Dealing with Singletons which have to subclass

Question

In the question What is an efficient way to implement a singleton pattern in Java? the answer with the most upvotes says, to use a Enum for implementing a singleton.

That is fine and I understand the arguments, respectively the language advantages.

I have, however, a set of classes which I define singleton but which need to extend other classes, this is not possible with the enum approach, since enums cannot subclass.

Joshua Bloch says in his slides:

• But one thing is missing—you can’t extend an enum type
  • In most cases, you shouldn’t
  • One compelling use case—operation codes

In most cases you shouldn't: could someone elaborate on that? I have implemented several servlets and they extend HttpServlet, why shouldn't these be singletons? I only want one instance of them in my application.

Answer 1

A Singleton class can extend other classes; actually by default in Java it would anyway extend Object. However what Josh is referring to is that you shouldn't extend a Singleton class because once you extend it, there is more than 1 instance present.

Answering the comment:

Actually the best way to implement the Singleton is:

From Effective Java

// Singleton with static factory
public class Elvis {
private static final Elvis INSTANCE = new Elvis();
private Elvis() { ... }
public static Elvis getInstance() { return INSTANCE; }
public void leaveTheBuilding() { ... }
}

Here Elvis can extend any other class.

Answer 2

You shouldn't care about the actual instance(s) of your servlets - lifecycle management is handled by the servlet container according to the Servlet specification contract to which you have agreed. If it makes sense to implement parts of you server-side functionality as a singleton, then go ahead and do that any way you like and use it from your servlet.

Answer 3

Josh is referring to extending the enum type, not to having the singleton type extend something else.