code: public static void assertOnlyOneMethod(final Collection<Method> methods, Class<? extends Annotation> annotation)
     {
        if (methods.size() > 1)
        {
           throw annotation == null ? MESSAGES.onlyOneMethodCanExist() : MESSAGES.onlyOneMethodCanExist2(annotation);
        }
     }
docs: Asserts only one method is annotated with annotation.
  
  @param method collection of methods to validate
  @param annotation annotation to propagate in exception message