Software Engineering for Students A Programming Approach

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