Object-oriented Design

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Object-oriented Design
Andrew Ireland School of Mathematical Computer
Sciences Heriot-Watt University Edinburgh
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Outline

• Characteristics of OOD
• Basic Ingredients of OOD Methods
• An example of OOD

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Characteristics OOD

• Abstraction Objects provide a useful and natural
  abstraction for many engineered artefacts and
  real-world systems
• Encapsulation Hiding within an Object those
  details which are not essential to an abstraction
• Modularity Separation of concerns (often merged
  with encapsulation)
• Hierarchy Ranking between abstractions class
  structure, includes,

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Many OOD Methods!

• The Booch Method
• The Coad Yourdon Method
• The Rambaugh Method
• The Wirfs-Brock Method

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OOD Common Ingredients

• Identification of objects (classes), e.g.
• Use cases
• Grammatical analysis
• Class-Responsibility-Collaborator modelling
• Identification of static perspective, e.g.
• Class diagrams
• Identification of dynamic perspective, e.g.
• Communication diagrams
• Activity diagrams
• State machine diagrams

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Grammatical Analysis

• Nouns suggest objects
• Verbs suggest operations
• Example
• An ATM will allow a customer to withdraw funds
  and display their account balance. Account
  details are held centrally. An account can be
  credited or debited. In addition, the bank can
  request for an account to be frozen at anytime.

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Grammatical Analysis

• Nouns suggest objects
• Verbs suggest operations
• Example
• An ATM will allow a customer to withdraw funds
  and display their account balance. Account
  details are held centrally. An account can be
  credited or debited. In addition, the bank can
  request for an account to be frozen at anytime.

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Grammatical Analysis
ATM
customer
account
bank
withdraw display
credit debit freeze
Note that the analysis only provides a starting
point
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Class-Responsibility-Collaborator

• Class-Responsibility-Collaborator (CRC) modelling
  provides a simple mechanism for identifying and
  organizing classes
• A CRC model contains a collection of index
  cards
• Class name
• Class type, e.g. device, property, role,
• Class characteristics, e.g. atomic, tangible,
  persistence, integrity, concurrent,
• Responsibilities, i.e. attributes operations
• Collaborations, i.e. where interaction with other
  classes is required

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Guidelines for Responsibility

• System intelligence should be evenly distributed
  promotes cohesion
• Each responsibility should be stated as generally
  as possible promotes the use of inheritance and
  polymorphism
• Information and the behaviour is related to it
  should reside within the same class promotes
  encapsulation/modularity

(WIR 90)
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Guidelines for Collaborations

• A responsibility that can not be met internally
  (attributes operations) will require
  collaboration with other classes
• Identify generic relationships between classes
• is-part-of relationship, e.g.
• brake is-part-of control-system
• has-knowledge-of, e.g.
• ATM has-knowledge-of customer-credit-limit
• depends-upon, e.g.
• If A provides C with X and C provides B with X
  then B depends-upon A

(WIR 90)
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Automatic Train Protection (ATP)
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Automatic Train Protection (ATP)

• The Ladbroke Grove rail disaster (London) took
  place on Oct 5, 1999
• 31 people were killed 520 injured as a result
  of a 2 train collision
• The collision was caused by a Signal Passed At
  Danger (SPAD)
• An ATP system would have prevented this disaster
  

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ATP Requirements

• The ATP controller is switched on via the
  drivers control panel
• The ATP controller is switched off via the
  control panel, and results in the brakes being
  activated
• When a train passes a track-side signal, signal
  sensors mounted on the train relay the signal
  aspect (proceed, caution, danger) to the
  controller
• If a proceed signal is reported then no action
  takes place
• If a caution signal is reported then an audio
  alarm is enabled. If the driver presses an
  acknowledgement button on the control panel
  within 5secs then the alarm is disable, otherwise
  a timeout occurs and the controller activates the
  trains brakes. If the alarm is disabled in time
  then the controller assesses the trains speed via
  speed sensors. If the trains speed is decreasing
  then the controller awaits further signals,
  otherwise the controller activates the trains
  brakes
• If a danger signal is reported then the
  controller activates the brakes

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ATP Requirements

• Sensing will be performed by majority vote, e.g.
  if 2 signal sensors report caution, while the
  remaining sensor reports proceed, then a
  caution signal will be reported to control

Sensor 1
Calculate majority
Sensor 2
caution
signal
Sensor 3
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Object Identification
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Use Cases
ATP
start system
driver
stop system
ltltincludesgtgt
auto brake
track-side signal sensors
ltltextendsgtgt
react to sensors/ driver
enable/disable alarm
speed sensors
ltltincludesgtgt
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Use Case Specification

• USE CASE ATP
• GOAL React to caution signal when speed
  deceasing
• ACTORS Driver, Track-side Signal Sensors, Speed
  Sensors
• MAIN SUCCESS SCENARIO
• The controller is informed of a signal aspect
• If a caution signal is reported then the
  controller enables the alarm
• If the driver presses the acknowledge (alarm)
  button within 5secs then the controller will
  disables the alarm
• The controller then requests the trains speed
  from the speed sensors, if the speed is
  decreasing then no action is taken
• EXTENSIONS
• 3a. If driver does not acknowledge alarm with
  5secs then enable automatic brakes
• 4a. If speed is not decreasing then enable
  automatic brakes

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Grammatical Analysis

• The ATP controller is switched on via the
  drivers control panel
• The ATP controller is switched off via the
  control panel, and results in the brakes being
  activated
• When a train passes a track-side signal, signal
  sensors mounted on the train relay the signal
  aspect (proceed, caution, danger) to the
  controller
• If a proceed signal is reported then no action
  takes place
• If a caution signal is reported then an audio
  alarm is enabled. If the driver presses an
  acknowledgement button on the control panel
  within 5secs then the alarm is disable, otherwise
  a timeout occurs and the controller activates the
  trains brakes. If the alarm is disabled in time
  then the controller assesses the trains speed via
  speed sensors. If the trains speed is decreasing
  then the controller awaits further signals,
  otherwise the controller activates the trains
  brakes
• If a danger signal is reported then the
  controller activates the brakes

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Grammatical Analysis

• The ATP controller is switched on via the
  drivers control panel
• The ATP controller is switched off via the
  control panel, and results in the brakes being
  activated
• When a train passes a track-side signal, signal
  sensors mounted on the train relay the signal
  aspect (proceed, caution, danger) to the
  controller
• If a proceed signal is reported then no action
  takes place
• If a caution signal is reported then an audio
  alarm is enabled. If the driver presses an
  acknowledgement button on the control panel
  within 5secs then the alarm is disable, otherwise
  a timeout occurs and the controller activates the
  trains brakes. If the alarm is disabled in time
  then the controller assesses the trains speed via
  speed sensors. If the trains speed is decreasing
  then the controller awaits further signals,
  otherwise the controller activates the trains
  brakes
• If a danger signal is reported then the
  controller activates the brakes

21
Grammatical Analysis

• The ATP controller is switched on via the
  drivers control panel
• The ATP controller is switched off via the
  control panel, and results in the brakes being
  activated
• When a train passes a track-side signal, signal
  sensors mounted on the train relay the signal
  aspect (proceed, caution, danger) to the
  controller
• If a proceed signal is reported then no action
  takes place
• If a caution signal is reported then an audio
  alarm is enabled. If the driver presses an
  acknowledgement button on the control panel
  within 5secs then the alarm is disable, otherwise
  a timeout occurs and the controller activates the
  trains brakes. If the alarm is disabled in time
  then the controller assesses the trains speed via
  speed sensors. If the trains speed is decreasing
  then the controller awaits further signals,
  otherwise the controller activates the trains
  brakes
• If a danger signal is reported then the
  controller activates the brakes

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Classes
Alarm
Panel
SigSen
enable disable
Brakes
SpdSen
Ctrl
on off ack sig_update time_out
activate
read_speed
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CRC for ATP Controller
Class name Ctrl Class type device Class
characteristics tangible, atomic

• Responsibilities
• decide if alarm should be
  enabled/disabled
• maintain record of speed
• decide if brakes should be activated
• manage timeouts

• Collaborators
• signal sensor
• speed sensor
• alarm
• brakes

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Static Perspective
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Class Diagrams
Alarm
Panel
SigSen
disable
sig_update
enable
on
off ack
activate
read_speed
Brakes
SpdSen
Ctrl
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Class Diagrams
Sensor

• calc_majority
• read_sensors

SigSen
SpdSen
- read_sensors
- read_sensors
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Dynamic Perspective
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Sequence Diagrams
SigSen
Panel
Alarm
SpdSen
Ctrl
Brakes
1. pass_signal
1.1 sig_update
1.2 enable
2 press_ack_button
2.1 ack
2.1.1 disable
2.1.2 read_speed
Caution signal received along with an
acknowledgement and a speed reduction
2.1.3 chk_speed
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Sequence Diagrams
SigSen
Panel
Alarm
SpdSen
Ctrl
Brakes
1 pass_signal
1.1 sig_update
1.2 enable
2 press_ack_button
2.1 ack
2.1.1 disable
2.1.2 read_speed
Caution signal received along with an
acknowledgement but, no speed reduction
2.1.3 chk_speed
2.1.4 activate
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Communication Diagrams
1 pass_signal
SpdSen
SigSen
2.1.2 read_speed
1.1 sig_update
1.2 enable
2.1.1 disable
Alarm
Ctrl
2.1 ack 3.1 on 4.1 off
2.14 activate
2.13 chk_speed
Panel
Brakes
2 press_ack_button
3 press_on_button
4 press_off_button
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State Machine Diagrams
proceed signal
speed check speed decreasing
caution signal
driver ack
proceed state
caution state I
caution state II
time_out
speed check speed not decreasing
danger signal
danger state
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Summary

• Learning outcomes
• Basic ingredients of an OOD method
• Object identification
• Structural perspective
• Dynamic perspective
• Recommended reading
• D. Budgen, Software Design, Addison-Wesley 2003
• I. Sommerville, Software Engineering,
  Addison-Wesley 2007
• R. Wirfs-Brock, B. Wilkerson, L. Weiner,
  Deigning Object-Oriented Software,
  Prentice-Hall 1990