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Inheritance in Action

Inheritance

A truly object-oriented language must support inheritance, the capability of a class to reuse

(

inherit

) methods and properties from another class. In the previous chapter, you learned how to

define properties and methods of a class, but what if you want two classes to use the same meth-

ods? This is where inheritance comes in.

Inheritance in Action

The easiest way to describe inheritance is through a classic example, geometric shapes. There are

really two types of shapes: ellipses (which are rounded) and polygons (which have a certain num-

ber of sides). Circles are a type of ellipse with one focus; triangles, rectangles, and pentagons are

types of polygons with a different number of sides. A square is a type of rectangle with all sides

equal. This describes a perfect inheritance relationship.

In this example, Shape is the

base class

(the class to be inherited from) of Ellipse and Polygon. An

Ellipse has one property called

foci

, indicating the number of foci the Ellipse has. Circle inherits

from Ellipse, so Circle is a

subclass

of Ellipse and Ellipse is a

superclass

of Circle. Likewise, Triangle,

Rectangle, and Pentagon are subclasses of Polygon and Polygon is a superclass to each of these

shapes. Finally, Square inherits from Rectangle.

The inheritance relationship is best explained through diagrams, which is where the Universal

Modeling Language (UML) comes in. One of UML’s many purposes is to visually represent com-

plex object relationships such as inheritance. Figure 4-1 is a UML diagram explaining the relation-

ship of Shape to its subclasses:

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