Uber Ride

import java.util.HashSet;
import java.util.Set;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.Semaphore;
import java.util.concurrent.locks.ReentrantLock;

class Demonstration {
  public static void main(String[] args) throws Exception {
    UberRideProblem uberRide = new UberRideProblem();
    Set<Thread> threads = new HashSet<Thread>();

    for (int i = 0; i < 10; i++) {
      Thread thread =
          new Thread(
              new Runnable() {

                @Override
                public void run() {
                  try {
                    uberRide.requestForDemocrat();
                  } catch (InterruptedException e) {

                  } catch (BrokenBarrierException e) {

                  }
                }
              });

      thread.setName("Democrat_" + i);
      threads.add(thread);
    }

    for (int i = 0; i < 14; i++) {
      Thread thread =
          new Thread(
              new Runnable() {

                @Override
                public void run() {
                  try {
                    uberRide.requestForRepublican();
                  } catch (InterruptedException e) {

                  } catch (BrokenBarrierException e) {

                  }
                }
              });

      thread.setName("Republican_" + i);
      threads.add(thread);
    }

    for (Thread thread : threads) {
      thread.start();
    }

    for (Thread thread : threads) {
      thread.join();
    }
  }
}

class UberRideProblem {
  private int republicanRequests = 0;
  private int democratRequests = 0;
  private Semaphore republicanSemaphore = new Semaphore(0);
  private Semaphore democratSemaphore = new Semaphore(0);
  private ReentrantLock lock = new ReentrantLock();
  private CyclicBarrier barrier = new CyclicBarrier(4);

  private void seated() {
    System.out.println(Thread.currentThread().getName() + " is seated");
  }

  private void drive() {
    System.out.println("Uber rider is started by instructor: " + Thread.currentThread().getName());
  }

  public void requestForRepublican() throws InterruptedException, BrokenBarrierException {
    lock.lock();
    republicanRequests++;
    boolean instructor = false;

    if (republicanRequests >= 4) {
      republicanSemaphore.release(3);
      republicanRequests -= 4;
      instructor = true;
    } else if (republicanRequests >= 2 && democratRequests >= 2) {
      republicanSemaphore.release(1);
      democratSemaphore.release(2);
      republicanRequests -= 2;
      democratRequests -= 2;
      instructor = true;
    } else {
      lock.unlock();
      republicanSemaphore.acquire();
    }

    seated();
    barrier.await();

    if (instructor) {
      drive();
      lock.unlock();
    }
  }

  public void requestForDemocrat() throws InterruptedException, BrokenBarrierException {
    lock.lock();
    democratRequests++;
    boolean instructor = false;

    if (democratRequests >= 4) {
      democratSemaphore.release(3);
      democratRequests -= 4;
      instructor = true;
    } else if (democratRequests >= 2 && republicanRequests >= 2) {
      democratSemaphore.release(1);
      republicanSemaphore.release(2);
      democratRequests -= 2;
      republicanRequests -= 2;
      instructor = true;
    } else {
      lock.unlock();
      democratSemaphore.acquire();
    }

    seated();
    barrier.await();

    if (instructor) {
      drive();
      lock.unlock();
    }
  }
}