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    <p>In the original librarian problem you described, the problem doesn't care which student is in which room, therefore uses a simple thread safe counter (i.e. a Semaphore) to implement control of the resources. Following that description of the problem there still needs to be an alteration of your implementation. One approach is to 2 methods on the librarian class, one for requesting the SME, the other for returning it.</p> <pre><code>class Librarian { Semaphore sme = new Semaphore(NUMBER_OF_SMES); void requestSme() throws InterruptedException { sme.acquire(); } void releaseSme() { sme.release(); } } class Student { Librarian librarian; public void run() { libarian.requestSme(); try { // Do something finally { librarian.releaseSme(); } } } </code></pre> <p>However if you do need to know which Student is working with which SME, then you need a different construct for managing the resources, a Semaphore is no longer sufficient. One example could be a Queue.</p> <pre><code>class Librarian { BlockingQueue&lt;SubjectMatterExpert&gt; q = new ArrayBlockingQueue&lt;SubjectMatterExpert&gt;(NUMBER_OF_SMES); public Librarian() { for (int i = 0; i &lt; NUMBER_OF_SMES; i++) q.put(new SubjectMatterExpert(String.valueOf(i)); } SubjectMatterExport requestSme() throws InterruptedException { q.take(); } void releaseSme(SubjectMatterExpert toRelease) { q.put(toRelease); } } class Student { Librarian librarian; public void run() { SubjectMatterExpert sme = libarian.requestSme(); try { System.out.println("Student: " + this + ", SME: " sme); finally { if (sme != null) librarian.releaseSme(sme); } } } </code></pre>
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    1. USMichael Barker
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    1. COMichael, Thank you very much for the detailed explanation. Before I begin to experiment further, a quick observation: Once a Student is assigned to a SME, the SME runs some sequence of task on the Student's behalf (Details of this is part 2 of the question). Once s/he is done with the task, how will the librarian queue know to add the SME back to the queue? Will the SME class call the releaseSme(), passing itself to the method? Would that be a safe call? Thank you for your answer again! Really appreciate it.
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    2. COI would leave the responsibility to release the SME to the Student. The Student can ask the question of the SME and then return the SME to the librarian once completed. I wouldn't have the SME run as a thread, just represent the work to be done for the Student as a simple method call.
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    3. COMichael, I understand why you'd suggest that the Student release the SME. That makes perfect sense.
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