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,make module, remake, etc.
2. Project Setting-modules-sources, then add src and gen as my source folder.

But they are not take effect.

Answer 1

Goto File -> Settings -> Compiler now check use external build

then rebuild project