An Example Class Design in VB.NET

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23 An Example Class Design
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A use-case diagram for a simple BankAccount model
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The BankAccount class
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The BankAccount class with properties and methods
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Receiving the balance message
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single class, we cannot show collaborations with other objects explicitly, but we can show the BankAccount end of a collaboration quite easily (Figure 218) Note the use of brackets around the class name to show we have an instance of the BankAccount class Note also that the current property value ( 15000) is shown this is not strictly necessary, and can sometimes cause confusion The
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The Deposit and Withdraw methods
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A BankAccount object and its interactions with an ATM
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balance message passed to the account will implicitly retrieve the current account balance (ie the value of that property) All we have done with these diagrams is to show what messages an object of the class can respond to, and what information (if any) these messages bring with them In the case of deposit and withdraw operations (Figure 219), additional information is required to indicate the amount to be deposited or withdrawn This is, of course, the Amount parameter, and is indicated by the word in brackets shown next to the message arrow We can always describe the action that the messages will have as a short text paragraph to resolve any possibilities of ambiguity In designing a single class as we have done here, it is not possible to show full interactions between objects At some later stage, we will have to incorporate this and other classes into a design diagram that shows the interactions, and shows which object sends a message and which receives it Figure 220 shows a very simple example of this, using speci c object instances rather than classes The object that interacts with the BankAccount object is an Automatic Teller Machine (or ATM) The interactions shown are very simple examples, since there are only two objects involved and all interactions are commands directed from the ATM to the BankAccount object Most systems will have to support more complex interactions involving more objects and messages being sent in either direction between them
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Exercise 24
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Assume the BankAccount class has been extended to include information on transaction operations As a part of this, it is now necessary for a BankAccount to determine the Date and Time that a transaction (deposit or withdrawal) is being made at It can do this by sending the Time and Date messages to the ATMMachine object Show the interactions that happen during a withdrawal operation on a diagram
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24 Locating Class Code in VB NET
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When you create a new Console or Windows application project in VB NET, it will be organized so that you will enter program code into a code module, which is a le for containing program text In the default module supplied with a new console project, initially called Module1, you can add new class code by entering class de nitions inside the module code that is, between the keywords Module Module1End Module In this case, you will be creating one or more new classes that are part of the module de nition In a Windows application, you will nd a form module, initially called Form1 and stored in a le Form1vb In this le you will nd a class de nition for the form (initially named Public Class Form1) To add a new class to this module, it can be located either inside or outside the form s class de nition In either type of project, you can also add class de nitions to a project by adding a new class module to it Select Project/Add Class from the menus, and when the dialog box appears, enter the name of the new class (to name the le the class code will be stored in) You can then proceed to add code to the class in the new Code window that appears In fact, you can add as many different classes as you like to this or any other code le When we create an object-oriented program or component, where we locate class code within it is an important issue, since we usually must balance the competing requirements of encapsulation and availability Encapsulation, which requires us to restrict the accessibility of classes and objects that we wish to maintain precise control over, can be enhanced by hiding classes within other classes; for example, locating a BankAccount class within the class de nition of a form that will be used as a user-interface for manipulating BankAccount objects Accessibility means being able to create objects of a class or work with them from some other code, and this is restricted when we de ne one class within another Truly independent classes, which can be used from any code in an application without restriction, are best placed in separate class modules Maximum independence can be realized by placing each class de nition in its own le, but this would lead to very complex organizations of les within an application A better approach is to locate related classes within the same le For example, we might create the following module (Listing 26) as a class library that could be used by applications that dealt with vehicles (note that the actual working code has been omitted from the inside of the de nitions)
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Public Class Wheel 'Wheel member definitions End Class Public Class Car Private mvarWheels(4) As Wheel 'Other definitions End Class
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