When Inheritance and when Aggregation?

• Inheritance should be used only if the relationship is-a is maintained throughout the lifetime of the objects involved; otherwise, aggregation is the best choice.

• In other words, a subclass should always be a valid instance of the superclass. This implies that you should be able to use objects of the subclass wherever you use objects of the superclass without any issues or unexpected behavior.

For example, if you have a "Vehicle" superclass and a "Car" subclass, you should always be able to treat a "Car" object as a "Vehicle" object because a car is a type of vehicle. If at some point in your program's execution, a "Car" object stops being a "Vehicle" (e.g., it loses essential vehicle characteristics), then using inheritance for this relationship may not be appropriate.

Using inheritance inappropriately or in situations where the "is-a" relationship breaks down can lead to design issues, inflexibility, and confusion in your code. In such cases, it might be better to consider other design principles, like composition or interfaces, to model relationships between classes and objects.