Java How to Program Fourth Edition - DCC

184 Control Structures: Part 1 Chapter 4
Consider the attributes of some real-world objects: A person’s attributes include height
and weight, for example. A radio’s attributes include its station setting, its volume setting
and whether it is set to AM or FM. A car’s attributes include its speedometer and odometer
readings, the amount of gas in its tank, what gear it is in, etc. A personal computer’s
attributes include its manufacturer (e.g., Sun, Apple, IBM or Compaq), type of screen (e.g.,
monochrome or color), main memory size (in megabytes), hard disk size (in gigabytes), etc.
We can identify the attributes of the classes in our system by looking for descriptive
words and phrases in the problem statement. For each descriptive word or phrase we find,
we create an attribute and assign that attribute to a class. We also create attributes to represent
any additional data that a class may need (as the need for this data becomes clear
throughout the design process).
We begin examining the problem statement looking for attributes distinct to each class.
Figure 4.17 lists the words or phrases from the problem statement that describe each class.
The sentence “The user can create any number of people in the simulation” implies that the
model will introduce several Person objects during execution. We require an integer
value representing the number of people in the simulation at any given time, because we
may wish to track, or identify, the people in our model. As mentioned in Section 2.9, the
ElevatorModel object acts as the “representative” for the model (even though the
model consists of several classes) for interactions with other parts of the system (in this
case, the user is a part of the system), so we assign the numberOfPeople attribute to
class ElevatorModel.
Class Elevator contains several attributes. The phrases “is moving” and “is summoned”
describe possible states of Elevator (we introduce states in the next “Thinking
About Objects” section), so we include moving and summoned as boolean attributes.
Elevator also arrives at a “destination floor,” so we include the attribute destinationFloor,
representing the Floor at which the Elevator will arrive. Although the
problem statement does not mention explicitly that the Elevator leaves from a current
Floor, we may assume another attribute called currentFloor representing on which
Floor the Elevator is resting. The problem statement specifies that “both the elevator
and each floor have capacity for only one person,” so we include the capacity attribute for
class Elevator (and class Floor) and set the value to 1. Lastly, the problem statement
specifies that the elevator “takes five seconds to travel between floors,” so we introduce the
travelTime attribute and set the value to 5.
Class Person contains several attributes. The user must be able to “create a unique
person,” which implies that each Person object should have a unique identifier. We
assign integer attribute ID to the Person object. The ID attribute helps to identify that
Person object. In addition, the problem statement specifies that the Person can be
“waiting on that floor to enter the elevator.” Therefore, “waiting” is a state that Person
object may enter. Though not mentioned explicitly, if the Person is not waiting for the
Elevator, the Person is moving to (or away from) the Elevator. We assign the
boolean attribute moving to class Person. When this attribute is set to false, the
Person is “waiting.” Lastly, the phrase “on that floor” implies that the Person occupies
a floor. We cannot assign a Floor reference to class Person, because we are interested
only in attributes. However, we want to include the location of the Person object in the
model, so we include the currentFloor attribute, which may have a value of either 1
or 2.
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