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CHAPTER 7 CREATING CLASS HIERARCHIES
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oCheckAccount.AccountNumber = 1000 dblBalance = oCheckAccount.GetBalance() oCheckAccount.Withdraw(500)
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Creating an Abstract Class
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At this point in the example, a client can access the GetBalance method through an instance of the derived CheckingAccount class or directly through an instance of the base Account class. Sometimes, you may want to have a base class that cannot be instantiated by client code. Access to the methods and properties of the class must be through a derived class. In this case, you construct the base class using the MustInherit modifier. The following code shows the Account class definition with the MustInherit keyword added: Public MustInherit Class Account This makes the Account class an abstract class. An abstract class is a class that defines the interfaces of the methods and properties that will be inherited by the derived classes. Because an abstract class does not contain any implementation code, only the interface definitions, it cannot be instantiated directly. For clients to gain access to the GetBalance method, they must instantiate an instance of the derived CheckingAccount class.
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Creating a Sealed Class
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By default, all classes can be inherited. When creating classes that can be inherited, you must take care that they are not modified in such a way that derived classes no longer function as intended. If you are not careful, you can create complex inheritance chains that are hard to manage and debug. For example, suppose you create a derived CheckingAccount class based on the Account class. Another programmer can come along and create a derived class based on the CheckingAccount and use it in ways you never intended. (This could easily occur in large programming teams with poor communication and design.) By using the NotInheritable modifier, you can create classes that you know will not be derived from. This type of class is often referred to as a sealed or final class. By making a class not inheritable, you avoid the complexity and overhead associated with altering the code of base classes. The following code demonstrates the use of the NotInheritable modifier when constructing a class definition: Public NotInheritable Class CheckingAccount
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Using Access Modifiers in Base Classes
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When setting up class hierarchies using inheritance, you must manage how the properties and methods of your classes are accessed. Two access modifiers you have looked at so far are Public and Private. If a method or property of the base class is exposed as Public, it is accessible by both the derived class and any client of the derived class. If you expose the property or method of the base class as Private, it is not accessible directly by the derived class or the client. You may want to expose a property or method of the base class to a derived class, but not to a client of the derived class. In this case, you use the Protected access modifier. The following code demonstrates the use of the Protected access modifier:
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CHAPTER 7 CREATING CLASS HIERARCHIES
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Protected Function GetBalance() As Double 'Code to retrieve account balance from database End Function By defining the GetBalance method as Protected, it becomes accessible to the derived class CheckingAccount but not to the client code accessing an instance of the CheckingAccount class.
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Activity 7-1. Creating and Using Base and Derived Classes with VS
In this activity, you will become familiar with the following: Creating a base class and derived classes Using access modifiers in the base class Creating an abstract base class
Creating the Account Class
To create the Account class, follow these steps: 1. Start VS. Select File Open Project. 2. Navigate to the Activity7_1Starter folder, click the Act7_1.sln file, and then click Open. When the project opens, it will contain a teller form. You will use this form later to test the classes you will create. 3. Select Project Add Class. 4. In the Add New Item dialog box, rename the class file to Account.vb, and click Open. The Account.vb file is added to the project, and the Account class definition code is added to the file. 5. Add the following code to the class definition file to create the private instance variable: Private _intAccountNumber As Integer 6. Add the following GetBalance method to the class definition: Public Function GetBalance(ByVal AccountNumber As Integer) As Double _ intAccountNumber = AccountNumber 'Data normally retrieved from database. Hardcoded for demo only If _intAccountNumber = 1 Then Return 1000 ElseIf _intAccountNumber = 2 Then Return 2000 Else Throw New Exception("Account number incorrect.") End If End Function
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