Observer Pattern VS Owner Referencing. Which is more correct? (Java)

Question

Within Java you can create an Observer-Observable set of classes in which the Observable can call the Observer. You can also in java explicitly reference an owning class instance in a child instance of another class and call the owning class instance's public functions.

Which is the better approach to take? Which is more beneficial in different scenarios, one example being Multi-Threading?

Answer 1

The Observer Pattern should be used whenever you don't know or don't care who is observing you. This is the key-concept in event-driven programming. You don't have any control of who is observing or what they do when you broadcast your events. Like you already mentioned in your comments, this is great for decoupling classes.

An example of a usage could be in a plugin-architecture:

You write a basic mail-server that broadcasts whenever a mail is received. You could then have a spam-plugin that validates the incoming mail, an auto-reply service that sends a reply, a forward service that redirects the mail and so on. Your plain mail server (the observable) doesn't know anything about spam, replies or forwarding. It just shouts out "Hey, a new mail is here" not knowing if anyone is listening. Then each of the plugins (the observers) does their own special thing, not knowing anything about each other. This system is very flexible and could easily be extended.

But the flexibility provided by the Observer Pattern is a two-edged sword. In the mail-server example, each plugin handles the incoming mail in total isolation from each other. This makes it impossible to setup rules like "don't reply or forward spam" because the observers doesn't know about each other - and even if they did, they wouldn't know in what order they are executed or has completed. So for the basic mail-server to solve this problem, It'll need to have references to the instances that does the spam/reply/forward actions.

So the Observer Pattern provides flexibility. You could easily add a new anti-virus plugin later, without having to modify your plain mail server code. The cost of this flexibility is loss of control of the flow of actions.

The reference approach gives you total control of the flow of actions. However you would need to modify your plain mail server code if you ever need to add support for an anti-virus plugin.

I hope this example gives you some ideas of the pros and cons of each approach.

In regards to multi-threading, one approach isn't favorable over the other.