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The UML Class Diagram

The UML Class Diagram

The class diagram is an excellent aid to model the attributes, operations, and associations of the classes in your applications. This article, the second in a series of articles that introduce the reader to the concepts of object-oriented design and the Unified Modeling Language, focuses on UML class diagrams. It reviews UML class diagramming notation and the use of Visio to construct a class diagram.

The Need for Modeling Classes
Class structures are fundamental to any object-oriented programming language. The class structure is a template for the objects that will implement the functionality of your applications. As a .NET developer, you need to become increasingly familiar with the classes available in the .NET Framework. As you gain experience developing .NET applications you will discover the need to develop your own class libraries. Good design of object-oriented applications dictates a clear separation of functionality between the user interface, business logic, and data access logic. Well-designed applications achieve this separation of functionality through the development of class libraries that encapsulate internal processing. By developing class libraries, your applications become more manageable, scalable, and extendable.

An all-too-common problem in many object-oriented applications is a lack of preplanning. Class libraries evolve haphazardly. As a result, the distinction of roles and responsibilities among the various classes in the library becomes muddled and convoluted. These class libraries are difficult to manage, revise, and employ. This is especially problematic when large teams of developers build an application. Employing a tool to model your class libraries is imperative to ensure that the application remains consistent, manageable, and clearly defined. A UML class diagram is an excellent tool for modeling the structure of your class libraries.

Modeling Class Structure Using UML
In a UML class diagram, a rectangle divided into three parts represents the class. The top section contains the name of the class, the middle section contains the attributes of the class, and the lower section contains the operations performed by the class. Operations may take the form of methods, constructors, destructors, properties, or events. Figure 1 illustrates a UML class diagram representing a loan class. This class is part of a library loan application that tracks loan, member, and inventory information for a small library.

Examining the class diagram reveals that along with the name of the attribute, it defines the attribute's accessibility. A minus sign (-) in front of the attribute name indicates it is private. A plus sign (+) indicates it is public, and a pound sign (#) indicates it is protected. A private element is only accessible from within the class. A public element is accessible from any code in the same project or any project with a reference to the project containing the class. A protected element is accessible from within the class and any class derived from the class.

Although the UML specifications do not define specific data types, when developing classes for use in .NET assemblies you can strongly type the attributes using the supported types of your implementation language. Visio allows the choice of either VB, C#, C++, or IDL supported data types. Data types can also be complex data types defined by your class library or the .NET class library. For example, a catalog class may have an item attribute that is of type System.Data.DataRow. Another possible scenario would be to create a catalog class as a collection class whose item attribute is an Item class type.

The lower section of the UML class shape defines the operations of the class. Typically, the class diagram will define the name of the operation, its accessibility, any parameters passed in to the operation, and any return value passed back when the operation executes. The loan class represented in Figure 1 contains two constructors (Sub New), a function (CalculateDueDate), an event procedure (OverDueNotice), and a property (LoanID). Unfortunately, the UML specifications do not define a way to identify the various operation types in the diagram. You can, however, add your own notation to indicate the various types depending on the modeling tool you use. Figure 2 shows the loan class shape with custom stereotypes added to identify the various operations defined in the diagram.

Modeling Class Associations
When a class refers to or uses another class, the classes form an association. For example, in the library loan application we have identified the need to construct a Loan class and a Book class. The responsibility of the Loan class is to maintain information about a book out on loan. This includes the member who has the book, the book, and the due date. The responsibility of the Book class is to maintain relevant information about the book. This includes ISBN, title, author, and publisher. Since the Loan class maintains a reference to the Book class, the book class is a collaborator of the Loan class. A line connecting the two classes represents this association in the class diagram. The association line often includes a label to describe the association.

Association lines can also indicate the direction of the association. In the previous case, the Loan class maintains a reference to the Book class, but the book class does not maintain a reference to the Loan class. To depict a one-way association, you add an arrow to the association line. The class diagram shown in Figure 3 documents the association between the loan class and the book class.

Sometimes a single instance of one class associates with multiple instances of another class. For example, a member may have multiple books out on loan. In this case, a single instance of a member class is associated with multiple instances of the Loan class. Multiplicity is included in the class diagram by placing the appropriate symbol on the association line. The class diagram in Figure 4 indicates that an instance of the Member class may be associated with multiple instances of the Loan class.

Using Visio to Construct a Class Diagram
Now that you're familiar with some of the notations used to construct a class diagram, you are ready to investigate the use of Visio to create class diagrams. If you do not have Visio installed, you can still benefit from following along and completing the diagram with paper and pencil.
1.   Start up Visio. From the File menu choose New -> Software -> UML Model Diagram.
2.   Locate the Shapes window along the left side of the screen. This window includes a tab for the common UML diagrams. Select the UML Static Structure tab (see Figure 5).
3.   From the Shapes window click and drag the Class shape onto the design surface. Right-click the Class shape on the design surface and choose Properties. A UML Class Properties window becomes visible (see Figure 6). Change the name to "Book" and add the description "Responsible for maintaining book information" in the Documentation text box.
4.   Select Attributes on the Categories list box in the left side of the window. Click on the New button on the right side of the window to add a new attribute to the class. Click on the Properties button to display the UML Attribute Properties window. Change the name of the attribute to "mISBN". Change the type to "VB::Char" and verify that the visibility is "private". Click the OK button to return to the UML Class Properties window.
5.   Select Operations in the Categories list box on the left side of the window. Click on the New button on the right side of the window to add a new operation to the class. Click on the Properties button to display the UML Operation Properties window. Change the name of the operation to "ISBN". Change the return type to "VB::Char" and verify that the visibility is "public". Select "Code Generation Options" in the Categories list box on the left side of the window. Change the operation kind to "Property" and check the "Create Get Method" and "Create Set Method" check boxes to indicate it is a readable/writable property. Click the OK button to return to the UML Class Properties window.
6.   Select Operations in the Categories list box on the left side of the window. Click on the New button on the left side of the window to add a new operation to the class. Click on the Properties button to display the UML Operation Properties window. Change the name of the operation to "CheckAvailability". Change the return type to "VB::Boolean" and verify that the visibility is "public". Select "Parameters" in the Categories list box on the left side of the window. Click on the New button on the right side of the window to add a new parameter to the operation. Click on the Properties button to display the UML Parameter Properties window. Change the name of the parameter to "ReservationDate" and the type to "VB::Date". Click the OK button to return to the UML Operation Properties window. Click the OK button to return to the UML Class Properties window. Click OK to close the UML Class Properties window. The Book class diagram should look similar to the one shown in Figure 7.
7.   In order to demonstrate the creation of class associations, add a Member class and a Loan class to the class diagram. If you want some practice, add some attributes and operations to these classes.
8.   From the Shapes window, click and drag the Binary Association shape onto the design surface. Attach end 1 of the Binary Association to the Member class and end 2 to the Loan class. The Binary Association line will turn red if it is not attached properly. Right-click the Binary Association shape on the design surface and choose Properties. The UML Association Properties window will display. Change the name of the association to "Takes Out". Change the Name Reading Direction to "forward". Change End 1's multiplicity to 1 and check the IsNavigable check box. When you are finished, click OK to close the window.
9.   Right-click the "Takes Out" Binary Association shape on the design surface and choose Shape Display Options. The UML Shape Display Options window displays. Under the General section, click the Name check box. Under the Suppress section, click on the First end and Second end check boxes. When finished, click OK.
10.   Figure 8 shows the completed diagram, including an association between the Loan class and the Book class. For additional practice, add this association to your diagram.

This article has introduced you to using the Unified Modeling Language to model the class structure of your application. The class diagram is a great aid for modeling the attributes and operations of the classes that make up your application. The class diagram also shows how the various classes in your application associate with each other. A CASE tool such as Visio, although not necessary, is beneficial in the creation of the class diagram.

A future article will continue the study of UML and class associations, focusing on modeling class inheritance hierarchies. In addition, you will be introduced to modeling interfaces, meta classes, and utility classes, which are all commonly used in class library applications. Finally, you will experience using Visio to generate a Visual Studio .NET class library from the UML model.

More Stories By Dan Clark

Dan is a Microsoft Certified Trainer, Microsoft Certified Solution Developer, and a Microsoft Certified Database Administrator. For the past seven years he has been developing applications and training others how to develop applications using Microsoft technologies. Dan has been developing and training Microsoft's .NET technologies since the early betas. He has recently authored the book "An Introduction to Object-Oriented Programming with Visual Basic .NET," published by Apress.

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