Visual Basic.NET How to Program (PDF)

Chapter 9 Object-Oriented Programming: Inheritance 369
Testing and Debugging Tip 9.1
When possible, avoid including Protected data in a base class. Rather, include non-
Private properties and methods that access Private data, ensuring that the object
maintains a consistent state. 9.1
We reexamine our point-circle hierarchy example once more; this time, attempting to
use the best software engineering technique. We use CPoint (Fig. 9.4), which declares
variables mX and mY as Private, and we show how derived class CCircle4 (Fig. 9.12)
can invoke base-class methods and properties to manipulate these variables.
1 ' Fig. 9.12: Circle4.vb
2 ' CCircle4 class that inherits from class CPoint.
3
4 Public Class CCircle4
5 Inherits CPoint ' CCircle4 Inherits from class CPoint
6
7 Private mRadius As Double
8
9 ' default constructor
10 Public Sub New()
11
12 ' implicit call to CPoint constructor occurs here
13 Radius = 0
14 End Sub ' New
15
16 ' constructor
17 Public Sub New(ByVal xValue As Integer, _
18 ByVal yValue As Integer, ByVal radiusValue As Double)
19
20 ' use MyBase reference to CPoint constructor explicitly
21 MyBase.New(xValue, yValue)
22 Radius = radiusValue
23 End Sub ' New
24
25 ' property Radius
26 Public Property Radius() As Double
27
28 Get
29 Return mRadius
30 End Get
31
32 Set(ByVal radiusValue As Double)
33
34 If radiusValue > 0
35 mRadius = radiusValue
36 End If
37
38 End Set
39
40 End Property ' Radius
Fig. 9.12 CCircle4 class that inherits from class CPoint, which does not provide
Protected data (part 1 of 2).