CSC/ECE 517 Fall 2009/wiki3 1 co

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Software Design Anti-Patterns

  Not all patterns are good. Anti-patterns are patterns that initially seem effective, but over time you learn that they lead you into traps. 
      - Scott Klement, Anti-Patterns: Avoid the Programming Dark Side

In this article, we will describe several software design patterns which have been categorized as "anti patterns": Programming approaches which are not uncommon and which can lead to poor program design and performance.

What are Anti-Patterns?

Different texts define the term anti-patterns in different ways:

In each definition, the implementer of the anti-pattern is trying to solve a problem in a common way, however the common way is a bad design that can lead to unintended consequences. Some of the reasons cited in the AntiPatterns book as to why these anti-patterns have been used include: Haste, sloth (or laziness), ignorance, apathy, and pride. Studying anti-patterns can at least help assist the potential anti-pattern implementer in the ignorance category.

Considerations When Writing Anti-Patterns

When writing patterns, there are various templates to from which to choose, for example, the pattern template from the Gang of Four's Design Pattern book:

  • Pattern Name and Classification
  • Intent
  • Also Known As
  • Motivation
  • Applicability
  • Structure
  • Participants
  • Collaboration
  • Consequences
  • Implementation
  • Sample Code
  • Known Uses
  • Related Patterns

Brown et al in their book AntiPatterns suggest 3 different possible templates for writing anti-patterns:

  • Pseudo-AntiPattern Template: Only the name and problem are described
  • Mini-AntiPattern Template: Includes the name, AntiPattern Problems, and Refactored Solution
  • Full AntiPattern Template: Includes 18 different sections such as Root Causes, Background, Known Exceptions, and Examples.

For the purposes of brevity, this wiki page will use the Mini-AntiPattern Template form with the addition of an Example section.

Anti-Pattern Examples

This section presents six anti-patterns from the book AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis.

The Blob or God Class

Anti-Pattern Problem

A design which includes a class that has the majority of attributes and/or methods in the system could a sign of the Blob anti-pattern. According to Brown et al, this anti-pattern could be the result of using a procedural design in an object-oriented language, and therefore imbuing one of the classes with the majority of the methods and seeing that class as a "controller" class. Another sign that the Blob anti-pattern may be present is if a class is designated as a "utility" class, which indicates a possible catch-all class for unrelated classes.

Refactored Solution

Brown et al suggests refactoring responsibilities into appropriate classes using the following steps:

  • Identify cohesive sets of operations and attributes that relate to a common focus, behavior, or function.
  • Review the system to see if there are "natural homes" for these collections in existing classes.
  • Remove redundant or "far-coupled" associations.
  • Migrate associates to derived classes to a common base class.
  • Remove all transient associations, replacing them as appropriate with type specifiers to attributes and operations arguments.

Example

An inappropriate use of the Rails framework, where a majority of the methods reside in a single controller class or model class. To resolve the issues, helper classes or other classes could be created which would more appropriately contain related methods that were inappropriately kept in a controller or model.

Lava Flow

Anti-Pattern Problem

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Refactored Solution

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Example

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Poltergeists

Anti-Pattern Problem

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Refactored Solution

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Example

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Golden Hammer

Anti-Pattern Problem

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Refactored Solution

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Example

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Spaghetti Code

Anti-Pattern Problem

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Refactored Solution

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Example

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Cut-and-Paste Programming

Anti-Pattern Problem

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Refactored Solution

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Example

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Conclusion

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References