The following example shows how the Controller class would work, given specific calls to receiveCar and getCar.

Step Operation Effect
1 Constructor Creates 3 priority queues. Empty Queue Empty Queue
2 getCar() Raises an exception since all three queues will be empty. Empty Queue Empty Queue
3 receiveCar(a, low) Places car a into the low queue. Queue with 1 car Queue with 1 car
4 receiveCar(b, low) Places car b into the low queue. Queue with 2 cars Queue with 2 cars
5 receiveCar(f, high) Places car f into the high queue. Queue with 3 cars Queue with 3 cars
6 receiveCar(d, medium) Places car d into the medium queue. Queue with 4 cars Queue with 4 cars
7 receiveCar(g, high) Raises an exception since the high queue is already full. Queue with 4 cars Queue with 4 cars
8 receiveCar(e, medium) Places car e into the medium queue. Queue with 5 cars Queue with 5 cars
9 getCar() Returns car f from the high queue. Queue with 4 cars Queue with 4 cars
10 getCar() Returns car d from the medium queue. Queue with 3 cars Queue with 3 cars
11 getCar() Returns car e from the medium queue. Queue with 2 cars Queue with 2 cars
12 getCar() Returns car a from the low queue. Queue with 1 car Queue with 1 car
13 getCar() Returns car b from the low queue. Queue with 0 cars Queue with 0 cars
14 getCar() Raises an exception since there are no more cars available in any of the three queues. Queue with 0 cars Queue with 0 cars
15 isEmpty() Returns true since all three queues are empty. Queue with 0 cars Queue with 0 cars