UML object diagrams depict instances and their relationships at a point in time. You create a UML object diagram to

1. ■ explore “real-world” examples of objects and the relationships between them;

2. ■ explain complex relationships between classes to people who find class diagrams too abstract;

3. ■ become input into creating a UML class diagram.

Indicate Attribute Values to Clarify Instances

Figure 52 depicts a UML object diagram, taken from The Object Primer 3rd Edition (Ambler 2004), which explores the relationships between students and seminars. The student objects have been given understandable names, such as John Smith and Sally Jones, which immediately identify them. The seminar objects also have understandable names, such as CSC 100a, but also have attribute values such as Term = “Fall” to clarify which seminar is truly meant.

Prefer Object Names over Attribute Values

Whenever possible try to find a good, understandable name for an object without resorting to indicating attribute values. This makes your object diagrams concise.

Indicate Roles to Distinguish Different Relationships

In Figure 52 you see that it is possible for students to be both teaching assistants and simply students of a seminar; for example, Sarah McGrath is a teaching assistant in CSC 100b, whereas Scott Ambler is a student of that seminar. Identifying roles in this manner can help you to identify different associations between the same classes.