Abstract Classes and Methods

Abstract Classes and Methods

In the examples we've been using so far in this chapter, Person , Employee , and Customer have all been classes that can be created using the New operator. However, there may be situations where a base class should never be created ”perhaps there should only be instances of the Employee type and Customer type and never an instance of the Person type. It's possible just to add a comment saying Person should never be created, or Person might have a Private constructor to make it impossible to create. However, Person can also be designated as an abstract type. An abstract type is the same as a regular (or concrete ) type in all respects except for one: An abstract type can never directly be created. In the following example, Person is now declared as an abstract type, using the MustInherit modifier.

 MustInherit Class Person   Public Name As String   Public Address As String   Public City As String   Public State As String   Public ZIP As String   Sub Print()     Console.WriteLine(Name)     Console.WriteLine(Address)     Console.WriteLine(City & ", " & State & " " & ZIP)   End Sub End Class Class Customer   Inherits Person   Public CustomerID As Integer End Class Class Employee   Inherits Person   Public Salary As Integer End Class 

NOTE

Just because a class is abstract and cannot be created, it does not mean that it cannot have constructors. An abstract class may have constructors to initialize methods or pass values along to base class constructors.

Abstract classes are special in that they can also define abstract methods . Abstract methods are overridable methods that are declared with the MustOverride keyword and provide no implementation. A class that inherits from a class with abstract methods must provide an implementation for the abstract methods or must be abstract itself. For example, the Person class could define an abstract PrintName method that each derived class has to implement to display the person's name correctly.

 MustInherit Class Person   Public Name As String   Public Address As String   Public City As String   Public State As String   Public ZIP As String   MustOverride Sub PrintName()   Sub Print()     PrintName()     Console.WriteLine(Address)     Console.WriteLine(City & ", " & State & " " & ZIP)   End Sub End Class Class Customer   Inherits Person   Overrides Sub PrintName()     Console.Write("Customer ")     Console.WriteLine(Name)   End Sub   Public CustomerID As Integer End Class Class Employee   Inherits Person   Overrides Sub PrintName()     Console.Write("Employee ")     Console.WriteLine(Name)   End Sub   Public Salary As Integer End Class 

In this example, Person.Print can call the PrintName method, even though Person supplies no implementation for the method, because it is guaranteed that any derived class that can be instanced must provide an implementation.