Is this a good reason to use a Singleton?

Question

I'm making an Android app that will have the timetables of a local bus. There are more than one timetable, the one that will be use depends on the day.

If it's a holiday I must use a special timetable, so I want to know when is a holiday and when is not.

The thing is that I'm creating a class that will handle this, it will try to retrieve information from memory or from a web api. Then some other classes will be able to communicate with this class, but it doesn't seem necessary to me to have more than one instance of this class, I could create just one instance and share it with the rest of the classes.

Could this class be a Singleton or it would be better if I create a normal class ?

Answer 1

In your case (retrieving info from memory), definitely avoid using a singleton class because it will highly likely be tied to your Activity context.

Your class will have a static reference to a class, therefore

1. it will be kept in memory when not needed.
2. singleton may be reinstantiated, or may use obsolete instance, with new instations of activities. You will lose control of the current variables.
3. diffent instances of the same activity class are highly likely to conflict with this class.

Examples of the same activity class several instantiation:

1. Change device orientation.
2. Running app from the webbrowser's, Google Play, file browser intent.

Besides, at some point, when you add functionality based on user reviews, your app will grow, you are likely want to refactor your class, break it into subclasses, put some of its methods into separate threads. It will no longer be easy to do.

It might seem fun while the app is small, and untested, but later, in Android specifically, you will run into a nightmite with unpredictable and hard to detect errors.

Because of Android's special way to recreate activity class, through onCreate, onResume etc. you will run into a nightmare, when the app will start living its own life.

You will no longer be able to rely on the assumption that the current singleton instantiation actually belongs to your current activity.

You may swap between orientations or run your app from different entry points (launcher, recent apps, google play), and it may reuse the variables actually prepared for a different activity instantiation.

If you need only one instance of the class, just create one instance of the class in the onCreate method - and that will make the app much more manageable.

Answer 2

One of the main advantages a Singleton class brings you is the fact that you are sure to have one and only one instance of an object doing some thing, and that it is instantiated only once (preferably at a specific point of your application, for instance at startup or only after certain other operations have been performed)

An example could be for instance a cache implementation: you want to make sure that all classes that need a certain cache read and write from the same object, that maybe is created and filled with information at startup time.

Your does not seem to be the case, unless you fetch the information you need when your application starts and then you keep them memorized for some reason: in this case you want to make sure your information is fetched one and only one time, to avoid wasting memory and elaboration time. Also, a Singleton is fine if you need to do some kind of operation when your class is instantiated, like opening a connection that then stays open.

On the other hand, if you just need a class with some method to call some external apis or database and you don't need to memorize any information in it, there is no reason to initialize a singleton.

If this is your case, why don't you try some static class/methods? They can be called like normal methods directly on the class with no need to instantiate objects or keeping a state, saving memory and avoiding side effects.