Visual Basic.NET How to Program (PDF)

Chapter 9 Object-Oriented Programming: Inheritance 373
The first class that we use in our case study is class CPoint (Fig. 9.4). We declared
CPoint’s instance variables as Private. Class CPoint also contains properties X and
Y for accessing mX and mY and method ToString (which CPoint overrides from class
Object) for obtaining a String representation of the x-y coordinate pair.
We also created class CCircle4 (Fig. 9.12), which inherits from class CPoint.
Class CCircle4 contains the CPoint functionality, in addition to providing property
Radius, which ensures that the mRadius member variable cannot hold a negative value,
and methods Diameter, Circumference, Area and ToString. Recall that method
Area was declared Overridable (line 53). As we discussed in Section 9.4, this keyword
enables derived classes to override a base-class method. Derived classes of class
CCircle4 (such as class CCylinder, which we introduce momentarily) can override
these methods and provide specific implementations. A circle has an area that is calculated
by the equation
πr 2
in which r represents the circle’s radius. However, a cylinder has a surface area that is calculated
by a different equation:
(2πr 2 ) + (2πrh)
in which r represents the cylinder’s radius and h represents the cylinder’s height. Therefore,
class CCylinder must override method Area to include this calculation, so we declared
class CCircle4’s method Area as Overridable.
Figure 9.14 presents class CCylinder, which inherits from class CCircle4 (line
5). Class CCylinder’s Public services include the inherited CCircle4 methods
Diameter, Circumference, Area and ToString; the inherited CCircle4 property
Radius; the indirectly inherited CPoint properties X and Y; the CCylinder constructor,
property Height and method Volume. Method Area (lines 43–45) overrides
method Area of class CCircle4. Note that, if class CCylinder were to attempt to
override CCircle4’s methods Diameter and Circumference, syntax errors would
occur, because class CCircle4 did not declare these methods Overridable. Method
ToString (lines 53–55) overrides method ToString of class CCircle4 to obtain a
String representation for the cylinder. Class CCylinder also includes method
Volume (lines 48–50) to calculate the cylinder’s volume. Because we do not declare
method Volume as Overridable, no derived class of class CCylinder can override
this method.
1 ' Fig. 9.14: Cylinder.vb
2 ' CCylinder class inherits from class CCircle4.
3
4 Public Class CCylinder
5 Inherits CCircle4
6
7 Protected mHeight As Double
Fig. 9.14 CCylinder class inherits from class CCircle4 and Overrides
method Area (part 1 of 2).