Organizing Game Objects - Learning C# by Programming Games - page 161

10.5.5 Sealed Methods
If you do not want a subclass of the class that you programmed to be able to override
a virtual function, you can define the method with the keyword sealed . Sealed meth-
ods cannot be changed anymore by a subclass. For example, suppose the header of
the HandleInput method of the Ball class would be given as follows:
public sealed override void HandleInput(InputHelper inputHelper)
In this case, we can no longer override the method in a subclass of Ball .Soifwe
were to try this in the class BouncingBall (which inherits from Ball ), we would get a
compiler error.
10.6 Hierarchies of Classes
10.6.1 A Subclass 'Is a Kind of' Base Class
We've seen several examples of classes inheriting from a base game object class in
this chapter. A class should only inherit from another class if the relationship be-
tween these two classes can be described as 'is a kind of'. For example: a Ball is
akindof ThreeColorGameObject , and a Painter is a kind of Game . In fact, the hierar-
chy does not end there. We could write another class that inherits from the Ball class,
such as BouncingBall , which could be a special version of a standard ball that bounces
off canisters instead of only colliding with them. And we could make another class
BouncingElasticBall that inherits from BouncingBall , which is a ball that deforms ac-
cording to its elastic properties when it bounces against a canister. Every time we
inherit from a class, we get the data (encoded in member variables) and the behavior
(encoded in methods and properties) from the base class for free.
Commercial games will have a class hierarchy of different game objects with
many different levels. For example, you could imagine that for a game that takes
place in a city with a harbor, we would need many different classes for objects
commonly occurring in such an environment:
class GameObject {a very generic class for describing game objects with a position and a
velocity.}
class StationaryGameObject : GameObject {...}
class Tree : StationaryGameObject {...}
class House : StationaryGameObject {...}
class MovingGameObject : GameObject {...}
class Vehicle : MovingGameObject {...}
class MotorizedVehicle : Vehicle {...}
class Car : MotorizedVehicle {...}
class Truck : MotorizedVehicle {...}