code: public static String findMainClass(File rootFolder) throws IOException {
          return doWithMainClasses(rootFolder, new ClassNameCallback<String>() {
              @Override
              public String doWith(String className) {
                  return className;
              }
          });
      }
docs: Find the main class from a given folder.
  
  @param rootFolder the root folder to search
  @return the main class or {@code null}
  @throws IOException if the folder cannot be read