1. Optional Case Study - Chapter 4
Outline
4.1 Introduction
4.2 Class Operations
4.3 Creating Class Operations
4.4 Class Clock, Building
4.5 Class Scheduler
4.6 Class Person
4.7 Class Floor, FloorButton, ElevatorButton
4.8 Class Door
4.9 Sequence Diagrams
4.10 Conclusion

2. 4.1 Introduction Previous chapters This chapter Chapter 5
Chapter 2 - identified classes
Chapter 3 - determined many attributes of our classes
This chapter
Determine operations (behaviors)
Chapter 5
Focus on interactions between classes

3. 4.2 Class Operations Class operations Deriving operations
Service class provides to clients
Radio - setting station, volume
Objects usually do not perform operations spontaneously
Operations invoked by sending object (client object)
Sends message to receiving object (server object)
Requests object perform specific operation
Deriving operations
Examine problem statement for verbs and verb phrases
Relate phrases to particular classes

4. 4.2 Class Operations (II)

5. 4.3 Creating Class Operations
Creating operations
Examine verb phrase
"moves" verb is listed with Elevator
Should "moves" be an operation?
No message tells elevator to move
Moves in response to a button, on condition that door is closed
"Moves" should not be an operation
"Arrives at floor" should not be an operation
Elevator itself decides when to arrive, based on time
"resets elevator button" - implies elevator sends message to elevator button, telling it to reset
ElevatorButton needs an operation to provide this service to the elevator

6. 4.3 Creating Class Operations (II)
Format
Write operation name in bottom compartment in class diagram
Write operations as function names, include return type, parameters
resetButton() : void
Function takes no parameters, returns nothing
Other verbs
Bell - provides service of ringing
Floor - signal arrival of elevator
Door - open and close

7. 4.3 Creating Class Operations (III)
open : bool = false
Door
openDoor( ) : void
closeDoor( ) : void
<none yet>
Bell
ringBell( ) : void
on : bool = false
Light
turnOff( ) : void
turnOn( ) : void
pressed : bool = false
FloorButton
pressButton( ) : void
resetButton( ) : void
time : int = 0
Clock
getTime( ) : int
tick( ) : void
occupied : bool = false
Floor
elevatorArrived( ) : void
isOccupied( ) : bool
Person
ID : int
stepOntoFloor( ) : void
exitElevator( ) : void
enterElevator( ) : void
ElevatorButton
Elevator
currentFloor : int = 1
direction : enum = up
capacity : int = 1
arrivalTime : int
moving : bool = false
processTime( time : int ) : void
personEnters( ) : void
personExits( ) : void
summonElevator( ) : void
prepareToLeave( ) : void
Scheduler
floor1ArrivalTime : int
floor2ArrivalTime : int
Building
runSimulation( ) : void

8. Building, once per second, will
4.4 Class Clock, Building
Class Clock
Has phrase "ticks every second"
"Getting the time" is an operation clock provides
Is the ticking also an operation?
Look at how simulation works
Building, once per second, will
Get time from clock
Give time to scheduler
Give time to elevator
Building has full responsibility for running simulation, therefore it must increment clock
getTime and tick therefore operations of Clock
processTime operation of Scheduler and Elevator

9. 4.5 Class Scheduler Class Scheduler
"randomly schedules times", "delays creating a person by one second"
Scheduler performs these actions itself, does not provide service to clients
"create person" - special case
A Person object cannot respond to a create message because it does not yet exist
Creation left to implementation details, not an operation of a class
More Chapter 7
"tells a person to step onto a floor" - class Person should have a function that the scheduler can invoke
stepOntoFloor - operation of class Person

10. 4.6 Class Person More verb phrases Class Person
"verifies a floor is unoccupied" - class Floor needs a service to let other objects know if a floor is occupied or not
isOccupied returns true or false
Class Person
"presses floor button", "presses elevator button"
Place operation pressButton under classes FloorButton and ElevatorButton
"enter elevator", "exit elevator" - suggests class Elevator needs operations to correspond to these actions
Discover what, if any, actions operations perform when concentrate on implementation

11. 4.7 Class Floor, FloorButton, ElevatorButton
"resets floor button" - resetButton operation
"turns off light", "turns on light" - turnOff and turnOn in class Light
Class FloorButton and ElevatorButton
"summons elevator" - summonElevator operation in class Elevator
"signals elevator to move" - elevator needs to provide a "move" service
Before it can move, must close door
prepareToLeave operation (performs necessary actions before moving) in class Elevator

12. 4.8 Class Door Class Door Notes
Phrases imply door sends message to person to tell it to exit or enter elevator
exitElevator and enterElevator in class Person
Notes
We do not overly concern ourselves with parameters or return types
Only want a basic understanding of each class
As we continue design, number of operations may vary
New operations needed
Some current operations unnecessary

13. 4.9 Sequence Diagrams Sequence Diagram Format
Model our "Simulation loop"
Focuses on how messages are sent between objects over time
Format
Each object represented by a rectangle at top of diagram
Name inside rectangle (objectName)
Lifeline - dashed vertical line, represents progression of time
Actions occur along lifeline in chronological order, top to bottom
Line with arrowhead - message between objects
Invokes corresponding operation in receiving object
Arrowhead points to lifeline of receiving object
Name of message above message line, includes parameters
Parameter name followed by colon and parameter type

14. 4.9 Sequence Diagrams (II)
: Building
{currentTime < totalTime}
tick( )
getTime( )
processTime( currentTime : int )
processTime( currentTime : int )
time
: Scheduler
: Clock
: Elevator

15. 4.9 Sequence Diagrams (III)
Flow of control
If object returns flow of control or returns a value, return message (dashed line with arrowhead) goes back to original object
Clock object returns time in response to getTime message received from Building object
Activations
Rectangles along lifelines - represent duration of an activity
Height corresponds to duration
Timing constraint
In our diagram, text to far left
While currentTime < totalTime objects keep sending messages as in the diagram

16. building sends processTime message to scheduler
processTime( time )
bool
isOccupied( ) : bool
: Person
scheduleArrival( floor1 )
personArrives( )
[occupied = false]
create
[occupied = true]
scheduleArrival( floor2 )
building : Building
scheduler : Scheduler
floor1 : Floor
floor2 : Floor
delayArrival( floor1 )
delayArrival( floor2 )
building sends processTime message to scheduler
scheduler decides whether to create a new person Can only create a new person if floor unoccupied Checks by sending isOccupied message to floor object
The two lifelines converge
scheduler handles second floor same way as the first When finished, returns control to building
floor1 returns true or false
scheduler's lifeline splits - conditional execution of activities. Condition supplied for each lifeline.
If true - scheduler calls delayArrival Not an operation - not invoked by another object
If false - scheduler creates new Person object When new objects created, rectangle corresponds to time. Message "create" sent from one object to another (arrowhead points to new object).
After new Person object created, it steps on first floor
Person object sends personArrives message to floor1 object
scheduler schedules new arrival for floor1 calls its own scheduleArrival function (not an operation)

17. Discussed operations of classes Chapter 5
4.10 Conclusion
Discussed operations of classes
Introduced sequence diagrams to illustrate operations
Chapter 5
Examine how objects interact with each other