Android Studio don't generate R.java for my import project

Question

I import a project to Android Studio , but the R.java is always empty.

public final class R {
}

I have tried:

1. Make project,ma

Answer 1

I failed even if I tried to update the SDK tools as mentioned here.

Here is how I solved the problem (for the library projects imported).

In File -> Project Structure -> Modules, I removed the problematic projects, and added them again by selecting New Module -> Android Library Module.

After rebuilding the project, I got all the R.java files back.