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    <p>The <a href="http://en.wikipedia.org/wiki/Factory_method_pattern" rel="nofollow noreferrer">factory method pattern</a> can be useful for times when there is no need to create a new instance of an object in order to perform some action.</p> <p>Here are a couple of general cases I can think of where a static factory method which returns the same object can come in handy:</p> <ol> <li><p><strong>The object is expensive to create</strong> -- There is a lot of processing when the object is instantiated, so instantiating the object more than once is not desirable. (This is also related to the <a href="http://en.wikipedia.org/wiki/Singleton_pattern" rel="nofollow noreferrer">Singleton pattern</a>.)</p></li> <li><p><strong>The object keeps no state</strong> -- If there is no state difference between the instances, there is no good purpose to create a new object every time.</p></li> </ol> <p>The <a href="http://en.wikipedia.org/wiki/Factory_method_pattern" rel="nofollow noreferrer">Wikipedia page on the factory method pattern</a> has more information on this topic.</p> <hr> <p>Let's take a look at a concrete example.</p> <p>The <a href="http://java.sun.com/javase/6/docs/api/java/text/DateFormat.html" rel="nofollow noreferrer"><code>DateFormat</code></a> class uses the <a href="http://java.sun.com/javase/6/docs/api/java/text/DateFormat.html#getInstance()" rel="nofollow noreferrer"><code>getInstance</code></a> static method to return a <code>DateFormat</code> instance, which can be used to format a <code>Date</code> into a preset formatting depending on the locale of the machine.</p> <p>Since the <code>DateFormat</code> that is returned is using the same formatting for every date formatting action, there is no real reason for creating a new <code>DateFormat</code> instance every time.</p> <p>In general, the way this is implemented is to create an instance if an instance does not already exist, and then keep a reference to that instance. If the instance is needed again, the reference is returned. (This is generally how the Singleton pattern is implemented as well.)</p> <p>For example:</p> <pre><code>class MySingleInstanceObject { private MySingleInstanceObject instance; private MySingleInstanceObject() { // Initialize the object. // This may be expensive. } public MySingleInstanceObject getInstance() { if (instance == null) { instance = new MySingleInstanceObject(); } return instance; } } </code></pre> <p>(FYI, the above code is an example of a singleton. Also, it is not thread-safe.)</p>
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    1. POStatic Factory Methods to avoid Duplicate Objects
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    1. COIf the factory method is invoked multiple times, in what way does it ensure that only one object is returned? Does it hold a reference to the object and check it for null?
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      1. USKevin Boyd
    2. COYes. There are code examples at the wiki for Singleton pattern: http://en.wikipedia.org/wiki/Singleton_pattern
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    3. CONote that you can get even more flexibility (plus testability) by using an IoC Container (dependency injection) framework and letting it control the lifetimes of objects.
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