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POWhen to use the abstract factory pattern?
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I'm trying to succinctly describe when to use a factory, for both myself and my team. I ran across the following related questions, which helped somewhat: <ul> <li><a href="https://stackoverflow.com/questions/4067271/when-to-use-factory-patterns">When to use factory patterns?</a> <ul> <li>(useful pdf link is broken)</li> </ul></li> <li><a href="https://stackoverflow.com/questions/2181269/how-do-you-create-your-factories">How do you create your Factories?</a> <ul> <li>(more 'how' rather than 'when')</li> </ul></li> <li><a href="https://stackoverflow.com/questions/489623/what-is-your-threshold-to-use-factory-instead-of-a-constructor-to-create-an-objec">What is your threshold to use factory instead of a constructor to create an object?</a> <ul> <li>(some general answers)</li> </ul></li> <li><a href="https://stackoverflow.com/questions/69849/factory-pattern-when-to-use-factory-methods">Factory Pattern. When to use factory methods?</a> <ul> <li>(more about factory methods than factory classes)</li> </ul></li> <li><a href="https://stackoverflow.com/questions/1519358/when-to-use-factory-method-pattern">When to use Factory method pattern?</a> <ul> <li>(again more about factory methods)</li> </ul></li> </ul> Based on these links, and a bunch of other sources (listed at the bottom), I've come up with the following: When to use the abstract factory pattern: <ul> <li>when you use an interface var or the 'new' operator <ul> <li>e.g. User user = new ConcreteUserImpl(); </li> </ul></li> <li>and the code you are writing should be testable / extensible at some point </li> </ul> Explanation: <ul> <li>interfaces by their very nature imply multiple implementations (good for unit testing) </li> <li>interface vars imply OCP- and LSP-compliant code (support sub-classing) </li> <li>use of the 'new' operator breaks OCP/DI, because highly-coupled classes are hard to test or change </li> </ul> "Do I create a factory for every object type? That seems excessive." <ul> <li>no, you can have one (or a few) factories that produce a lot of (usually related) object types </li> <li>e.g. appFactory.createUser(); appFactory.createCatalog(); etc. </li> </ul> When NOT to use a factory: <ul> <li>the new object is very simple and unlikely to be sub-classed <ul> <li>e.g. List list = new ArrayList();</li> </ul></li> <li>the new object is not interesting to test <ul> <li>has no dependencies</li> <li>performs no relevant or long-running work</li> <li>e.g. Logger log = new SimpleLogger();</li> </ul></li> </ul> References: <ul> <li><a href="http://googletesting.blogspot.com/2008/08/where-have-all-singletons-gone.html" rel="nofollow noreferrer">http://googletesting.blogspot.com/2008/08/where-have-all-singletons-gone.html</a></li> <li><a href="http://misko.hevery.com/2008/07/08/how-to-think-about-the-new-operator/" rel="nofollow noreferrer">http://misko.hevery.com/2008/07/08/how-to-think-about-the-new-operator/</a></li> <li><a href="http://misko.hevery.com/2008/08/17/singletons-are-pathological-liars/" rel="nofollow noreferrer">http://misko.hevery.com/2008/08/17/singletons-are-pathological-liars/</a></li> <li><a href="http://en.wikipedia.org/wiki/Dependency_Injection" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Dependency_Injection</a></li> <li><a href="http://en.wikipedia.org/wiki/Open_Closed_Principle" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Open_Closed_Principle</a></li> <li><a href="http://en.wikipedia.org/wiki/Liskov_substitution_principle" rel="nofollow noreferrer">http://en.wikipedia.org/wiki/Liskov_substitution_principle</a></li> </ul> <hr/> My question is: is my summary accurate, and does it make sense? Is there anything I've overlooked? Thanks in advance.
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<oop><design-patterns><language-agnostic><factory><factory-pattern>
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