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4.7 ● Use cases 4.7 Use cases 45 One widely used approach to documenting requirements is “use cases”. These are textual descriptions which can be augmented by UML use case diagrams. Use cases take the point of view of the user or users of the system. A user who is carrying out a particular role is called an actor. A use case is a task that an actor needs the system to carry out. For example, in the ATM system (Appendix A), one of the things that a user does is withdraw cash. This is a use case. As part of withdrawing cash, the user will have to carry out subtasks, such as offering up their card and entering a PIN, but these smaller tasks are not use cases. It is the overall user task that constitutes a use case. A use case both specifies what the user does and what the system does, but says nothing about how the system performs its tasks. In the ATM system, the use case for withdrawing cash is: withdraw cash. The user offers up their card. The system prompts for the PIN. The user enters the PIN. The system checks the PIN. If the card and PIN are valid, the system prompts the user for their choice of function. The user selects dispense cash. The user prompts for the amount. The user enters the amount. The system ejects the card. When the user has withdrawn the card, the system dispenses the cash. We see that the user’s task requires a whole number of detailed steps. Sometimes the user’s objective is not achieved, for example, if the PIN is wrong. However, the overall name of the use case describes what normally happens. Other use cases for the ATM are check balance and transfer money. SELF-TEST QUESTION 4.2 Write a use case for checking a balance. You will see that sometimes different use cases have parts in common. This is no problem. In the above example, and in most cases, the actor is a person, but an actor can be anything that interacts with the system. This could be, for example, another software system or another computer communicating across the internet. For example, in a web server (program), the actor is a web browser program running on another computer. It is sometimes difficult to identify distinct use cases. In the ATM, for example, is entering and validating the PIN a use case? The answer is no because it is not a useful function from the user’s point of view, whereas withdrawing cash is. Suppose a person
46 Chapter 4 ■ Requirements engineering carries out a series of transactions, inserting their card, withdrawing cash, checking their balance and then transferring money. Is this collection a single use case? No, because it constitutes a number of useful user functions. One way to identify distinct use cases is to identify a goal that an actor wishes to accomplish. Another viewpoint is identifying some outcome of value to the user. The task of correctly entering a PIN is neither a goal nor a valuable outcome. It is only a part of some complete and useful function. It is therefore not a valid use case in itself. For a large system, there will be many use cases. In order to control complexity, use cases are grouped into use case packages. Each package contains a set of related use cases. For example, a word processor has many commands, but the commands are in groups, such as filing, editing text, setting styles and printing. The set of use cases constitutes the functional specification of a system. This in itself is valuable, but, as we shall see, use cases can also be used to: ■ derive the software structure ■ create test cases ■ help write a user manual ■ predict software cost. In some approaches to development, such as Agile Methods and the Unified Process (both discussed later in this book), use cases are the driving force behind the development process. 4.8 ● Use case diagrams We can document use cases, such as those we have met, as a UML use case diagram. Figure 4.1 shows the use case diagram for the ATM. There is a single actor, shown as a stick figure. The name of the role of the user is shown below. Arrows lead from the actor to the use cases, shown as ovals with their function named beneath. You will see that a use case diagram does not contain the detail associated with a (textual) use case. However, it does give an overall picture of the actors and the use cases. Thus a use case diagram is an informal graphical representation of requirements. Bank customer Figure 4.1 Use case diagram for the ATM withdraw cash check balance
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Software Engineering for Students D
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We work with leading authors to dev
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Pearson Education Limited Edinburgh
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vi Contents Part D ● Verification
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viii Detailed contents 3 The feasib
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x Detailed contents 9 Data flow des
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xii Detailed contents 14.7 Repetiti
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xiv Detailed contents 19.7 Unit tes
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Page 32 and 33: Analysis and design 1 /3 Coding 1 /
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Page 42 and 43: Exercises 19 • Exercises These ex
Page 44 and 45: Further reading 21 Analyses of the
Page 46 and 47: ■ documentation ■ maintenance
Page 48 and 49: 2.2 The tasks 25 An important examp
Page 50 and 51: 2.4 Methodology 27 reality. Like an
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Page 54 and 55: 3.2 ● Technical feasibility 3.3 C
Page 56 and 57: 3.5 Case study 33 The hardware cost
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Page 72: Further reading 49 Further reading
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7.4 Selecting control structures 95
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while do if endif then else endWhil
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• Exercises 7.1 Review the argume
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count = 0 loop: count = count + 1 i
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> 8.2 Case study 103 A statement th
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start button event create defender
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8.3 ● Discussion Abstraction One
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Exercises 109 skill. On the other h
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CHAPTER 9 This chapter explains: 9.
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9.2 Identifying data flows 113 Noti
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9.3 Creation of a structure chart 1
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SELF-TEST QUESTION 9.4 Discussion 1
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Exercises 119 During the second sta
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CHAPTER 10 This chapter explains:
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In English, this reads: 10.2 A simp
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10.2 A simple example 125 Now comes
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10.4 Multiple input and output stre
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Process header Process issue 10.4 M
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10.5 Structure clashes 131 As seen
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10.5 Structure clashes 133 Let us r
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10.6 Discussion 135 ■ teachable -
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Exercises 137 2. a control block, s
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CHAPTER 11 Object-oriented design T
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Figure 11.1 The cyberspace invaders
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SELF-TEST QUESTION 11.1 Derive info
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11.5 Class-responsibility-collabora
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11.7 ● Discussion Summary 147 OOD
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11.11 Compare and contrast the prin
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CHAPTER 12 This chapter explains: 1
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12.3 Delegation 153 The concepts of
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12.5 Factory method 155 The followi
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12.8 Model, view controller (observ
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Figure 12.4 Pipe and Filter pattern
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Figure 12.6 Layers in a distributed
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Answers to self-test questions 163
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CHAPTER 13 Refactoring This chapter
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13.3 ● Move Method 13.6 Inline Cl
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class Sprite Instance variables x y
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Summary Summary 171 it is making po
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PART C PROGRAMMING LANGUAGES
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176 Chapter 14 ■ The basics and a
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178 Chapter 14 ■ The basics > > >
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180 Chapter 14 ■ The basics > Ear
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182 Chapter 14 ■ The basics > Cas
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186 Chapter 14 ■ The basics > } }
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188 Chapter 14 ■ The basics Unfor
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190 Chapter 14 ■ The basics Ada d
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192 Chapter 14 ■ The basics The w
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194 Chapter 14 ■ The basics In a
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196 Chapter 14 ■ The basics > str
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198 Chapter 14 ■ The basics Answe
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CHAPTER 15 Object-oriented programm
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202 Chapter 15 ■ Object-oriented
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222 Chapter 16 ■ Programming in t
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238 Chapter 17 ■ Software robustn
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260 Chapter 18 ■ Scripting GNU/Li
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262 Chapter 18 ■ Scripting In sum
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PART D VERIFICATION
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268 Chapter 19 ■ Testing We begin
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270 Chapter 19 ■ Testing within a
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272 Chapter 19 ■ Testing Test num
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274 Chapter 19 ■ Testing if (a >=
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276 Chapter 19 ■ Testing 3. apply
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278 Chapter 19 ■ Testing made con
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280 Chapter 19 ■ Testing 19.3 Dev
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282 Chapter 19 ■ Testing 19.2 The
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284 Chapter 20 ■ Groups The term
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286 Chapter 20 ■ Groups Of course
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288 Chapter 20 ■ Groups • Exerc
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CHAPTER 21 This chapter explains: 2
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Stage Input Output 21.3 Feedback be
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Summary The essence and the strengt
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CHAPTER 22 This chapter: 22.1 ● I
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22.2 The spiral model 299 to try to
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22.4 ● Discussion Exercises 301 A
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CHAPTER 23 Prototyping This chapter
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Therefore, in summary: ■ the prod
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23.5 Evolutionary prototyping 307 U
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Reuse components 23.6 Rapid prototy
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Pitfalls For users, the problems of
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Answers to self-test questions 313
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24.2 ● Big-bang implementation 24
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Tested component Figure 24.1 Top-do
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24.7 ● Use case driven implementa
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■ middle-out ■ use case based.
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SELF-TEST QUESTION 25.1 What is the
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sharing of software or their own re
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Summary 327 Inappropriate patches,
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Further reading 329 Cathedral and t
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These are qualified by the statemen
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26.3 Extreme programming 333 develo
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SELF-TEST QUESTION 26.3 Which of th
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CHAPTER 27 This chapter explains:
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Figure 27.1 The phases of the unifi
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27.5 ● Iteration 27.6 Case study
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The transition phase Summary 343 Th
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PART F PROJECT MANAGEMENT
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348 Chapter 28 ■ Teams The commun
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350 Chapter 28 ■ Teams Level of s
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352 Chapter 28 ■ Teams A chief pr
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356 Chapter 28 ■ Teams • Furthe
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358 Chapter 29 ■ Software metrics
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31.3 Case study - assessing verific
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31.5 A single development method? 3
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Further reading 391 31.2 Draw up a
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32.3 ● The world of programming l
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32.5 ● The real world of software
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32.6 Control versus skill 397 Final
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Formal methods 32.7 Future methods
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Summary 401 In the short-term futur
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Further reading 403 An extensive tr
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APPENDIX A Case studies are used th
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Figure A.1 Cyberspace invaders A.4
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APPENDIX B Glossary Within the fiel
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C.2 ● Class diagrams C.2 Class di
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util Figure C.6 A package diagram S
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References to books and websites ar
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abstraction 99, 107 acceptance test
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fork 324 formal methods 276, 388, 3
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quality 18, 362 quality assurance 1
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