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The use case diagram of include and extend

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I remember when I first learned UML last year, I wrote a blog on use case diagrams - UML Diagram - Use Case Diagram

The afternoon questions of the May 2011 soft test, the test to the use case diagram, suddenly feel some ambiguity about the concept of include and extend in the use case diagram, so move out, with everyone to learn.

In fact, after careful consideration, the two concepts are still very well understood, as the name implies, include is the meaning of "contain", extend is the meaning of "extend".


Although include means to include, I believe that include in the use case diagram, mainly reflects the idea of "abstraction". That is, when two or more use cases have the same parts, the set of actions can be abstracted out as a separate sub-use case in which the common parts are abstracted.

Because the sub use case is abstracted, neither it nor the base use case is a complete use case; only when they are combined is it a complete use case.

We often design a query screen like this when we are working on a system.

This query interface and delete and modify is a typical example of an extend (extend) relational use case. If you want to perform modify and delete operations, you must first perform a query operation, so you can pull out the query function. Its use case can be expressed as follows.

(Include is indicated by an arrow with a dashed header pointing in the direction of the sub-use case)


The extend relationship is an extension of a base use case, and differs from include in that the base use case with the extend relationship is a complete use case, and the use case from which it extends is another feature (use case) on top of it.

In the extend relationship, the base use case

Also on this query screen above.

This query and the import to Excel is an EXTEND relationship, the export to Excel is an extension of the query on top of the query.

include is indicated using an arrow with a dashed header, with the direction of the arrow pointing to the base use case.

To sum up.

The main idea behind the include relationship is the idea of "abstraction", where the base use case and the child use case are an inseparable whole and must be executed simultaneously.

The extend relationship mainly refers to the fact that from one use case another use case can be extended to complete a complete function even without the participation of a child use case.

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