Can an observable class be constructed as a singleton?

Question

I\'m making a program in Java with the Observer pattern (with the help of the Java API). If some of the observables had more than one instance, the program could crash. Should I

Answer 1

Should I implement them as singleton? Is it recommended?

Of course you can do this. Whether it is a good idea depends on the actual context.

• Are these things conceptually singleton, or is this just a hack to try to make bugs disappear? (Or to put it another way, is the real problem that your application design is wrong ...)

• Singletons do have issues with respect to unit testing. This is not a show stopper, but it is one of the reasons that people tend to avoid them.

Answer 2

Yes, the GoF lists the Singleton as a related pattern (along with the Mediator) to the Observer:

The ChangeManager may use the Singleton pattern to make it unique and globally accessible.

ChangeManager is their Observable in the sample code.

I have used a Singleton Observable (here) to update the GUI everytime relevant changes occur to the Persistence Unit. The program won't crash but it won't function as expected if there were more instances.

Answer 3

I did it before and didn't have any problems exactly because I used a singleton. The pattern is there to use it.