Design Patterns

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Design Patterns
Yes, they are important Robert Cotton April 23,
2009
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Roadmap

• History of Design Patterns
• Explore some more common patterns
• Frameworks or Design Patterns?
• Discus the consequences of patterns

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History
Did not originate in Computer Science Thought up
by Christopher Alexander Formalized for Computer
Science by the 'Gang of Four' who wrote Design
Patterns Elements of Reusable Object-Oriented
Software
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Gang of Four
Erich Gamma Richard Helm Ralph Johnson John
Vlissides referred to as 'Gamma et al'
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What is a Design Pattern

• Four elements
• Pattern Name
• Problem
• Solution
• Consequences

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Common Patterns

• Abstract Factory
• Builder
• Chain of Responsibility
• Facade
• Iterator
• Observer
• Singleton

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Design Patterns in Use
Abstraction
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Design Patterns in Use
Iterator
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Design Patterns in Use
Singleton
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Design Patterns in Use
Observer
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Three Major Categories

• Creational Patterns
• Structural Patterns
• Behavioral Patterns

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Design Patterns vs. Frameworks

• Frameworks contain and utilize design patterns.
• Frameworks are not design patterns

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Frameworks

• Which use Design Patterns?
• Spring MVC
• Symfony
• Zend
• Hibernate (NHibernate)?
• Castle Windor
• Ruby on Rails
• .... (all of them)?

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Why Use Patterns

• Promotes good software design
• Need coupling/decoupling?
• Need encapsulation?
• Need to handle events?

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Not Always Better

• Does not guarantee better code
• Can be hard to implement
• Can be hard to maintain
• Can be hard to understand

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Errors
Implementing some Design Patterns can actual lead
to more errors!!
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Why Learn Them

• Often times frameworks seem to operate on sheer
  magic.
• Usually it is utilizing a specific framework.
• Become a better software engineer
• Become a more valuable employee
• Become a better coder.