Object-Oriented Design

Subclasses must pass the "is-a" test

• A subclass passes the "is-a" test when ALL of the public methods of the base class hierarchy make sense for the subclass. Ideally, a subclass wouldn't have any public methods other than those inherited from the parent, some possibly overridden. That way, the behavior of the subclass can be invoked through a reference to the base class, thus achieving true polymorphism.

• When only a subset of the base class' public methods make sense for the subclass, then use delegation instead of inheritance. If delegation is too tedious, then consider extracting a new class from the methods that do make sense, making both the former base class and the target class a subclass of the new one.

Favor Composition Over Inheritance

• Inheritance is very powerful. Like Lord Acton said, "Power corrupts, and absolute power corrupts absolutely."

• Use inheritance to achieve polymorphism. When you extend one class from another, do so with the idea that you'll be invoking the behavior of that class through a reference to the base class.

• In general, try to use composition to extend behavior. Composition amounts to creating classes with references to other classes responsible for the specialized behavior. One class delegates behavior to another.

• Strive to design each class so that it "does one thing and one thing well." It should be readily apparent from the class' source file what that one thing is. Ideally, it should be readily apparent from the class' public interface. Even ideally-er, it should be readily apparent from its name.

• Extending the behavior of a class can be accomplished in two ways: inheritance and composition. Inheritance (used only when all base class methods make sense for the subclass) locks your behavior into a static structure. Sometimes this is reasonable, but you should darn well know what the reason is before you do it. In general, composition provides opportunites for reuse that inheritance does not.