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What are factory and singleton classes?
Do you find some of the OO terminolgy baffling? Once you've learnt about constructors and methods, inheritance, overloading and polymorphism and statics, you might think you're there. Then someone mentions a "factory class" or a "singleton" ... Fear not - factory and singleton classes are posh names for something you would naturally write anyway, applied to certain techniques so that they can be described in OO design texts, project specifications, and articles such as this. A SINGLETON is a class for which only one object member ever exists; you can create a class that's a singleton by writing the constructor so that it returns a truely fresh object the first time you call it, and a reference to the same object should you call it again. We have an example (in Python, but it applies to any language in principle) here A FACTORY is a class who's constructor returns an object which might be in one of several different classes, not necessarily including the class in which it is, in theory, the contsructor. I could, for example, have a base class called "accom" for accommodation, subclassed to "hotel" and "campsite". If calls to the "accom" constructor return a hotel or a campsite (depending on constructor parameters), then that accom class is a factory. We have an example of a factory (in PHP, but it applies to any language in principle) here. There's also an exanple in Perl ... the class containing the factory method is available here, sample code that calls it here and the other class that may be built here (written 2007-06-04 17:59:27) Associated topics are indexed under Q907 - Object Orientation and General technical topics - Object Orientation: Design TechniquesH108 - Objects in PHP Y112 - Python - Objects - Intermediate P218 - Perl - More Objects C233 - C and C based languages - OO in C++ - beyond the basics R108 - Ruby - More Classes and Objects J710 - Java - Extending Classes and More
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Bathtubs and pecking birdsfor loop - how it works (Perl, PHP, Java, C, etc) Judging the quality of contributed Perl code Sunday afternoon What are factory and singleton classes? Five of the best - pictures from London An update on Perl - where is it going? New Serieses for the summer on TV MySQL - the order of clauses and the order of actions What brought YOU to our web site? |
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