Chapter 21 Design Patterns Reviewed from the New Perspective of Object-Oriented Design

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Chapter 21Design Patterns Reviewedfrom the New
Perspective of Object-Oriented Design

• Ku-Yaw Chang
• canseco_at_mail.dyu.edu.tw
• Assistant Professor, Department of Computer
  Science and Information Engineering
• Da-Yeh University

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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A Summary ofObject-Oriented Principles

• Objects are things with well-defined
  responsibilities.
• Objects are responsible for themselves.
• Encapsulation means any kind of hiding
• Data hiding
• Class hiding
• Implementation hiding
• Abstract out variations in behavior and data with
  commonality/variability analysis.
• Design to interfaces.

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A Summary ofObject-Oriented Principles

• Think of inheritance as a method of
  conceptualizing variation, not for making special
  cases of existing objects.
• Keep variations in a class decoupled from other
  variations in the class.
• Strive for loose coupling.
• Strive for high cohesion.
• Be absolutely meticulous in applying the once and
  only once rule

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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How Design PatternsEncapsulate Implementations

• Most patterns described by GoF
• Give ways to hide specific implementations
• Allow for easily adding new implementations

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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Commonality/Variability Analysis and Design
Patterns

• An example
• Draw a square with drawing program one
• Draw a circle with drawing program two
• Draw a rectangle with drawing program one
• Tow commonalities
• Drawing programs
• Shapes to draw
• Patterns
• Provide the backdrop in analysis and design
• Give a common vocabulary for discussion
• Incorporate best-practice approaches into your
  code

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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Decomposing a Problem Domain into Responsibilities

• In the Bridge pattern
• Problem domain composed of two different types of
  entities
• Abstractions
• Implementations
• Decompose the problem domain
• Objects
• Which objects are needed
• Responsibilities
• How these objects are instantiated

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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Relationships Within a Pattern

• Alexander
• At this final stage, the patterns are no longer
  important.
• Shalloway
• The patterns have taught your to be receptive to
  what is real.
• Relationships and forces described by the
  patterns
• It is not the patterns themselves that are
  important

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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Patterns and Contextual Design

• Patterns are microcosmic examples of contextual
  design
• Designing to an interface is designing within a
  context

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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Field Notes

• What implementations does this pattern hide?
• Allow for changing them
• What commonalities are present in this pattern?
• Identify them
• What are the responsibilities of the objects in
  this pattern?
• Easier to do my decomposition by responsibility

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Field Notes

• What are the relationships between these objects?
• More information about the forces
• How may the pattern itself be a microcosmic
  example of designing by context
• A better understanding of why the pattern is good
  design

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Outline

• A Summary of Object-Oriented Principles
• How Design Patterns Encapsulate Implementations
• Commonality/Variability Analysis and Design
  Patterns
• Decomposing a Problem Domain into
  Responsibilities
• Relationships Within a Pattern
• Patterns and Contextual Design
• Field Notes
• Summary

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Summary

• Look at patterns by seeing
• What they encapsulate
• How they use commonality/variability analysis
• How they decompose a problem domain into
  responsibilities
• How they specify relationships between objects
• How they illustrate contextual design

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The End