How it works...

Let's write a test program:

public static void main(String... arg) {  double timeSec = 10.0;  int engineHorsePower = 246;  int vehicleWeightPounds = 4000;  Vehicle vehicle = new Car(4, vehicleWeightPounds, engineHorsePower);  System.out.println("Passengers count=" +                                  ((Car)vehicle).getPassengersCount());  System.out.println("Car speed (" + timeSec + " sec) = " +                                vehicle.getSpeedMph(timeSec) + " mph");  vehicle = new Truck(3300, vehicleWeightPounds, engineHorsePower);  System.out.println("Payload=" +                            ((Truck)vehicle).getPayload() + " pounds");  System.out.println("Truck speed (" + timeSec + " sec) = " +                                vehicle.getSpeedMph(timeSec) + " mph");}

Notice that the vehicle reference of the Vehicle type points to the object of the Car

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