code: public static String findSingleMainClass(File rootFolder, String annotationName)
  			throws IOException {
  		SingleMainClassCallback callback = new SingleMainClassCallback(annotationName);
  		MainClassFinder.doWithMainClasses(rootFolder, callback);
  		return callback.getMainClassName();
  	}
docs: Find a single main class from the given {@code rootFolder}. A main class annotated
  with an annotation with the given {@code annotationName} will be preferred over a
  main class with no such annotation.
  @param rootFolder the root folder to search
  @param annotationName the name of the annotation that may be present on the main
  class
  @return the main class or {@code null}
  @throws IOException if the folder cannot be read