Visual Basic.NET How to Program (PDF)

370 Object-Oriented Programming: Inheritance Chapter 9
41
42 ' calculate CCircle diameter
43 Public Function Diameter() As Double
44 Return mRadius * 2
45 End Function ' Diameter
46
47 ' calculate CCircle4 circumference
48 Public Function Circumference() As Double
49 Return Math.PI * Diameter()
50 End Function ' Circumference
51
52 ' calculate CCircle4 area
53 Public Overridable Function Area() As Double
54 Return Math.PI * mRadius ^ 2
55 End Function ' Area
56
57 ' return String representation of CCircle4
58 Public Overrides Function ToString() As String
59
60 ' use MyBase reference to return CPoint String representation
61 Return "Center= " & MyBase.ToString() & _
62 "; Radius = " & mRadius
63 End Function ' ToString
64
65 End Class ' CCircle4
Fig. 9.12 CCircle4 class that inherits from class CPoint, which does not provide
Protected data (part 2 of 2).
For the purpose of this example, to demonstrate both explicit and implicit calls to baseclass
constructors, we include a second constructor that calls the base-class constructor
explicitly. Lines 17–23 declare the CCircle4 constructor that invokes the second
CPoint constructor explicitly using the base-class constructor-call syntax (i.e., reference
MyBase followed by a set of parentheses containing the arguments to the base-class constructor).
In this case, xValue and yValue are passed to initialize the base-class members
mX and mY. The insertion of the MyBase reference followed by the dot operator
accesses the base-class version of that method—in this constructor, MyBase.New invokes
the CPoint constructor explicitly (line 21). By making this explicit call, we can initialize
mX and mY to specific values, rather than to 0. When calling the base-class constructor
explicitly, the call to the base-class constructor must be the first statement in the derivedclass-constructor
definition.
Common Programming Error 9.2
It is a syntax error if a derived class uses MyBase to call its base-class constructor, and the
arguments do not match exactly the parameters specified in one of the base-class constructor
definitions. 9.2
Class CCircle4’s ToString method (line 58–63) overrides class CPoint’s
ToString method (lines 54–56 of Fig. 9.4). As we discussed earlier, overriding this
method is possible, because method ToString of class System.Object (class
CPoint’s base class) is declared Overridable. Method ToString of class
CCircle4 displays the Private instance variables mX and mY of class CPoint by