If You create new xml file with prefix _ , for example _style.xml and You to clean project (Project->Clean), than package in folder project\gen will deleted with R.java class and new R.java not be created.
For to solving this problem You have to rename file without prefix _ as style.xml or name what You need and rebuild project.
If some ID cannot be resolved or is not a field get error occurence
You have to delete import android.R; in Activity.class if was inserted,
when this error is displayed.