Java™ Application Development on Linux - Dator

284Chapter 12Analysis and Design: Seeking the Objects12.6ESSENTIAL AND NONESSENTIALSo far, we have walked you through a very simple example, and we have madesound choices at every step. In more complex cases, even the best of us willmake mistakes. We will head down blind alleys. We will group things togetherthat might belong in separate abstract categories, but should, perhaps, share aninterface. These are not so much errors as judgment calls, and skill at recognizingthem and making the correct decisions comes only with experience.For now, the most important questions to ask include:• Do I need this class?We are often tempted to create too many inherited classes. When weseek more generic, higher level abstractions, it is often possible to use onlythe more abstract class. Of course, it is possible to carry that tendency toofar. If your methods contain a lot of “if’s” to handle various subtypes, thatmight be a case where you should inherit and overload the method.• Should I get functionality by inheritance or composition?Inheritance should be reserved only for cases where a class is a morespecific variety of the base class. For example, you might have a Personclass, and then you might have Employee and Customer classes inheritcommon attributes and methods from Person. This is frequently called an“is-a” relationship, as in “A User is a Person.” If your proposed inheritancerelationship makes sense phrased that way, it might well be a goodcandidate for inheritance.Composition is when you use a class as an attribute. To extend ourexample, you might have an Address class. You might be tempted to havePerson inherit from Address. But a Person is not an Address. Try it: “APerson is an Address.” Nope. Instead, you should just have an instance ofthe Address class as an attribute of Person. Such a relationship is oftencalled a “has-a” relationship, as in “A Person has an Address.” If therelationship makes sense phrased that way, it is a good candidate forcomposition. Another way to recognize that you’ve wrongly used inheritanceis if you end up having a radically different class inherit from thesame base class. For example, suppose you have a class, Building. Wouldit make sense for Building and Person to inherit from Address? AreBuildings and Persons more specific instances of the same general type ofthing? No, they are not. Building and Person should get Addressfunctionality by composition.