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Inheritance Hierarchy for Shapes

publish date: 1969/12/31 00:00:00 UTC

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A shape can be classified as either 2-D or 3-D. Design an inheritance hierarchy. Which arrangement correctly represents the hierarchy with at least five shape classes?

2DShape and 3DShape as independent classes with no common superclass

Circle → Rectangle → Triangle → Sphere → Cube (linear inheritance chain)

Shape → Circle → Rectangle (with Triangle as a subclass of Circle)

Shape → 2DShape (Circle, Rectangle, Triangle) and Shape → 3DShape (Sphere, Cube)

Correct Answer

Shape → 2DShape (Circle, Rectangle, Triangle) and Shape → 3DShape (Sphere, Cube)

Explanation

The correct hierarchy has Shape as the root superclass with two direct subclasses: 2DShape and 3DShape. 2DShape is then subclassed into at least Circle, Rectangle, and Triangle. 3DShape is subclassed into at least Sphere and Cube. This gives a minimum of seven classes (Shape, 2DShape, 3DShape, Circle, Rectangle, Triangle, Sphere/Cube) with a clean generalization hierarchy. Common attributes (e.g., colour, area()) go in Shape; 2D-specific attributes (e.g., perimeter()) in 2DShape; 3D-specific (e.g., volume()) in 3DShape.

Reference

Software Engineering, Ian Sommerville, 10th edition

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