Creating Class Hierarchies in Font

Encoder ECC200 in Font Creating Class Hierarchies

n the previous chapter, you looked at how to create classes, add attributes and methods, and instantiate object instances of the classes in client code. This chapter introduces you to inheritance. Inheritance is one of the most powerful and fundamental features of any OOP language. Using inheritance, you create base classes that encapsulate common functionality. Other classes can be derived from these base classes. The derived classes inherit the properties and methods of the base classes, and extend the functionality as needed. Another fundamental OOP feature introduced in this chapter is polymorphism. Polymorphism allows a base class to define methods that will be implemented by any derived classes. The base class defines the message signature that derived classes must adhere to, but the implementation code of the method is left up to the derived class. The power of polymorphism lies in the fact that clients know they can implement methods of classes of the base type in the same fashion. Even though the internal processing of the method may be different, the client knows the inputs and outputs of the methods will be the same. After reading this chapter, you should be familiar with the following: How to create and use base classes How to create and use derived classes How access modifiers control inheritance How to override base class methods How to implement interfaces How to implement polymorphism through inheritance and through interfaces

One of the most powerful features of any OOP language is inheritance. Inheritance is the ability to create a base class with properties and methods that can be used in classes derived from the base class.

The purpose of inheritance is to create a base class that encapsulates properties and methods needed in multiple derived classes of the same type. For example, you could create a base class Account. A GetBalance method is defined in the Account class. You can then create two separate classes: a SavingsAccount and a CheckingAccount class. Because the SavingsAccount and the CheckingAccount class use the same logic to retrieve balance information, they inherit the GetBalance method from the base class Account. This enables you to create one common code base that is easier to maintain and manage. Derived classes are not limited to the properties and methods of the base class, however. The derived classes define methods and properties that must be made unique to them. For example, when withdrawing money from a checking account, the business rules require that a minimum balance be maintained. A minimum balance is not required when withdrawing money from a savings account. In this case, each of the derived classes contains its own unique definition of a Withdraw method. To create a derived class in VB code, you use the Inherits keyword along with the name of the base class when defining the derived class. The following code demonstrates the creation of a CheckingAccount class derived from an Account base class. Public Class Account Private _lngAccountNumber As Long Public Property AccountNumber() As Long Get Return _lngAccountNumber End Get Set(ByVal Value As Long) _lngAccountNumber = Value End Set End Property Public Function GetBalance() As Double 'Code to retrieve account balance from database End Function End Class Public Class CheckingAccount Inherits Account Private _dblMinBalance As Double Public Sub Withdraw(ByVal Amount As Double) 'Code to withdraw from account End Sub End Class The following code could be implemented by a client creating an object instance of CheckingAccount. Notice that to the client, there is no distinction made between the call to the GetBalance method and the call to the Withdraw method. In this case, the client has no knowledge of the Account class; instead, both methods appear to have been defined by CheckingAccount. Dim oCheckAccount As CheckingAccount = New CheckingAccount() Dim dblBalance as Double