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7 n Inheritance and Polymorphism
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The Visual Studio IDE providing a list of MotorHome members
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As a consequence of using an interface, members of this class will now be compatible with both the Dwelling class and the IVehicle interface In code that works with objects of the Dwelling class, it will behave like a Dwelling object because of its inherited behaviour In code that works with IVehicle objects, it will behave like an IVehicle because we have provided it with the members every IVehicle object provides VB NET (and Visual Studio) is aware of both compatibilities Consequentially, VS can pop up a list of members of MotorHome that includes the IVehicle interface (see Figure 73)
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731 The IComparable Interface
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As a concrete example of interface inheritance, recall the example in 6, Listing 69, in which an earlier version of the BankAccount class was made sortable The relevant part of the class is reproduced in Listing 724
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Public Class BankAccount Implements IComparable Public Overridable Overloads Function CompareTo( _ ByVal obj As Object) As Integer _ Implements IComparableCompareTo 'Compare the name of this object with the 'parameter's Name If objName = mvarName Then 'These are the same Return 0 ElseIf objname < mvarName Then 'obj is smaller than (comes before) this one Return 1
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73 Interface Inheritance
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Else 'This one must be smaller Return -1 End If End Function End Class Listing 724: Using the IComparable interface to make a class sort-able
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In this case, our BankAccount class contains code that makes it compatible with an interface specially de ned to allow any objects that implement it to be compared The IComparable interface contains a single method de nition (Listing 725)
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Interface ICompareTo Function CompareTo(ByVal obj As Object) As Integer End Interface Listing 725: The IComparable interface de nition
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Any class that implements this interface must provide its own de nition of the
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CompareTo() method The reward for this is that an array of objects of the class will be compatible with the ArraySort() method, and can be sorted into an order de ned by the programmer s implementation of the method In the BankAccount example, the ArraySort() operation would sort an array of accounts into order
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of the account name The NET CLR de nes a large number of standard interface classes for many different purposes There is a standard ICollection interface, for creating Collection classes that are compatible with the standard collections and interfaces to allow you to de ne your own encryption classes, connect objects to databases, de ne new types of events, create formatting classes, interact with web browsers and even extend the features of Visual Studio Code inheritance allows us to re-use code already written, and therefore put less effort into the development of an application, but interface inheritance is unbeatable as a way of allowing us to hook our own classes into existing code
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Exercise 77
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Interface classes have no need for Public, Private, Protected or Friend access scope keywords or the use of Overridable, MustOverride or NotOverridable modi ers for member signatures Why is this A number of classes in a complex business application are to be given the ability to collect data from a mail-server An interface, called IMailable is to de ne two methods CheckMail, a function that will return a Boolean result indicating whether mail is available, and GetMailData, a function that will return an Integer value De ne this interface
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7 n Inheritance and Polymorphism
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In the previous chapter on data structures, I mentioned the CollectionBase class as being the starting point for creating a custom collection; a collection class that would collect only a speci c class of object Microsoft calls this a strongly typed collection The process of creating a strongly typed collection relies on inheritance, and because of this it is very simple to do The CollectionBase class is de ned as MustInherit, and includes an internal collection based on a protected ArrayList member called List It also provides a public Count property and a Clear() method to remove all items from the collection The other methods and properties necessary for managing a collection Add, Remove and Item must be implemented in the inheriting class As an example, assume we wished to create a strongly typed BankAccountCollection The implementation of this is as follows 1 Create the BankAccount class; preferably in a separate class module (we have already met several versions of this) We can add the new Collection class to the same module and therefore guarantee that whenever we use the Collection class, the objects that it collects are already available 2 Add a new class to the class module called BankAccountCollection This should inherit from CollectionBase, which is found in SystemCollections:
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