I meet the problem Cannot resolve symbol AppCompatActivity
I solved the problem by deleting .idea folder and syncing project with gradle files by the acticle Cannot resolve symbol AppCompatActivity - Support v7 libraries aren't recognized?
Could tell me what .idea folder is ? Why can I delete it and sync ?
.idea folder is storing the settings for your IDE (Development Environment) - so the settings of Android Studio how to visualize your project. You can check more about .idea folder here
The .idea folder (hidden on OS X) in the solution root contains IntelliJ's project specific settings files. These include per-project details such as VCS mapping and run and debug configurations, as well as per-user details, such as currently open files, navigation history and currently selected configuration.
Some files should be committed to source control, some should be excluded.
See here: https://rider-support.jetbrains.com/hc/en-us/articles/207097529-What-is-the-idea-folder-
It stores project specific settings that pertain to AndroidStudio. It is called .idea because AndroidStudio is derived from JetBrains' Intellij IDEA. Most if not all IDEs in this family create such a directory. For example, JetBrains' Rider, an IDE for writing .NET applications, also creates such a directory.
Basically it is for project scoped IDE configuration.
