This app implements your own collection class: the priority queue. A priority queue is a modified version of a queue in which elements are not dequeued in the order in which they were inserted. Instead, elements are removed from the queue in order of priority. For example, you could use a priority queue to model a hospital emergency room: patients enter in any order, but more critical patients are seen before less critical patients. Similarly, if you were building a self-driving car that needed to process messages from multiple sensors, you might use a priority queue to respond to extremely important messages (say, that a pedestrian has just walked in front of the car) before less important messages (say, that a car two lanes over has just switched on its turn signal).