UML Tutorial: Part 1 -- Class Diagrams

By Robert C. Martin

UML stands for Unified Modeling Language. It represents a unification of the concepts and notations presented by the three amigos in their respective books. The goal is for UML to become a common language for creating models of object oriented computer software.

In its current form UML is comprised of two major components: a Meta-model and a notation. In the future, some form of method or process may also be added to; or associated with, UML.

UML is unique in that it has a standard data representation. This representation is called the metamodel. The meta-model is a description of UML in UML. It describes the objects, attributes, and relationships necessary to represent the concepts of UML within a software application.

This provides CASE manufacturers with a standard and unambiguous way to represent UML models. Hopefully it will allow for easy transport of UML models between tools. It may also make it easier to write ancillary tools for browsing, summarizing, and modifying UML models.

A deeper discussion of the metamodel is beyond the scope of this column. Interested readers can learn more about it by downloading the UML documents from the rational web site.

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UML basics Part III: The class diagram

by Donald Bell

In June 2003, I began a series of articles titled "UML Basics," designed as an introduction to the Unified Modeling Language. The first article in this series provided high-level introductions to the most widely used diagrams in the UML; the second article offered an in-depth look at the activity diagram.

In this third article, I will focus on the class diagram. Although almost every UML-knowledgeable person claims to understand this diagram, very few actually know the diagram's proper notation set and consequently do not know how to use the diagram. The discussion that follows should enable you to understand and draw a proper class diagram using the UML v1.4 notation set.

This article assumes you have a rudimentary understanding of object oriented design. For those of you who need a little assistance with OO concepts, you might try the Sun brief tutorial about Object Oriented Programming at http://java.sun.com/docs/books/tutorial/java/concepts/. Reading the sections "What Is a Class?" and "What Is Inheritance?" should give you enough understanding to read this article. In addition, David Taylor's book, Object-oriented Technologies: A Manager's Guide, offers an excellent, high-level explanation of object-oriented design without requiring an in-depth understanding of computer programming.

The purpose of the class diagram is to show the static structure of the system being modeled. The diagram specifically shows the entities in the system -- and I literally mean entities, as in "discrete things," not to be confused with "database entities" -- along with each entity's internal structure and relationships with other entities in the system. Because class diagrams only model the static structure of a system, only types of entities
are shown on a class diagram; specific instances are not shown. For example, a class diagram would show an Employee class, but would not show actual employee instances such as Donald Bell, Mike Perrow, or Jimmy Buffett.

Developers typically think of the class diagram as a diagram specifically meant for them, because they can use it to find out details about the system's coded classes or soon-to-be-coded classes, along with each class's attributes and methods.

Class diagrams are particularly useful for business modeling, too. Business analysts can use class diagrams to model a business's current assets and resources, such as account ledgers, products, or geographic hierarchy.

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