restart -" in the toolbar

IDEA's cache will then be cleared and the project will be reloaded.