OO design: should collections always be stored?

Question

I've wondered this in many different situations, so here I come, looking for the experts knowledge. Let's say I have to model something that requires a collection. A simple example: an application that stores famous quotes along with their author and a set of tags or keywords. The user should be able to enter a tag or keyword and get matching quotes for it. My question is: do I really need a class that contains my collection of quotes? Something like this:

Or would this also be correct?

I'm asking this in the more abstract way possible (after all, UML should never depend on the implementation). I've always thought the second example (just 1 class) was incorrect, but now I'm thinking that maybe the user can press a button on some interface and that button executes some code that gets a quote stored somewhere, and the second example would also be correct?

Basically, should I always have a collection stored somewhere, even if the storing class does nothing else but just store the collection (and provide the methods to access it)?

Answer 1

I definitelly prefer only one class, if there is no strong reason to have another container class (especially on abstract conceptual level). Then I add the collection methods as static functions. A separate container class would only bring more complexity, more dependencies and doubts like yours. :) Doubts often indicate the lack of a real need. When you really need something, you know it.

Here an example with some explanations. I find it simple, clear, elegant and abstract, meaning non-restrictive, easy to transform to any implementation you like:

When it comes to relationships of this class to other class, then you actially have your collection, without introducing new class. This diagram shows two examples. "Other class" actually sees a collection "quotes" which is ordered, like Vector. "One more class" also has a collection of Quotes with different characteristics.

Later on implementation level you can implement it directly like this or eventually adding a Factory or Container class, according to concrete, implemention resctrictions and special reqs.