In every instance in all of my classes where I reference R.id.something, the R is in red and it says "cannot resolve symbol R". Also every time there is R.layout.something it is underlined in red and says "cannot resolve method setContentView(?)". The project always builds fine. It is annoying to see this all the time. I have read many other questions on here about something similar but most involved importing projects from Eclipse. I am using what I believe to be the most recent version of Android Studio and the project was created with Android Studio and worked without any "cannot resolve R" problems. I would like to know what causes this if anyone knows.

The error “Cannot resolve symbol R”  in Android studio has faced mostly, when you shift your code to another Computer or send the code to another person. The “R” turns red and can’t run the Application and throw the error in “Logcat”. Most often “R cannot be resolved” error appears if there is an issue with some of your resource files. Due to this error, you are unable to build your application. That’s why we need to solve this error as it not getting away by just doing a simple restart or hitting Alt+Enter.

Now the point that comes here is how we can fix “cannot resolve symbol R” in Android Studio. So in this article, we are going to discuss six different methods to fix “cannot resolve symbol R” in Android Studio. 

In the latest versions of Android Studio, at least for me, the following works:

"Tools" -> "Android" -> "Sync Project with Gradle Files"

In latest Android Studio 3.1.3 (July 2018), "Sync Project with Gradle Files" is available in main menu bar.

I was using gradle 3.3.0. It was the main culprit. Wasted 6.50 hours from my life. Gradle 3.2.1 removed the error.

classpath 'com.android.tools.build:gradle:3.2.1'