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 had this problem after update gradle...

There are several reasons that causes the project not to be built:

1-Unknown error in drawable or xml files

2-Update gradle or libraries and etc ...

Solution :

1-Clean and rebuild project

2-Delete .idea and build folders in project file(shown in picture) then goto "File/Invalidate catch-restart"

3-Roll back to previous gradle version and libraries.