Sequence diagrams are rather like collaboration diagrams, in that they represent the same information. The focus of a sequence diagram is not spatial, as it is with a collaboration diagram, however. Its focus is on the communication between objects in process. A sequence diagram is shown in Figure 5.4.
Each object is represented by a vertical bar. Time elapses from the top to the bottom. Because of this, they can be more straightforward to read than collaboration diagrams. Also, message numbering is optional because the message placement on the page indicates its ordinality.
Sequence diagrams are most useful for representing complex interactions within a system. Sequence diagrams are generally written after collaboration diagrams.
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