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    <p>You are on the right track using <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="noreferrer"><code>ConcurrentHashMap</code></a>. For each point:</p> <ol> <li>Check out the methods <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html#putIfAbsent(K%2C%20V)" rel="noreferrer"><code>putIfAbsent</code></a> and <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html#replace(K%2C%20V)" rel="noreferrer"><code>replace</code></a> both are threadsafe and combine checking current state of hashmap and updating it into one atomic operation.</li> <li>The <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html#get(java.lang.Object)" rel="noreferrer">get</a> method is not synchronized internally but will return the most recent value for the specified key available to it (check the <a href="http://docs.oracle.com/javase/1.5.0/docs/api/java/util/concurrent/ConcurrentHashMap.html" rel="noreferrer">ConcurrentHashMap class Javadoc for discussion</a>). </li> </ol> <p>The benefit of <code>ConcurrentHashMap</code> over something like <a href="http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Collections.html#synchronizedMap(java.util.Map)" rel="noreferrer"><code>Collections.synchronizedMap</code></a> is the combined methods like <code>putIfAbsent</code> which provide traditional Map <code>get</code> and <code>put</code> logic in an internally synchronized way. Use these methods and <em>do not</em> try to provide your own custom synchronization over <code>ConcurrentHashMap</code> as it will not work. The <code>java.util.concurrent</code> collections are internally synchronized and other threads will not respond to attempts at synchronizing the object (e.g. <code>synchronize(myConcurrentHashMap){}</code> will not block other threads).</p>
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