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copied!<p>The Gang Of Four "Design Patterns; Elements of Reusable Object-Oriented Software" book contains two entries, "Abstract Factory" (aka 'Virtual Constructor') and "Factory Method"... I dunno about "Concrete Factory." I've heard the term but never given it too much thought.</p> <h2>Factory Method</h2> <p>In "Factory Method" an object has a method which is responsible for the instantiation of another object. A common example would be the JavaScript document object and the creation of <code>HtmlElement</code> objects:</p> <pre><code>var newDiv = document.createElement('div'); </code></pre> <p>This isn't a great example though, as an important part of the Factory Method is polymorphism. If I could extend <code>document</code> to define another class which defines another <code>createElement</code> this would be prime Factory Method material.</p> <h2>Abstract Factory</h2> <p>An abstract factory is meant to "provide an interface for creating <em>families of related or dependent objects</em> without specifying concrete classes.</p> <p>The typical straight-out-of-the-book example is a Widget Factory; back in the day when the GoF was published, cross-platform GUI development was a bit of a hassle, so you could define an abstract widget factory class.</p> <p>That class could have methods <code>createWindow</code>, <code>createButton</code>, <code>createScrollBar</code> etc. In turn, several implementations would be defined to produce Swing widgets or AWT or whatever. Then, depending on configuration, the different class would be instantiated.</p> <h2>Addendum - Concrete Factory</h2> <p>I believe that a Concrete Factory is any non-abstract implementation of Abstract Factory or Factory method.</p> <p>So, when I write my own generalization of <code>document</code> which overrides <code>createElement</code>, the class I create is a Concrete Factory.</p> <p>Likewise, while <code>WidgetFactory</code> would be an Abstract Factory, <code>SwingWidgetFactory</code> would be a concrete factory.</p>

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