TABLE OF CONTENTS This section introduces the second of the 13 UML diagrams: the object diagram. Object diagrams realise a structural aspect of the model and are heavily related to class diagrams. In fact, many people confuse the terms 'object diagram' and 'class diagram' and use them interchangeably, but they are in fact two separate diagrams. Indeed, many CASE tools that are available do not make any distinction between the two diagrams, often combining them into something known as a 'structural diagram' or 'object model' or something similar.
The definition of an object is that it is an instance of a class; therefore, it seems logical that object diagrams will represent instances of class diagrams, which is exactly what they do. Objects represent real-life examples of the abstract classes found in class diagrams and relate very closely to real life. Object diagrams also have strong relationships with component and deployment diagrams (both structural) as well as all types of interaction diagrams (communication, sequence, interaction overview and timing diagrams).
Object diagrams are made up of two main elements: objects and links. Objects are defined as an instance, or many instances, of a class. In the same way, links are instances of associations.
It can be seen from the diagram in Figure 5.13 that an 'Object diagram' is made up of one or more 'Instance' and zero or more 'Link'. Also, each 'Link' relates together two 'Instance'. When instances are identified, it is usual to assign a...
