Java How to Program Fourth Edition - DCC

704 Graphical User Interface Components: Part 1 Chapter 12
draw and transfer funds). A person is an external entity that can play the part of a Bank-
Customer. In the same manner as an object is an instance of a class, a person playing the
part of a BankCustomer performing one of its roles (such as making a deposit) is an
instance of actor BankCustomer. For example, when a person named Mary plays the
part of a BankCustomer making a deposit, Mary—in the role of the depositor—becomes
an instance of actor BankCustomer. Later in that day, another person named Jon can be
another instance of actor BankCustomer. In the course of a day, several hundred people
might use the ATM machine—some are “depositors”, some are “withdrawers” and some
are “transferrers,” but all of these people are instances of actor BankCustomer.
The problem statement in our elevator simulation supplies the actors—“The user
requires the ability to create a person in the simulation and situate that person on a given
floor.” Therefore, the actor of our system is the user who controls the simulation (i.e., the
user who clicks the buttons to create new Persons in the simulation). An external entity—
a real person—plays the part of the user to control the simulation. In our system, the use
case is “Create Person,” which encompasses creating a Person object, then placing that
Person on either the first or second Floor. Figure 12.28 models one actor called “User.”
The actor’s name appears underneath the actor.
The system box (i.e., the enclosing rectangle in the figure) contains the use cases for
the system. Notice that the box is labeled “Elevator Simulation.” This title shows that this
use-case model focuses on the one use case that our simulation provides to users (i.e.,
“Create Person”). The UML models each use case as an oval. The system box for a system
with multiple use cases would have one oval per use case.
There is a reasonable alternate view of the use case of our elevator simulation. The
problem statement from Section 2.9 mentioned that the company requested the elevator
simulation to “determine whether the elevator will meet the company’s needs.” We are
designing a simulation of a real-world scenario—the Person object in the simulation represents
an actual human being using an actual elevator. Thus, we may view the user of the
elevator simulation as the user of the elevator. Therefore, specifying a use case from the
Person object’s perspective helps model how a real person uses a real elevator system.
We offer the use case of Fig. 12.29, titled “Relocate Person.” This use case describes the
Person moving (relocating) to the other Floor. (The Person travels to the second
Floor if starting on the first Floor and to the first Floor if starting on the second
Floor.) This use case encompasses all actions that the Person performs along his or her
journey, such as walking across a Floor to the Elevator, pressing Buttons and riding
the Elevator to the other Floor.
User
Elevator Simulation
Create Person
Fig. Fig. 12.28 12.28 Use-case diagram for elevator simulation from user’s perspective.
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