11.4 Abstract ClassesEach type of Window has a different shape and appearance. Drop-down listboxes look very different from buttons . Clearly, every subclass of Window should implement its own DrawWindow( ) method ”but so far, nothing in the Window class enforces that they must do so. To require subclasses to implement a method of their base, you need to designate that method as abstract . An abstract method has no implementation. It creates a method name and signature that must be implemented in all derived classes. Furthermore, making one or more methods of any class abstract has the side effect of making the class abstract. Abstract classes establish a base for derived classes, but it is not legal to instantiate an object of an abstract class. Once you declare a method to be abstract, you prohibit the creation of any instances of that class.
Thus, if you were to designate DrawWindow( ) as an abstract method in the Window class, the Window class would thus become abstract. Then you could derive from Window, but you could not create any Window objects/instances. If the Window class is an abstraction, there is no such thing as a simple Window object; only objects derived from Window. Making Window.DrawWindow( ) abstract means that each class derived from Window would have to implement its own DrawWindow( ) method. If the derived class failed to implement the abstract method, that derived class would also be abstract, and again no instances would be possible. Designating a method as abstract is accomplished by placing the MustOverride keyword at the beginning of the method definition, as follows : MustOverride Public Sub DrawWindow( ) If one or more methods of the derived class are abstract ( MustOverride ), the base class definition must be marked MustInherit , as in the following: MustInherit Public Class Window Example 11-3 illustrates the creation of an abstract Window class and an abstract DrawWindow( ) method. Example 11-3. An abstract class and methodOption Strict On Imports System MustInherit Public Class Window ' constructor takes two integers to ' fix location on the console Public Sub New(top As Integer, left As Integer) Me.top = top Me.left = left End Sub 'New ' simulates drawing the window ' notice: no implementation Public MustOverride Sub DrawWindow( ) Protected top As Integer Protected left As Integer End Class 'Window ' ListBox derives from Window Public Class ListBox Inherits Window ' constructor adds a parameter Public Sub New(top As Integer, left As Integer, contents As String) MyBase.New(top, left) ' call base constructor listBoxContents = contents End Sub 'New ' an overridden version implementing the ' abstract method Public Overrides Sub DrawWindow( ) Console.WriteLine("Writing string to the listbox: {0}", listBoxContents) End Sub 'DrawWindow Private listBoxContents As String ' new member variable End Class 'ListBox Public Class Button Inherits Window Public Sub New(top As Integer, left As Integer) MyBase.New(top, left) End Sub 'New ' implement the abstract method Public Overrides Sub DrawWindow( ) Console.WriteLine("Drawing a button at {0}, {1}" + ControlChars.Lf, top, left) End Sub 'DrawWindow End Class 'Button Public Class Tester Shared Sub Main( ) Dim winArray(3) As Window winArray(0) = New ListBox(1, 2, "First List Box") winArray(1) = New ListBox(3, 4, "Second List Box") winArray(2) = New Button(5, 6) Dim i As Integer For i = 0 To 2 winArray(i).DrawWindow( ) Next i End Sub 'Main End Class 'Tester Output: Writing string to the listbox: First List Box Writing string to the listbox: Second List Box Drawing a button at 5, 6 In Example 11-3, the Window class has been declared MustInherit and therefore cannot be instantiated . If you replace the first array member: winArray(0) = New ListBox(1, 2, "First List Box") with this code: winArray(0) = New Window(1, 2) the program will generate the following error: C:\...Module1.vb(63): 'New' cannot be used on class 'DebuggingVB.Window' because it contains a 'MustOverride' member that has not been overridden. You can instantiate the ListBox and Button objects because these classes override the MustOverride method, thus making the classes concrete (i.e., not abstract). |