Yes, I am sorry, I got the facts wrong there. HttpServlet indeed is abstract class but the functions still have their own generic implementation. A class being abstract doesn't mean we _must_ override the functions. Such a requirement is imposed only if the functions themselves are abstract which I don't think they are. Read this.
Although (at least according to the "normal" rules for class extension) it will compile just fine without overriding any of them (AFAIK, I haven't actually tried it), it just wouldn't do anything other than return the http error message that the requested http service/method is not supported/implemented.