Use Cases

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UML Use Case Diagrams and Specifications

UML Use Cases have quickly become a defacto way of gathering Functional Requirements because they can remove much of the ambiguity from communication between a Clients/StakeHolders/Domain Expert and Developers.

In other words, it's an effective way to doodle the project's functional (but only functional) requirements in a way that both parties can understand.

Intentional Simplicity=Separation of Concerns:
What makes it especially useful for the functional requirements gathering phase of a project is that it achieves two things elegantly:

it's not especially difficult to pick up, and therefore allows Clients/StakeHolders/Domain Experts to get involved -- suggesting use cases they wish added and reviewing Use Cases already on paper -- and therefore diminish ambiguities and misunderstandings (ie cost) later.
Use Cases do not have notations for process flow -- only functionality -- neatly separating roles.
This allows stakeholders to clearly state what they want, while leaving how that will be achieved to be figured out by the developers and engineers later (the actual implementation logic is then notated with different UML diagrams, specifically designed for the Design Phase).

A Special Note for Developers:
Note that the intentional simplicity of Use Cases are part of its effectiveness for discussing with StakeHolders -- but also part of the reason that some developers have a hard time coming to grips with using them effectively, as they tend to try to add implementation logic to Use Cases. Developers must restrain themselves from this temptation and remember that whereas most diagrams in the UML Specifications are for Design and Deployment purposes, Use Cases are for the Requirement phase, focused outwards, rather than inwards -- it's way too early to write in stone how the Use Case will be fulfilled.

Don't Forget the Use Case come with Specifications:
Whereas creating Use Case Diagrams are a moderately fun exercise, one must not forget that the full formal UML process involves backing each oval with a written Specification of how the Use Case works.
Not only do most UML diagraming packages completely omit this part (hence why the process of making Use Case Specifications is not more widely used/understood) but writting Specifications is moderately hard since it can be quite difficult to write Specifications that only commit requirements, and not implementation logic.

Links below:
To help you learn how to use Use Cases effectively in your next project, I've put together a set of links that I've found useful to get up to speed as quickly as possible.

Learn

Tutorial on Use Cases

This is a gentle introduction to UML Use Cases, but be forewarned that the diagrams have some mistakes -- namely that they use Generalization relationships (solid line + clear arrow) marked with <>, when they should be dashed lines with open arrow (dependency).
But other than that slight mistake, the tutorial is good.

Reuse in Use-Case Models: <>, <>, and Inheritance

An excellent article covering all the types of connectors you could ever use in a Use Case.
A must read if you are trying to get a grasp on Use Cases.

Writting Effective Use Cases

Once you've read an intro on UML Diagrams, and understand how to draw one, lets put it in working context...try this tutorial.
It will also show clearly how one gets from the Doodling/Diagram part to the Written/Specifications part...

Writing Good User Case Specifications

You've now learnt how to draw Use Cases -- and seen that you need to think about Specifications as well...but lets make sure you can do those well.
If you can grit your teeth and not end up pulling your hair out while listening to one of the slowest presentations I have ever heard -- you'll be rewarded.
The content of this video is really complete, covering how to write Use Case Specifications (and not much on Use Case diagrams).
What suprised me about the presentation was how focusing on doing Use Case Specifications correctly, cleared up most of the questions that arise when trying to determine how to draw Use Case diagrams.
Very useful (but coffee and Patience required).

UML 2 Use Case Diagramming Guidelines

Finally, some advice on how to lay out your Use Case Diagrams clearly.

IBM:Rational Rose: Capturing application requirements in a use case model

By now, with the above links, you should be a pro at Use Case Diagrams and Specifications -- but here's a link to help docs written by IBM for its products. A bit dry, but since they practically invented UML, the link is certainly worth knowing about.

Tools

Although the means to draw Use Cases Diagrams are provided in most UML packages, parts of the process (eg: Lists and Specifications) are usually not provided in the same package.
Here is a list of tools and other links that may help in this regard...

Use Case Maker

Although I am currently (Nov08) a big proponent of ArgoUML (its very capable...and its free), I think this application has a lot of merit.
Most people like the doodling side of Use Cases -- and conveniently like forgetting as long as possible that Use Cases always should be accompanied with Use Case Specifications (the written part...).
Technically, Use Cases are incomplete without both halves.
Unfortunately, most (almost all) UML diagraming tools only offer the diagramming half of Use Cases, expecting you to use Word or some other solution to handle the putting together of Use Case Specifications.
This application stands out from the rest because it handes the dodling side of things, and attempts to address the written part as well...
Its IDE needs work for clarity's sake...but its useable as it stands.
Another point I (sortof) like about the concept is that it handles only Use Cases. And that's interesting in a way -- Use Cases are done with client/StakeHolder -- and are not Design related. In other words, there is arguably some logic in using a seperate app just for Use Cases, in order to keep them distinct from the more design related UML diagrams.
That said...I still prefer having everything kept together, and just would prefer UML packages growing to be able to handle Requirement gathering better...
But check Use Case Maker out...

Visual Use Case

If you want more than what freeware solutions can provide, and you have $600 to blow, you should consider this very complete application: this will handle all the steps of making Use Cases Diagrams -- backed by Specifications -- and do a damn good job of it.
A very complete package...

Resources

Right. Once you've learnt what Use Cases are, and want to use them on a daily basis, you'll probably want some Real-World information.
Try these links...

Use Cases: Requirements in Context, 2 edition (by Daryl Kulak and Eamonn Guiney)

This a very approachable, down to earth book that will give grounded advice.