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    <blockquote> <p>while a singleton is instantiated by a static method</p> </blockquote> <p>While this is the usual way of doing it, this is by no means the only way.</p> <p>In Java 1.5 a new version of Singleton is the enum singleton pattern:</p> <pre><code>public enum Elvis{ INSTANCE // this is a singleton, no static methods involved } </code></pre> <p>And since enums can have constructors, methods and fields, you can give them all the immutable state you want.</p> <p><strong>Reference:</strong> </p> <ul> <li><a href="http://download.oracle.com/javase/tutorial/java/javaOO/enum.html" rel="nofollow">Java tutorial: Enum Types</a></li> <li><a href="http://java.sun.com/docs/books/effective/" rel="nofollow" title="You must!!! read this book">Effective Java</a>, Item 3</li> <li><a href="http://en.wikipedia.org/wiki/Singleton_pattern#The_Enum-way" rel="nofollow" title="The Enum Singleton Pattern">Singleton (the enum way)</a> (WikiPedia)</li> </ul> <hr> <p>Also, the term Singleton leaves some room for interpretation. Singleton means that there is exactly one object per defined scope, but the scope can be a number of things:</p> <ul> <li><s>Java VM</s> <strong>Classloader</strong> (thanks @Paŭlo Ebermann for reminding me): in this case use <a href="http://en.wikipedia.org/wiki/Singleton_pattern#The_Enum-way" rel="nofollow" title="The Enum Singleton Pattern">enums</a> or the <a href="http://en.wikipedia.org/wiki/Initialization_on_demand_holder_idiom#Example_Java_Implementation" rel="nofollow" title="aka: Initialization on demand holder idiom">initialize-through-static-inner-class pattern</a>. This is of course what is usually meant by a singleton.<br> Be Careful: enums and all other singletons are broken if loaded through multiple Classloaders.</li> <li><strong>Enterprise Application</strong> (in this case you need a container-managed singleton, e.g. a <a href="http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-factory-scopes-singleton" rel="nofollow" title="Spring 3.x Reference: The Singleton scope">Spring singleton bean</a>). This can be several objects per VM or one object per several VMs (or one Object per VM, of course)</li> <li><strong>Thread</strong> (use a <a href="http://download.oracle.com/javase/6/docs/api/java/lang/ThreadLocal.html" rel="nofollow"><code>ThreadLocal</code></a>)</li> <li><strong>Request</strong> / <strong>Session</strong> (again, you'll need a container to manage this, <a href="http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-factory-scopes-other" rel="nofollow" title="Spring 3.x Reference: Request, session, and global session scopes">Spring</a>, <a href="http://docs.jboss.org/seam/2.2.1.CR1/reference/en-US/html/concepts.html#d0e3960" rel="nofollow" title="Seam Reference: Defining the component scope">Seam</a> and several others can do that for you)</li> <li>did I forget anything?</li> </ul> <p>All of the above can be made immutable, each in their own way (although it's usually not easy for container-managed components)</p>
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    1. POHow to make an immutable singleton in Java?
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    1. USSean Patrick Floyd
    1. USSean Patrick Floyd
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    1. POHow to make an immutable singleton in Java?
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    1. CObeat by 12 seconds =[
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    2. COThis example is taken from Effective Java, I presume ;-)
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    3. CO@darioo: yes, but it's missing the `leaveTheBuilding()` method :-)
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      1. USSean Patrick Floyd
 

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