ObjectOriented Design

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Chapter 12

• Object-Oriented Design

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Objectives

• To explain how a software design may be
  represented as a set of interacting objects that
  encapsulate their own state and operations.
• To describe the activities in the object-oriented
  design process.

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Objectives

• To introduce various models used to describe an
  object-oriented design
• To show how the UML may be used to represent
  these models

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Topics covered

• Objects and object classes
• An object-oriented design process
• Design evolution

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Characteristics of OOD

• Allows designers to think in terms of interacting
  objects that maintain their own state and provide
  operations on that state instead of a set of
  functions operating on shared data.
• Objects hide information about the
  represen-tation of state and hence limit access
  to it.
• Objects may be distributed and may work
  sequentially or in parallel.

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A design strategy based on information hiding

• Another way to think about information hiding
• Potentially changeable design decisions are
  isolated (i.e., hidden) to minimize the impact
  of change.
• - David Parnas

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Interacting objects
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Advantages of OOD

• Easier maintenance. Objects may be understood
  as stand-alone entities (and ripple effect is
  reduced).
• Objects are appropriate reusable components.
• For some systems, there is an obvious mapping
  from real world entities to system objects.

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Object-oriented development

• OO Analysis concerned with developing an
  object model of the application domain.
• OO Design concerned with developing an
  object-oriented system model to implement
  requirements
• OO Programming concerned with realising an OOD
  using an OO pro-gramming language such as Java or
  C.

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The history of OO Development as reflected by
Sommervilles text

• 1st Edition (1982) No mention of OO design!
• 2nd Edition (1985) it has been sug-gested that
  a better design methodology is OO design To
  place such comments in perspective, many large
  systems have been built using top-down design.
  Few large systems have been built using an
  object-oriented approach.

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The history of OO Development as reflected by
Sommervilles text

• 3rd Edition (1989) It is only within the past
  few years that an alternative mode of
  decomposition, OO design, has been recognized as
  of value.

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The history of OO Development as reflected by
Sommervilles text

• 5th Edition (1995) Until relatively recently,
  the most commonly used software design strategy
  involved decomposing the design into functional
  components with system state infor-mation held in
  a shared data areaIt is only since the late
  1980s thatOO design has been widely adopted.

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The history of OO Development as reflected by
Sommervilles text

• 6th Edition(2001) An OO approach to the whole
  (of) software development is now commonly used
• Coverage of functional design is now included in
  the new chapter on legacy systems.

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The history of OO Development as reflected by
Sommervilles text

• Although many software developers consider
  function-oriented design to be an outdated
  approach, OO development may not offer
  significant advantages (in some) situations. An
  interesting challengeis to ensure that
  function-oriented and OO systems can work
  together.

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Objects and object classes

• Objects are entities with state and a defined set
  of operations on that state.
• State is represented as a set of object
  attributes.
• Operations provide services to other objects when
  requested.

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Objects and object classes

• Object classes are templates for objects.
• An object class definition includes declarations
  of all attributes and operations associated with
  an object of that class.
• They may inherit attributes and services from
  other object classes.

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The Unified Modeling Language

• Several different notations for OOD were proposed
  in the 1980s and 1990s. (Booch, Rumbaugh,
  Jacobson, Coad Yourdon, Wirfs, )
• UML is an integration of these notations.
• It describes a number of different models that
  may be produced during OO analysis and design
  (user view, structural view, behavioural view,
  implementation view, )
• The de facto standard for OO modelling.

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Employee object class (UML)
Object attributes
Services to other objects
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Object communication

• Conceptually, objects communicate by message
  passing.
• Messages include
• The name of the service requested,
• A copy of the information required to carry out
  the service, and
• the name of a holder for the result of the
  service.
• In practice, messages are often imple-mented by
  procedure calls

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Message examples

• // Call a method associated with a buffer //
  object that returns the next value // in the
  buffer
• v circularBuffer.Get ()
• // Call the method associated with a//
  thermostat object that sets the // temperature
  to be maintained
• thermostat.setTemp (20)

Holder for result
Name of service
Info required
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Generalization and inheritance

• Objects are members of classes which define
  attribute types and operations.
• Classes may be arranged in a hierarchy where one
  class (a super-class) is a generalization of one
  or more other classes (sub-classes)

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Generalization and inheritance

• A sub-class inherits the attributes and
  operations from its super class and may add new
  methods or attributes of its own.

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A UML generalisation hierarchy
Definition on p. 18
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Advantages of inheritance

• It is an abstraction mechanism which may be used
  to classify entities.
• It is a reuse mechanism at both the design and
  the programming level.
• The inheritance graph is a source of
  organisational knowledge about domains and
  systems. (OO Analysis)

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Problems with inheritance

• Object classes are not self-contained (i.e., they
  cannot be understood without reference to their
  super-classes).
• Designers have a tendency to reuse the
  inheritance graph created during analysis.
  (Inheritance graphs of analysis, design and
  implementation have different functions.)

Due to inherited attributes/ops
Discipline/education problem?
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Inheritance and OOD

• Inheritance is a useful implementation concept
  which allows reuse of attribute and operation
  definitions.
• Some feel that identifying an inheritance
  hierarchy or network is also a funda-mental part
  of object-oriented design. (Obviously, this can
  only be implemented directly using an OOPL.)

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Inheritance and OOD

• Others feel this places unnecessary restrictions
  on the implementation.
• Inheritance introduces complexity and this is
  undesirable, especially in critical systems.
• This appears to be Sommervilles view

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UML associations

• Objects and object classes participate in various
  types of relationships with other objects and
  object classes.
• In the UML, a generalized relationship is
  indicated by an association.

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UML associations

• Associations may be annotated with information
  that describes their nature.
• Associations can be used to indicate that an
  attribute of an object is an associated object or
  that a method relies on an associated object.

(semantics)
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An association model
annotations
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Concurrent objects

• The nature of objects as self-contained entities
  make them well suited for con-current
  implementation.
• The message-passing model of object
  communication can be implemented directly if
  objects are running on separate processors in a
  distributed system. (as opposed to using
  procedure calls)

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Concurrent object implementation servers and
active objects

• Servers (Passive objects) implemented as
  parallel processes with entry points
  correspon-ding to object operations. If no calls
  are made to it, the object suspends itself and
  waits for further requests for service.
• Active objects implemented as parallel
  processes and the internal object state may be
  changed by the object itself and not simply by
  external calls.

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Example an active transponder object

• A transponder object broadcasts an aircrafts
  position. (on demand)
• The object periodically updates the position by
  triangulation from satellites. (autonomously)

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An active transponder object
Java-based interface description
ON DEMAND
IN BACKGROUND
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Active object implementation Java threads, Ada
tasks, etc.

• Threads in Java are a simple construct for
  implementing concurrent objects.
• Threads must include a method called run() and
  this is started up by the Java run-time system.
• Active objects typically include an infinite loop
  so that they are always carrying out the
  computation.

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An object-oriented design process (an iterative,
boot-strapping process)

• Define the context and modes of use of the
  system.
• Design the system architecture.
• Identify the principal system objects.
• Develop design models (static and dynamic).
• Specify object interfaces.

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Weather system description
A weather data collection system is required to
generate weather maps on a regular basis using
data collected from remote, unattended weather
stations and other data sources such as weather
observers, balloons and satellites. Weather
stations transmit their data to the area computer
in response to a request from that machine.
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Weather system description
The area computer validates the collected data
and integrates it with the data from different
sources. The integrated data is archived and,
using data from this archive and a digitised map
database a set of local weather maps is created.
Maps may be printed for distribution on a
special-purpose map printer or may be displayed
in a number of different formats.
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Weather station description
A weather station is a package of software
controlled instruments which collects data,
performs some data processing and transmits this
data for further processing. The instruments
include air and ground thermometers, an
anemometer, a wind vane, a barometer and a rain
gauge. Data is collected every five minutes.
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Weather station description
When a command is issued to transmit the weather
data, the weather station processes and
summarises the collected data. The summarised
data is transmitted to the mapping computer when
a request is received.
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Define system context and modes of use

• Goal develop an understanding of the
  relationships between the software being designed
  and its external environment.
• System context a static model that describes
  other systems in the environ-ment.
• The context of the weather station is illustrated
  below using UML packages.

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Context of weather station
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Define system context and modes of
use (cont.)

• Modes of system use a dynamic model that
  describes how the system will interact with its
  environment.
• Modes of weather station use are illustrated
  below using a UML use-case model.

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Use-cases for the weather station
External entity (weather data collection sys)
Possible interactions
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Use-case description
basis for information hiding
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Design system architecture

• A layered architecture is appropriate for the
  weather station
• Interface layer for handling communications
• Data collection layer for managing instruments
• Instruments layer for collecting data
• Rule of Thumb There should be no more than 7
  entities in an architectural model. (See Miller,
  "The Magical Number Seven, Plus or Minus Two,
  WOR. )

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Weather station architecture
UML annotations
UML nested packages
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Identify principal system objects

• Identifying objects (or object classes) is the
  most difficult part OO design.
• There is no magic formula it relies on the
  skill, experience, and domain knowledge of system
  designers
• An iterative process you are unlikely to get it
  right the first time.

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Approaches to object identification

• Use a grammatical approach based on a natural
  language description of the system (Abbotts
  heuristic).
• Associate objects with tangible things in the
  application domain (e.g. devices).
• Use a behavioural approach identify objects
  based on what participates in what behaviour.

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Approaches to object identification (cont.)

• Use scenario-based analysis. The objects,
  attributes and methods in each scenario are
  identified.
• Use an information-hiding based approach.
  Identify potentially change-able design decisions
  and isolate these in separate objects.
• Bonus approach! (No extra charge.)

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Weather station object classes

• Weather station interface of the weather
  station to its environment. It reflects
  interactions identified in the use-case model.
• Weather data encapsulates summarised data from
  the instruments.
• Ground thermometer, Anemometer, Barometer
  application domain hardware objects related to
  the instruments in the system.
• hardware-controlling SOFTWARE

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Weather station object classes
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Other objects and object refinement

• Use domain knowledge to identify more objects,
  operations, and attributes.
• Weather stations should have a unique identifier.
• Weather stations are remotely situated so
  instrument failures have to be reported
  automatically. Therefore, attributes and
  operations for self-checking are required.

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Other objects and object refinement (cont.)

• Active or passive objects?
• Instrument objects are passive and collect data
  on request rather than autonomously. This
  introduces flexibility (how?) at the expense of
  controller processing time.
• Are any active objects required?
• Hint consider potentially changeable design
  decisions

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Develop design models

• Design models show the relationships among
  objects and object classes.
• Static models describe the static structure of
  the system in terms of object and object class
  relationships.
• Dynamic models describe the dynamic interactions
  among objects.

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Examples of design models

• Sub-system models show logical groupings of
  objects into coherent sub-systems. (static)
• Sequence models show the sequence of object
  interactions associated with system uses.
  (dynamic)

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Examples of design models

• State machine models show how indi-vidual objects
  change their state in re-sponse to events.
  (dynamic)
• Other models include use-case models, aggregation
  models, generalisation (inheritance) models, etc.

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Subsystem models

• In the UML, these are shown using packages, an
  encapsulation construct.
• This is a logical model the actual organization
  of objects in the system as implemented may be
  different.

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Weather station subsystems
Active object
Annotations go here
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Sequence models

• Objects are arranged horizontally across the top.
• Time is represented vertically models are read
  top to bottom.
• Interactions are represented by labelled arrows
  different styles of arrows represent different
  types of interaction.
• A thin rectangle in an object lifeline represents
  the time when the object is the controlling
  object in the system.

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Data collection sequence
Return of control
No reply expected
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Weather station state machine model
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Object interface specification

• Designers should avoid revealing data
  representation information in their interface
  design. (operations access and update all data)
• Objects may have several logical interfaces which
  are viewpoints on the methods provided.
  (supported directly in Java)
• The UML uses class diagrams for interface
  specification but pseudocode may also be used.

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Weather station interface (Java-based)
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Design evolution

• Hiding information in objects means that changes
  made to an object need not affect other objects
  in an unpredictable way.
• Assume pollution monitoring facilities are to be
  added to weather stations.
• Pollution readings are transmitted with weather
  data.

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Changes required

• Add an object class called Air quality as part
  of WeatherStation.
• Add an operation reportAirQuality to
  WeatherStation. Modify the control software to
  collect pollution readings.
• Add objects representing pollution monitoring
  instruments.
• and thats all.

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Pollution monitoring
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Key points

• OOD results in design components with their own
  private state and operations.
• Objects should have constructor and inspection
  operations. They provide services to other
  objects.
• Objects may be implemented sequentially or
  concurrently.
• The Unified Modeling Language provides different
  notations for defining different object models.

as do algebraic specs
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Key points

• A range of different models may be produced
  during an object-oriented design process. These
  include static and dynamic system models (See
  the OMG website for sources.)
• Object interfaces should be defined precisely.
• Object-oriented design simplifies system
  evolution.