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SSJ is a Java library for stochastic simulation, developed under the direction of Pierre L'Ecuyer, in the Département d'Informatique et de Recherche Opérationnelle (DIRO), at the Université de Montréal. It provides facilities for generating uniform and nonuniform random variates, computing different measures related to probability distributions, performing goodness-of-fit tests, applying quasi-Monte Carlo methods, collecting (elementary) statistics, and programming discrete-event simulations with both events and processes.

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import umontreal.iro.lecuyer.simevents.*;
import umontreal.iro.lecuyer.rng.*;
import umontreal.iro.lecuyer.randvar.*;
import umontreal.iro.lecuyer.stat.*;
import java.util.LinkedList;

public class QueueEv {

   RandomVariateGen genArr;
   RandomVariateGen genServ;
   LinkedList waitList = new LinkedList ();
   LinkedList servList = new LinkedList ();
   Tally custWaits     = new Tally ("Waiting times");
   Accumulate totWait  = new Accumulate ("Size of queue");

   class Customer { double arrivTime, servTime; } 

   public QueueEv (double lambda, double mu) {
      genArr = new ExponentialGen (new MRG32k3a(), lambda);
      genServ = new ExponentialGen (new MRG32k3a(), mu);
   }

   public void simulateOneRun (double timeHorizon) {
      Sim.init();
      new EndOfSim().schedule (timeHorizon);
      new Arrival().schedule (genArr.nextDouble());
      Sim.start();
   }

   class Arrival extends Event {
      public void actions() {
         new Arrival().schedule (genArr.nextDouble()); // Next arrival.
         Customer cust = new Customer();  // Cust just arrived.
         cust.arrivTime = Sim.time();
         cust.servTime = genServ.nextDouble();
         if (servList.size() > 0) {       // Must join the queue.
            waitList.addLast (cust);
            totWait.update (waitList.size());
         } else {                         // Starts service.
            custWaits.add (0.0);
            servList.addLast (cust);
            new Departure().schedule (cust.servTime);
         }
      }
   }

   class Departure extends Event {
      public void actions() {
         servList.removeFirst();
         if (waitList.size() > 0) { 
            // Starts service for next one in queue.
            Customer cust = waitList.removeFirst();
            totWait.update (waitList.size());
            custWaits.add (Sim.time() - cust.arrivTime);
            servList.addLast (cust);
            new Departure().schedule (cust.servTime);
         }
      }
   }

   class EndOfSim extends Event {
      public void actions() {
         Sim.stop();
      }
   }

   public static void main (String[] args) { 
      QueueEv queue = new QueueEv (1.0, 2.0);
      queue.simulateOneRun (1000.0);
      System.out.println (queue.custWaits.report());
      System.out.println (queue.totWait.report());
   }
}