Introduction to UML

UML, the Unified Modeling Language, is a language for object-oriented design. We use UML diagrams, because "a picture is worth 1K words." UML class diagrams can show the important elements of and relationships between classes in a concise and intuitive way. UML is much more than just class diagrams. Other kinds of UML diagrams can illustrate how classes collaborate with one another and how users interact with classes.We will only use a small subset of UML in this book.

Most of our diagrams were created with a design tool called Umbrello.[4] For a good overview of UML, we recommend The Umbrello UML Modeller Handbook, available from the help menu of Umbrello. Another reference that provides maximum content and minimal bulk is [Fowler04].

[4] http://uml.sourceforge.net

Figure 2.1 is a class diagram with only one class: Person. Notice that the declarations appear in the name : type, Pascal-style, rather than the more familiar C++/Java style, where the names come after the types. This is to help with readabilitybecause we tend to read from left to right, this syntax helps us find names faster. Notice also that public members are preceded by a plus sign (+) and private members are preceded by a minus sign (-).

Figure 2.1. Person class

2.5.1. UML Relationships

UML is especially good at describing relationships between classes. Looking ahead to Example 2.9, we describe the subobject relationship in UML. Because the subobjects are strictly part of the parent object and cannot possibly exist as stand-alone objects, we could use the composition relationship. As Figure 2.2 shows, the filled-in diamond indicates that the object on that side is composed (partially) of the object(s) from the other side of the relationship.

Figure 2.2. Composition