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  1. PO
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    <p>Here is how I would handle and avoid duplicates</p> <pre><code>import java.util.Collections; import java.util.Set; import java.util.concurrent.*; public class TestExecution implements Callable&lt;Void&gt; { private static final ThreadPoolExecutor TPE = new ThreadPoolExecutor(2, 5, 1, TimeUnit.MINUTES, new LinkedBlockingQueue&lt;Runnable&gt;()); private static final Set&lt;TestExecution&gt; TE_SET = Collections.newSetFromMap(new ConcurrentHashMap&lt;TestExecution, Boolean&gt;()); private final String key1; private final String key2; public static void main(String... args) throws InterruptedException { new TestExecution("A", "A").execute(); new TestExecution("A", "A").execute(); new TestExecution("B", "B").execute(); Thread.sleep(8000); new TestExecution("A", "A").execute(); new TestExecution("B", "B").execute(); new TestExecution("B", "B").execute(); TPE.shutdown(); } public TestExecution(String key1, String key2) { this.key1 = key1; this.key2 = key2; } void execute() { if (TE_SET.add(this)) { System.out.println("Handling " + this); TPE.submit(this); } else { System.out.println("... ignoring duplicate " + this); } } public boolean equals(Object obj) { return obj instanceof TestExecution &amp;&amp; key1.equals(((TestExecution) obj).key1) &amp;&amp; key2.equals(((TestExecution) obj).key2); } public int hashCode() { return key1.hashCode() * 31 + key2.hashCode(); } @Override public Void call() throws InterruptedException { if (!TE_SET.remove(this)) { System.out.println("... dropping duplicate " + this); return null; } System.out.println("Start processing " + this); Thread.sleep(4000); System.out.println("Finish processing " + this); return null; } public String toString() { return key1 + ':' + key2; } } </code></pre> <p>prints</p> <pre><code>Handling A:A ... ignoring duplicate A:A Handling B:B Start processing A:A Start processing B:B Finish processing A:A Finish processing B:B Handling A:A Handling B:B Start processing A:A Start processing B:B ... ignoring duplicate B:B Finish processing B:B Finish processing A:A </code></pre>
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    1. POThread Pool handling 'duplicate' tasks
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    1. USPeter Lawrey
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    1. COOK, thanks, some good pointers there, especially avoiding multithreading issues by using ConcurrentHashMap and overriding the toString method. A couple of questions. Why not use HashSet (is that because there is no equivalent thread safe object to use?) Also I don't understand the code for removal from the HashMap. You seem to do this at the start of the processing, shouldn't that be at the end of processing?
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    2. COYou can use `Collections.synchronizedSet(new HashSet())` This is thread safe, but not concurrent. Whether its at the start or the end depends on your requirements. Is it better to occasionally do something twice or occasionally not doing something (because the new task was added between the task ending and being removed)
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    3. COOK, I don't really know the difference between 'thread safe' and 'concurrent', maybe I have some investigation to do there. But for removing the element, if I don't want to re-execute (or queue) a job that has already started, then the TE_SET.remove should be moved to the end of the Call function, right? Am I correct in assuming the "dropping duplicate" case is an error case? If we are in the call function, then an element should always have been written to the HashSet right?
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