Inheritance 101 : Page 2

I Want My Inheritance
Looking at the Water and Fish classes, it is easy to see some class commonalities. Inheritance involves extracting that commonality into a separately defined class. That new class is called a superclass, parent class or base class. The original classes then inherit from the new base class and become child classes, which are also referred to as derived classes or subclasses.

In this example, you can define a new base class named OceanElement that defines any element that can be placed into the WA-TOR ocean. Both the location and image properties from the child classes are extracted from those classes and instead implemented in the OceanElement class:

   Public MustInherit Class OceanElement
      Protected myLocation As Point
      Public Property Location() As Point
         Get
            Return myLocation
         End Get
         Set(ByVal Value As Point)
            myLocation = Value
         End Set
      End Property
      Public MustOverride ReadOnly Property Image() _
         As Image
   End Class

Notice the MustInherit keyword on the Class declaration. This keyword identifies the class as an abstract class. An abstract class is one that defines properties and methods but cannot itself be instantiated. This keyword is frequently used in base classes when the base class defines common functionality but does not itself represent an object in the application.

The Protected keyword in the declaration for the location ensures that the location value can only be accessed through the inheritance hierarchy. This means that only classes that inherit from this class or inherit from classes that inherit from this class can access that value directly. All other classes must access the value through the defined Property statements.

The MustOverride keyword in the declaration for the Image property denotes that the child classes must override this property. This keyword is needed in this case because each child class defines its own Image object containing its visual representation for the simulation.

The Water, Fish, and Shark classes all then inherit from this OceanElement base class. The Water class is shown as an example:

   Public Class Water : Inherits OceanElement
      Private Shared myImage As Image _ 
         = Image.FromFile("../water.jpg")
      Public Overrides ReadOnly Property image() _
         As Image
         Get
            Return myImage
         End Get
      End Property
      Public Sub New(ByVal location As Point)
         myLocation = location
      End Sub
   End Class

Notice how much less code is here than in the earlier example of the Water class. The Image Property statement overrides the Image property implemented in the base class to define the unique image for the Water class. The constructor for the Water class sets the myLocation variable, which is now maintained by the base class. The Water class has access to this variable from the base class because it was defined in the base class with Protected scope.

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