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POUpdating (and Iterating) a Map in Java
 

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  1. POUpdating (and Iterating) a Map in Java
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    <p>I have seen this code in the book Java 7 recipes to update and iterate a Map concurrently: </p> <pre><code>ConcurrentMap&lt;Integer,String&gt; concurrentMap = new ConcurrentHashMap&lt;Integer, String&gt;(); for (int i =0;i &lt; 1000;i++) { startUpdateThread(i, concurrentMap); } try { Thread.sleep(1000); } catch (InterruptedException e) { e.printStackTrace(); } for (Map.Entry&lt;Integer, String&gt; entry : concurrentMap.entrySet()) { System.out.println("Key :"+entry.getKey()+" Value:"+entry.getValue()); } </code></pre> <p>And the method to update the map as follows: </p> <pre><code>private void startUpdateThread(int i, final ConcurrentMap&lt;Integer, String&gt; concurrentMap) { Thread thread = new Thread(new Runnable() { public void run() { while (!Thread.interrupted()) { int randomInt = random.nextInt(20); concurrentMap.put(randomInt, UUID.randomUUID().toString()); } } }); thread.setName("Update Thread "+i); updateThreads.add(thread); thread.start(); } </code></pre> <p>I have tried with just a HashMap instead of ConcurrentHasMap and the results are the same (monitored with Java VisualVM). Does anyone know why? </p> <p>Thanks, Agustin</p> <p>UPDATE: A few interesting questions: </p> <ol> <li>If the HashMap has a constant capacity, what of the following operations can be performed safely? <ul> <li>Two threads updating values in the HashMap. </li> <li>Two threads updating values in the HashMap while a third one is reading the HashMap. </li> </ul></li> <li>If I want speed in updating a Map, does it make sense to have more than 4 threads in a computer with only 4 processors? </li> </ol>
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    1. USAgustin Treceno
    1. USAgustin Treceno
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    1. COWhat do you mean with "monitored with Java VisualVM"?
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      1. POUpdating (and Iterating) a Map in Java
      1. USEnno Shioji
    2. COAs you are inserting random stuff, I don't understand how you conclude that results are identical?
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      1. USfvu
    3. COso what's your problem? What is the issue if the result are the same? What's your expectation then?
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      1. POUpdating (and Iterating) a Map in Java
      1. USAdrian Shum
 

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