Visual Basic.NET How to Program (PDF)

Chapter 9 Object-Oriented Programming: Inheritance 363
33 Set(ByVal radiusValue As Double)
34
35 If radiusValue > 0
36 mRadius = radiusValue
37 End If
38
39 End Set
40
41 End Property ' Radius
42
43 ' calculate CCircle2 diameter
44 Public Function Diameter() As Double
45 Return mRadius * 2
46 End Function ' Diameter
47
48 ' calculate CCircle2 circumference
49 Public Function Circumference() As Double
50 Return Math.PI * Diameter()
51 End Function ' Circumference
52
53 ' calculate CCircle2 area
54 Public Function Area() As Double
55 Return Math.PI * mRadius ^ 2
56 End Function ' Area
57
58 ' return String representation of CCircle2
59 Public Overrides Function ToString() As String
60 Return "Center = " & "[" & mX & ", " & mY & "]" & _
61 "; Radius = " & mRadius
62 End Function ' ToString
63
64 End Class ' CCircle2
Fig. 9.8 CCircle2 class that inherits from class CPoint (part 2 of 2).
Lines 12 and 20 in the CCircle2 constructors (lines 10–24) invoke the default
CPoint2 constructor implicitly to initialize the base-class portion (variables mX and mY,
inherited from class CPoint) of a CCircle2 object to 0. However, because the parameterized
constructor (lines 17–24) should set the x-y coordinate to a specific value, lines 21–
22 attempt to assign argument values to mX and mY directly. Even though lines 21–22
attempt to set mX and mY values explicitly, line 20 first calls the CPoint default constructor
to initialize these variables to their default values. The compiler generates a syntax
error for lines 21–22 (and line 60, where CCircle2’s method ToString attempts to use