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G. Object-Oriented Programming: Inherita... > G.9. Abstract Classes and Methods

G.9. Abstract Classes and Methods

When we think of a class, we assume that programs will create objects of that type. Sometimes it’s useful to declare classes—called abstract classes—for which you never intend to create objects. Because they’re used only as superclasses in inheritance hierarchies, we refer to them as abstract superclasses. These classes cannot be used to instantiate objects, because, as we’ll soon see, abstract classes are incomplete. Subclasses must declare the “missing pieces” to become “concrete” classes, from which you can instantiate objects. Otherwise, these subclasses, too, will be abstract. We demonstrate abstract classes in Section G.10.

Purpose of Abstract Classes

An abstract class’s purpose is to provide an appropriate superclass from which other classes can inherit and thus share a common design. In the Shape hierarchy of Fig. G.3, for example, subclasses inherit the notion of what it means to be a Shape—perhaps common attributes such as location, color and borderThickness, and behaviors such as draw, move, resize and changeColor. Classes that can be used to instantiate objects are called concrete classes. Such classes provide implementations of every method they declare (some of the implementations can be inherited). For example, we could derive concrete classes Circle, Square and Triangle from abstract superclass TwoDimensionalShape. Similarly, we could derive concrete classes Sphere, Cube and Tetrahedron from abstract superclass ThreeDimensionalShape. Abstract superclasses are too general to create real objects—they specify only what is common among subclasses. We need to be more specific before we can create objects. For example, if you send the draw message to abstract class TwoDimensionalShape, the class knows that two-dimensional shapes should be drawable, but it does not know what specific shape to draw, so it cannot implement a real draw method. Concrete classes provide the specifics that make it reasonable to instantiate objects.


  

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