February 24, 2021
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Visual Introduction to UML for Object-Oriented Design

  • By Mark Grand
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Class diagrams can include objects. Often, objects are drawn as shown in Figure 17. The object shown in this figure is an instance of a class named Area. The underline tells you that it is an object. A name may appear to the left of the colon (:). The only significance of the name is that it you can use it to identify the individual object.

Figure 17: Object

The lines that connect two objects are not associations. The lines that connect objects are called links. Links are run time connections between objects, whereas associations are relationships between classes. A link is an occurrence of an association, just as an object is an instance of a class. Links can have association names, navigation arrows, and most of the other embellishments that associations can have. However, because a link is a connection between two objects, links may not have multiplicity indicators or aggregation diamonds.

Some diagrams consist just of objects and links. Such diagrams are considered a kind of class diagram. However, there is a special name for this kind of diagram. A diagram that consists of just objects and links is called an object diagram. Figure 18 is an example of an object diagram.

Click here for a larger image.

Figure 18: An Object Diagram


This article has shown you how to use UML to describe classes, interfaces, objects, and the relationships among them. The next article will show you how to use UML to refine a class or object model.

About the Author

    Mark Grand is a consultant and book author with over 30 years of experience who specializes in Distributed Systems, Object-Oriented Design, and Java. He was the architect of the first commercial B2B e-commerce product for the Internet.

Mark is most widely known for his best selling design pattern books. Mark has taught for U.C. Berkeley, Sun, and other organizations.

Mark is based in the Atlanta area. He has been involved with object-oriented programming and design since 1982.

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This article was originally published on December 12, 2008

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