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16.3.3 Extend


   A relationship from an extending use case to an extended use case that specifies how and when the behavior defined in the extending use case can be inserted into the behavior defined in the extended use case.


   DirectedRelationship (from Kernel ) on page 62

Issue 8465 - add NamedElement as parent

   NamedElement (from Kernel , Dependencies ) on page 99


   This relationship specifies that the behavior of a use case may be extended by the behavior of another (usually supplementary) use case. The extension takes place at one or more specific extension points defined in the extended use case. Note, however, that the extended use case is defined independently of the extending use case and is meaningful independently of the extending use case. On the other hand, the extending use case typically defines behavior that may not necessarily be meaningful by itself. Instead, the extending use case defines a set of modular behavior increments that augment an execution of the extended use case under specific conditions.

   Note that the same extending use case can extend more than one use case. Furthermore, an extending use case may itself be extended.

   It is a kind of DirectedRelationship , such that the source is the extending use case and the destination is the extended use case. It is also a kind of NamedElement so that it can have a name in the context of its owning use case. The extend relationship itself is owned by the extending use case.


   No additional attributes



   [1] The extension points referenced by the extend relationship must belong to the use case that is being extended.

   extensionLocation->forAll (xp | extendedCase.extensionPoint->includes(xp))


   The concept of an extension location is intentionally left underspecified because use cases are typically specified in various idiosyncratic formats such as natural language, tables, trees, etc. Therefore, it is not easy to capture its structure accurately or generally by a formal model. The intuition behind the notion of extension location is best explained through the example of a textually described use case: Usually, a use case with extension points consists of a set of finer-grained behavioral fragment descriptions, which are most often executed in sequence. This segmented structuring of the use case text allows the original behavioral description to be extended by merging in supplementary behavioral fragment descriptions at the appropriate insertion points between the original fragments (extension points). Thus, an extending use case typically consists of one or more behavior fragment descriptions that are to be inserted into the appropriate spots of the extended use case. An extension location, therefore, is a specification of all the various (extension) points in a use case where supplementary behavioral increments can be merged.

   If the condition of the extension is true at the time the first extension point is reached during the execution of the extended use case, then all of the appropriate behavior fragments of the extending use case will also be executed. If the condition is false, the extension does not occur. The individual fragments are executed as the corresponding extension points of the extending use case are reached. Once a given fragment is completed, execution continues with the behavior of the extended use case following the extension point. Note that even though there are multiple use cases involved, there is just a single behavior execution.


   An extend relationship between use cases is shown by a dashed arrow with an open arrowhead from the use case providing the extension to the base use case. The arrow is labeled with the keyword «extend». The condition of the relationship as well as the references to the extension points are optionally shown in a Note attached to the corresponding extend relationship.(See Figure 16.3.)


   Perform ATM Transaction On-Line Help

   Figure 16.3 - Example of an extend relationship between use cases

   In the use case diagram above, the use case Perform ATM Transaction has an extension point Selection. This use case is extended via that extension point by the use case On-Line Help whenever execution of the Perform ATM Transaction use case occurrence is at the location referenced by the Selection extension point and the customer selects the HELP key. Note that the Perform ATM Transaction use case is defined independently of the On-Line Help use case.


   This relationship is intended to be used when there is some additional behavior that should be added, possibly conditionally, to the behavior defined in another use case (which is meaningful independently of the extending use case).

   Changes from previous UML

   The notation for conditions has been changed such that the condition and the referenced extension points may now be included in a Note attached to the extend relationship, instead of merely being a textual comment that is located in the vicinity of the relationship.