Android™ How to Program, Second Edition

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 ...