why does R does not exist error come in android?

Question

packages/apps/Myfolder/src/com/android/myfolder/MyFile.java:196: package R does not exist
                  addPreferencesFromResource(R.xml.myfile);
                                              ^
packages/apps/Myfolder/src/com/android/myfolder/MyFile.java:344: package R does not exist
        menu.add(0, MENU_SAVE, 0, R.string.menu_save)
                                   ^
packages/apps/Myfolder/src/com/android/myfolder/MyFile.java:346: package R does not exist
        menu.add(0, MENU_CANCEL, 0, R.string.menu_cancel)
                                     ^
packages/apps/Myfolder/src/com/android/myfolder/MyFile.java:454: package R does not exist
                     errorMsg = mRes.getString(R.string.error_empty);
                                                ^
packages/apps/Myfolder/src/com/android/myfolder/MyFile.java:458: package R does not exist
                     errorMsg = mRes.getString(R.string.error_empty);

Answer 1

1. Try Clean-> Build (If not just restart eclipse, it just worked!)
2. In case you are importing project, make sure you choose proper Level.