Sequence Diagrams
Sequence diagrams document the interactions between classes to achieve a
result, such as a use case. Because UML is designed for object-oriented
programming, these communications between classes are known as messages. The
Sequence diagram lists objects horizontally, and time vertically, and models
these messages over time.
Notation
In a Sequence diagram, classes and actors are listed as columns, with vertical
lifelines indicating the lifetime of the object over time.
| Object |
Objects are instances of classes, and are arranged horizontally. The pictorial
representation for an Object is a class (a rectangle) with the name prefixed by
the object name (optional) and a semi-colon. |
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| Actor |
Actors can also communicate with objects, so they too can be listed as a
column. An Actor is modeled using the ubiquitous symbol, the stick figure. |
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| Lifeline |
The LifeLine identifies the existence of the object over time. The notation for
a Lifeline is a vertical dotted line extending from an object. |
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| Activation |
Activations, modeled as rectangular boxes on the lifeline, indicate when the
object is performing an action. |
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| Message |
Messages, modeled as horizontal arrows between Activations, indicate the
communications between objects. |
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Following is an example of a Sequence diagram, using the default named objects.
You can imagine many instances where a user performs an action in the user
interface, and the system in turn calls another object for processing.
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