code: public static String findMainClass(JarFile jarFile, String classesLocation)
  			throws IOException {
  		return doWithMainClasses(jarFile, classesLocation, MainClass::getName);
  	}
docs: Find the main class in a given jar file.
  @param jarFile the jar file to search
  @param classesLocation the location within the jar containing classes
  @return the main class or {@code null}
  @throws IOException if the jar file cannot be read