public class Floor {
  enum State { NO_ONE_WAITING, PASSENGERS_WAITING, LOADING }
  
  private int passengersWaiting = 0;
  private State state = State.NO_ONE_WAITING;

  synchronized public void waitForElevator(String msg) {
    if (state == State.LOADING) {
      try {
        wait();
      } catch (InterruptedException ex) {
      }
    }

    passengersWaiting++;
    System.out.println(msg);

    if (state == State.NO_ONE_WAITING) {
      this.state = State.PASSENGERS_WAITING;
      notifyAll();
    }
  }

  synchronized public void getOnOffElevator(String msg) {
    if (state != State.LOADING) {
      try {
        wait();
      } catch (InterruptedException ex) { }
    }

    passengersWaiting--;
    System.out.println(msg);

    if (passengersWaiting == 0) {
      this.state = State.NO_ONE_WAITING;
      notifyAll();
    }
  }

  synchronized public void elevatorArrives(String msg) {
    if (state == State.LOADING) {
      try {
        wait();
      } catch (InterruptedException ex) { }
    }

    System.out.println(msg);

    if (state == State.PASSENGERS_WAITING) {
      this.state = State.LOADING;
    } else {
      this.state = State.NO_ONE_WAITING;
    }
    notifyAll();
  }

  synchronized public void elevatorLeaves(String msg) {
    if (state != State.NO_ONE_WAITING) {
      try {
        wait();
      } catch (InterruptedException ex) { }
    }

    System.out.println(msg);
  }
}