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. 9.3, 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 ...
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