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Diagramming use cases

From: Foundations of Programming: Object-Oriented Design

Video: Diagramming use cases

A use case diagram is another diagram that comes from UML. The name can be a little misleading. It sounds like a diagram of a use case, and it isn't. It's almost always a diagram of several use cases and multiple actors at the same time. The reason it exists is so we can get an overview of these and see how they interact all in context. So it's not a replacement or substitution for a written use case. It's not the same thing. We still need the written use cases, we have this so we can see a different perspective.

Diagramming use cases

A use case diagram is another diagram that comes from UML. The name can be a little misleading. It sounds like a diagram of a use case, and it isn't. It's almost always a diagram of several use cases and multiple actors at the same time. The reason it exists is so we can get an overview of these and see how they interact all in context. So it's not a replacement or substitution for a written use case. It's not the same thing. We still need the written use cases, we have this so we can see a different perspective.

So, imagine that we've been asked to write a simple knowledge base for an organization. We've talked to the customer, written a few casual use cases, brainstormed some actors of this new system that we plan to build. The use case titles are in no particular order, Search Articles, View an Article, Manage Users, Create an Article, and View Analytics. I am not trying to be exhaustive here, just list a few use case goals that make sense. Let's say our actors are all role-based. We've got a Visitor, a Contributor, and an Administrator.

Now I'll use stick figures to represent the primary actors, and this is how you represent actors in a UML use case diagram, because stick figures are easy, easy on paper, easy on a whiteboard, you don't have to have drawing skills. We're not trying to make this look impressive. Just as I show actors with stick figures, I'm going to draw ellipses around my use case titles. This is the way we make it obvious that this is a self-contained use case and not just a piece of text. Now I'm going to draw a box around all the use cases.

This box represents the boundaries of my system, my knowledge base application that I'm going to write. Anything inside the box is part of my system and anything outside is not. So the actors are all outside. Next, I'll draw a line between any of the actors and the use cases they will interact with. So let's say a Visitor can search and view articles, a Contributor can create articles, and an Administrator can manage users and view statistics. Now of course, the Administrator would likely be able to search articles and read articles, too, although you could argue that when doing that he's just the same as the Visitor role.

Now these lines don't need arrows. It's not really about direction, it's merely saying this actor uses this use case. I'm also going to say they we're using a separate external computer system to store our article analytics data. So I'll represent that external system over here on the right. Now I could use another stick figure, because it's an actor. But because it's a non-human actor, I prefer to use a box. Though in that box, I'm going to write the word actor with these two angle quotes just to make it obvious.

You can use to two less than signs and two greater than signs around the word actor, although officially it is the French style quote, the guillemots, sometimes called angle quotes or chevrons here. I'm going to connect lines between this external system and the two use cases I plan to use with it. The View Article use case, which would save some analytics data back to this external system, and the View Analytics use case, which would allow the administrator to read the details of them. Now typically you put the primary actors on the left-hand side, these are the ones who initiate any of the use cases and the secondary actors on the right-hand side, and these actors take more of a reactive role, but even that's not a hard and fast rule.

Now this diagram is not referring to sequence. There is no necessary order to it. Sometimes people will write the use cases in order from the top down if the application naturally flows that way, but very often the use cases themselves don't have a built-in sequence. It's a simple overview of multiple use cases and multiple actors at the same time without the details of each particular written use case. It can be useful as a Communication tool, even with business users as it's not particularly technical, and sometimes helpful to figure out if something is missing from the picture.

The use case diagrams can get a little deeper than this, but what we've done here will let you read the vast majority of use case diagrams and easily create your own.

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This video is part of

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Foundations of Programming: Object-Oriented Design

47 video lessons · 48254 viewers

Simon Allardice
Author

 
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  1. 11m 35s
    1. Welcome
      1m 25s
    2. Who this course is for
      1m 15s
    3. What to expect from this course
      3m 6s
    4. Exploring object-oriented analysis, design, and development
      1m 41s
    5. Reviewing software development methodologies
      4m 8s
  2. 26m 14s
    1. Why we use object-orientation
      2m 42s
    2. What is an object?
      5m 22s
    3. What is a class?
      4m 43s
    4. What is abstraction?
      2m 45s
    5. What is encapsulation?
      3m 45s
    6. What is inheritance?
      3m 35s
    7. What is polymorphism?
      3m 22s
  3. 12m 16s
    1. Understanding the object-oriented analysis and design processes
      4m 13s
    2. Defining requirements
      6m 9s
    3. Introduction to the Unified Modeling Language (UML)
      1m 54s
  4. 23m 35s
    1. Understanding use cases
      6m 11s
    2. Identifying the actors
      4m 16s
    3. Identifying the scenarios
      5m 7s
    4. Diagramming use cases
      4m 18s
    5. Employing user stories
      3m 43s
  5. 16m 36s
    1. Creating a conceptual model
      1m 59s
    2. Identifying the classes
      2m 27s
    3. Identifying class relationships
      2m 38s
    4. Identifying class responsibilities
      6m 43s
    5. Using CRC cards
      2m 49s
  6. 22m 25s
    1. Creating class diagrams
      6m 11s
    2. Converting class diagrams to code
      4m 57s
    3. Exploring object lifetime
      5m 55s
    4. Using static or shared members
      5m 22s
  7. 19m 49s
    1. Identifying inheritance situations
      6m 49s
    2. Using inheritance
      2m 43s
    3. Using abstract classes
      2m 2s
    4. Using interfaces
      4m 20s
    5. Using aggregation and composition
      3m 55s
  8. 9m 23s
    1. Creating sequence diagrams
      5m 18s
    2. Working with advanced UML diagrams
      2m 3s
    3. Using UML tools
      2m 2s
  9. 10m 39s
    1. Introduction to design patterns
      2m 40s
    2. Example: the singleton pattern
      4m 53s
    3. Example: the memento pattern
      3m 6s
  10. 21m 47s
    1. Introduction to object-oriented design principles
      2m 50s
    2. Exploring general development principles
      3m 55s
    3. Introduction to SOLID principles
      6m 43s
    4. Introduction to GRASP principles
      8m 19s
  11. 7m 1s
    1. Reviewing feature support across different object-oriented languages
      3m 50s
    2. Additional resources
      2m 27s
    3. Goodbye
      44s

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