time += incLength; // increment time

    double lambda = avgGrocPerMin * incLength;
  Random rand = new Random();

    // loop through each queue
    for (int i = 0; i < lines.length; i++) {

       if (lines[i].isEmpty()) {   // skip empty queues
           continue;
        }

        // loop to find number of groceries processed in interval
       int k = 0;
       double prob = rand.nextDouble();
       while (true) {
          // calc probability of exactly k groceries during interval (Poisson dist.)
          double probK = Math.pow(lambda, k) * Math.exp(-lambda) / factorial(k);
          if (probK <= prob) { // increment k until cumulative probability, prob, reached
             k++;
             prob -= probK;
          } else {
             break;
          }
       }

       while (!lines[i].isEmpty() && k > 0) {
            lines[i].decFront(); // decrement grocery count
            k--;
       }
    }
    this.display();
}             double probK = Math.pow(lambda, k) * Math.exp(-lambda) / factorial(k);
          if (probK <= prob) { // increment k until cumulative probability, prob, reached
             k++;
             prob -= probK;
          } else {
             break;
          }
       }

       while (!lines[i].isEmpty() && k > 0) {
            lines[i].decFront(); // decrement grocery count
            k--;
       }
    }
    this.display();
}