There are two benefits to manipulating objects solely in terms of the interface defined by abstract classes:
Clients remain unaware of the specific types of objects they use, as long as the objects adhere to the interface that clients expect.
Clients remain unaware of the classes that implement these objects. Clients only know about the (...) [abstraction] defining the interface.
This so greatly reduces implementation dependencies between subsystems that it leads to the following principle ofreusable object-oriented design: Program to an interface, not an implementation.
[@Gof1994], p. 18.
That leads us to our second principle of object-oriented design: Favor object composition over class inheritance. Ideally, you shouldn’t have to create new components to achieve reuse. You should be able to get all the functionality you need just by assembling existing components through object composition. But this is rarely the case, because the set of available componentsis never quite rich enough in practice. Reuseby inheritancemakesit easier to make new components that can be composed with old ones. Inheritance and object composition thus work together. Nevertheless, our experience isthat designers overuse inheritance as a reuse technique, and designs are often made more reusable (and simpler) by depending more on object composition.
[@Gof1994], p. 20.
Een manier om de verantwoordelijkheid van een klasse in te richten is door te kijken naar de informatie waar deze over kan beschikken. Breng die dingen samen die dezelfde informatie moeten maken, aanpassen, lezen en verwijderen.
Dit staat ook wel bekend als het information expert-principe:
Assign a responsibility to the information expert—the class that has the information necessary to fulfill the responsibility.
[@Larman2004], p. 248
In plaats van allemaal lijsten bij te houden van namen en adressen, kan je vaak beter een lijst bijhouden met personen, waarin namen en adressen zijn opgenomen.