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POJava: is 'Three Stooges' example really immutable?
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  1. POJava: is 'Three Stooges' example really immutable?
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    copied!<p>In <code>Java Concurrency In Practice</code>, the following example is given to illustrate how to create an immutable class:</p> <p><a href="http://www.javaconcurrencyinpractice.com/listings/ThreeStooges.java" rel="nofollow">http://www.javaconcurrencyinpractice.com/listings/ThreeStooges.java</a></p> <p>This class has: <code>private final Set&lt;String&gt; stooges = new HashSet&lt;String&gt;();</code> which is initialized in its Constructor:</p> <pre><code>public ThreeStooges() { stooges.add("Moe"); stooges.add("Larry"); stooges.add("Curly"); } </code></pre> <p>and has a method</p> <pre><code>public boolean isStooge(String name) { return stooges.contains(name); } </code></pre> <p>to see if a name is one of the three stooges.</p> <p>But when I do this: <code>ThreeStooges ts = new ThreeStooges()</code>, is it guaranteed that the object will be properly constructed (i.e. the state of <code>stooges</code> correctly initialized) before its reference is set into <code>ts</code>?</p> <p>In other words, if I publish this object, is it possible that some thread will see it as incorrectly initialized (i.e. it will see <code>stooges</code> as empty when accessed through <code>isStooge()</code>)? </p> <p>My understanding is that an immutable object will be properly constructed and correctly visible when it's published - (because it uses final instance variables). Is my understanding correct? If yes, is this class still immutable?</p> <p>EDIT: it seems from the comments I saw that it's difficult to believe an object can be seen by other threads before its Constructor completes. Here's a link on that: <a href="http://jeremymanson.blogspot.in/2008/05/double-checked-locking.html" rel="nofollow">http://jeremymanson.blogspot.in/2008/05/double-checked-locking.html</a></p>
 

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