CSC/ECE 517 Fall 2009/wiki3 1 co

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:

• "An AntiPatern is a literary form that describes a commonly occurring solution to a problem that generates decidedly negative consequences." - AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis
• "Anti-patterns are patterns that initially seem effective, but over time you learn that they lead you into traps." - Anti-Patterns: Avoid the Programming Dark Side
• Over the last decade we have witnessed the maturity of both best practices...and worst practices (as manifested in the anti-patterns movement...). - Patterns of Anti-Patterns?, Journal of Object Technology

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

text

Refactored Solution

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Example

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Cut-and-Paste Programming

Anti-Pattern Problem

text

Refactored Solution

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Example

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Conclusion

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References

• www.antipatterns.com
• W. J. Brown et al., AntiPatterns: Refactoring Software, Architectures, and Projects in Crisis, John Wiley & Sons, Inc., New York, NY, 1998.
• S. Klement, Anti-Patterns: Avoid the Programming Dark Side, 2008
• Wikipedia entry: Anti-pattern
• Anti Patterns Catalog offered by Cunningham & Cunningham, Inc.
• What is an AntiPattern? from Sourcemaking.com
• Anti-Patterns and Worst Practices – You’re Doing it Wrong!
• M. H. Dodani,Patterns of Anti-Patterns?, Journal of Object Technology, Vol 5, No 6, pp 29-33, 2006
• E. Gamma et al,Design Patterns: Elements of Reusable Object-Oriented Software, Addison-Wesley Professional, 1994.
• G. Meszaros and J. Doble, A Pattern Language for Pattern Writing.