R.javaAndroid Studio is very different than Eclipse. Since you say this is an old project, I assume it was originally from Eclipse. The Eclipse is R.javaincluded in the source tree. However, in Android Studio it is hidden. From another stack overflow answer :
1. build
This has all the full output of the process make, i.e. classes.dex, compiled classes and resources, etc.
Only a few folders are displayed in the Android Studio GUI. Important that your R.java is here under build/source/<flavor>/r/<build type(optional)>/<package>/R.java
R.java . . , , - R.java , . , .