Visual Basic.NET How to Program (PDF)

358 Object-Oriented Programming: Inheritance Chapter 9
21 ' display new point value
22 output &= vbCrLf & vbCrLf & _
23 "The new location of point is " & point.ToString()
24
25 MessageBox.Show(output, "Demonstrating Class Point")
26 End Sub ' Main
27
28 End Module ' modPointTest
Fig. 9.5 modPointTest demonstrates class CPoint functionality (part 2 of 2).
We now discuss the second part of our introduction to inheritance by creating and
testing class CCircle (Fig. 9.6), which directly inherits from class System.Object
and represents an x-y coordinate pair (representing the center of the circle) and a radius.
Lines 7–8 declare the instance variables mX, mY and mRadius as Private data. The
Public services of class CCircle include two CCircle constructors (lines 11–27),
properties X, Y and Radius (lines 30–70), methods Diameter (lines 73–75), Circumference
(lines 78–80), Area (lines 83–85) and ToString (lines 88–91). These properties
and methods encapsulate all necessary features (i.e., the “analytic geometry”) of a
circle; in the next section, we show how this encapsulation enables us to reuse and extend
this class.
1 ' Fig. 9.6: Circle.vb
2 ' CCircle class contains x-y coordinate pair and radius.
3
4 Public Class CCircle
5
6 ' coordinate of center of CCircle
7 Private mX, mY As Integer
8 Private mRadius As Double ' CCircle's radius
9
10 ' default constructor
11 Public Sub New()
12
13 ' implicit call to Object constructor occurs here
14 X = 0
15 Y = 0
16 Radius = 0
17 End Sub ' New
18
Fig. 9.6 CCircle class contains an x-y coordinate and a radius (part 1 of 3).