Collection as a metaphor for real world containers

Question

I find modeling physical containers using collections very intuitive. I override/delegate add methods with added capacity constraints based on physical attributes such as volume of added elements, sort based on physical attributes, locate elements by using maps of position to element and so on.

However, when I read the documentation of collection classes, I get the impression that it's not the intended use, that it's just a mathematical construct and a bounded queue is just meant to be constrained by the number of elements and so forth.

Indeed I think that I unless I'm able to model this collection coherently, I should perhaps not expose this class as a collection but only delegate to it internally. Opinions?

Answer 1

Many structures in software development do not have a physical counterpart. In fact, some structures and algorithms are quite abstract, and do not model objects directly in the physical world. So just because an object does not serve as a suitable model for physical objects in the real world does not necessarily mean it cannot be used effectively to solve problems within a computer program.

Answer 2

Not entirely sure that I've understood you correctly. I gather that you want to know if you should expose the collection (subclassing) or wrap it (have a private field). As Robert says, it really depends on the case. It's pretty much your choice. Nonetheless I'd say that in many cases the better choice is to not expose the collection because the constraints define the object you are modelling and are not fully congruent with the underlying collection. In short: users of your object shouldn't need to know that they are dealing with a collection unless it is really a collection with some speciality e.g. has all properties of a collection but allows only a certain number of objects.

Answer 3

Indeed I think that I unless I'm able to model this collection coherently, I should perhaps not expose this class as a collection but only delegate to it internally. Opinions?

Firstly, you don't want to get too hung up with the modeling side of software engineering. UML style models (usually) serve primarily as a way of organizing and expressing the developer's high level ideas about how an application should be implemented. There is no need to have a strict one-to-one relationship between the classes in the model and the implementation classes in the application code.

Second, you don't want to get too hung up about modeling "real world" (i.e. physical) objects and their behavior. Most of the "objects" that are used in a typical applications have no real connection with the real world. For example, a "folder" or "directory" is really little more than an analogy of the physical objects with the same names. There's typically no need for the computer concept to be constrained by the physical behavior of the real world objects.

Finally, there are a number of software engineering reasons why it is a bad idea to have your Java domain classes extend the standard collection types. For example:

• The collections have a generic behavior that it is typically not appropriate to expose in a domain object. For instance, you typically don't want components of a domain object to be added and removed willy-nilly.

• By extending a collection type, you are implicitly giving permission for some part of your application to treat domain objects as just lists or sets or whatever.

• By extending collection classes, you would be hard-wiring implementation details into your domain APIs. For example, you would need to decide between extending ArrayList or LinkedList, and changing your mind would result (at least) in a binary API incompatibility ... and possibly worse.