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Lesson 2Review sequence diagram notation
ObjectiveReview the Notation for a Sequence Diagram
Sequence diagram notation
The sequence diagram illustrates objects talking to one another over time. To do so, the diagram uses objects, timelines, and event arrows. The diagram consists of a set of participating objects lined up across the top of the diagram. A timeline is drawn down from each object.

Sequence Diagram Example

You will note from the diagram above:
  1. The objects use standard object notation, that is, a rectangle containing the object name, a colon, and the class name of the object. All three of these elements are underlined. The object name is optional. You may use just the colon and class name:
    ObjectName:ClassName
  2. The timeline is a vertical dashed line.
  3. The events appear as arrows.
Event Shown Horizontal
An event is shown as a horizontal arrow from the timeline of the "sending" object to the timeline of the "receiving" object. A return from an event is shown as a horizontal arrow from the "receiving" object back to the "sending" object.

Return Arrow Line
Some tools support a different kind of line for the return arrow, for example, a dashed line instead of a solid line. This is extremely useful because the two events are handled so differently.
The order of events
The position on the timeline indicates a relative point in time. The top of the line represents the beginning of the timeline. The bottom represents the end of the timeline.