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Patterns in Software Patterns in Software DesignDesign

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ObjectivesObjectives

To explain why design patterns are important

To define software design patterns To present a taxonomy of design

patterns based on their granularity To discuss pattern catalogs

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TopicsTopics

Why design patterns are important

Christopher AlexanderSoftware design patternsPattern granularityPattern catalogs

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Why Design Patterns?Why Design Patterns?

Expert designers behave differently from novices—what do experts know that novices do not?

Among other things, experts have a store of successful design patterns from past experience that they apply to new problems.

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More Pattern More Pattern AdvantagesAdvantages

Promoting communication—Pattern names and knowledge of advantages and disadvantages speeds communication

Streamlining documentation—Pattern form and behavior need not be elaborated

Increasing efficiency—Tool support for patterns makes development faster

Supporting reuse—Patterns and their implementations can be reused extensively

Providing ideas—Patterns can be the starting point for design or a basis for improvements

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Design Patterns in Design Patterns in Building ArchitectureBuilding Architecture

In the 1970s architect Christopher Alexander introduced a new approach to building design based on design patterns.

Alexander argued that• patterns are consequences of human

anatomy, psychology, physiology, sociology, and politics;

• great architecture has always relied on patterns, but they have never been studied systematically; and

• anyone can make great buildings once the patterns are known and understood.

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Examples of Alexander’s Examples of Alexander’s PatternsPatterns

Scattered Work—Use laws and incentives to encourage work places throughout a city.

Four-Story Limit—In an urban area, keep most buildings four stories high or less.

South Facing Outdoors—Always build structures north of the outdoor spaces that go with them.

Warm Colors—Choose light and surface colors to achieve a yellow-red light tone in a room.

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Results of Alexander’s Results of Alexander’s WorkWork

Alexander’s work has not been widely accepted among building architects, and his efforts to get non-professionals to design great buildings have often failed, as he himself admits.

But Alexander has inspired a community of software developers to generate work in design patterns.

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Design Patterns Design Patterns DefinedDefined

A pattern is a model proposed for imitation. A software design

pattern is a model proposed for imitation in solving a software

design problem.

A pattern is a model proposed for imitation. A software design

pattern is a model proposed for imitation in solving a software

design problem.

The pattern community generally prefers a more complex definition of patterns involving context, resolved forces, etc.

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Design Pattern Design Pattern GranularityGranularity

Software design patterns have no inherent granularity.•Architectural styles or patterns are for

entire systems and sub-systems.•Design patterns proper involve several

interacting functions or classes.•Data structures & algorithms are low-

level patterns.•Idioms are ways of doing things in

particular programming languages.

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Pattern CatalogsPattern Catalogs

Realization of the importance of design patterns has spurred creation of catalogs of patterns.

These are much like the pattern books used in building architecture or interior design and the handbooks used in engineering.

We will consider a small collection of patterns, presenting our own catalog.

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SummarySummary

Design patterns capture expertise and make it available to novices and experts, providing many advantages.

Design patterns are models propose for imitation in solving software design problems.

Patterns come in various granularities including architectural styles, mid-level design patterns, data structures and algorithms, and programming language idioms.