One of your goals during analysis is to identify potential opportunities for reuse, a goal you can work toward as you are developing your use-case model. Potential reuse can be model through four generalization relationships supported by UML use-case models:  Extend dependencies between use cases Include dependencies between use cases Inheritance between use cases Inheritance between actors

Extend Dependencies Between Use Cases

An extend dependency, formerly called an extends relationship in UML v1.2 and earlier, is a generalization relationship where an extending use case continues the behavior of a base use case. The extending use case accomplishes this by conceptually inserting additional action sequences into the base use-case sequence. This allows an extending use case to continue the activity sequence of a base use case when the appropriate extension point is reached in the base use case and the extension condition is fulfilled. When the extending use case activity sequence is completed, the base use case continues. In the use case diagram of Figure 1 , you see the use case “Perform” extends the use case “Enroll in University,” the notation for doing so is simply a normal use-case dependency with the stereotype of <<extend>>. In this case, “Enroll in University” is the base use case and “Perform Security Check” is the extending use case.

An extending use case is, effectively, an alternate course of the base use case. In fact, a good rule of thumb is you should introduce an extending use case whenever the logic for an alternate course of action is at a complexity level similar to that of your basic course of action. I also like to introduce an extending use case whenever I need an alternate course for an alternate course; in this case, the extending use case would encapsulate both alternate courses. Many modelers avoid the use of extend dependencies as this technique has a tendency to make use-case diagrams difficult to understand. My preference is to use extend dependencies sparingly. Note, the extending use case¾in this case “Perform Security Check”¾would list “UC33 Enroll in University,” the base use case, in its “Extends” list.

Figure 1 . The opportunities for reuse in use-case models.

Just as you indicate the point at which the logic of an alternate course replaces the logic of a portion of the basic course of action for a use case, you need to be able to do the same thing for an extending use case. This is accomplished through the use of an extension point, which is simply a marker in the logic of a base use case indicating where extension is allowed.  The red text in Figure 2 presents an example of how an extension point would be indicated in the basic course of action of the “Enroll In University” use case. Notice how the identifier and the name of the use case is indicated. If several use cases extended this one from the same point, then each one would need to be listed. A condition statement, such as “Condition: Enrollee is an international student,” could have been indicated immediately following the name of the use case but, in this example, it was fairly obvious what was happening.

Figure 2. The Enroll in University Use Case (narrative style).

Include Dependencies Between Use Cases

A second way to indicate potential reuse within use-case models exists in the form of include dependencies. An include dependency, formerly known as a uses relationship in UML v1.2 and earlier, is a generalization relationship denoting the inclusion of the behavior described by another use case. The best way to think of an include dependency is that it is the invocation of a use case by another one. In Figure 1 , you see the use case “Enroll in University” includes the use case “Enroll in Seminar,” the notation for doing so is simply a normal use-case dependency with the stereotype of <<include>>.  The blue test in Figure 2 presents an example of how you would indicate where the use case is included in the logic of the including use case. Similar to calling a function or invoking an operation within source code, isn’t it?

You use include dependencies whenever one use case needs the behavior of another. Introducing a new use case that encapsulates similar logic that occurs in several use cases is quite common. For example, you may discover several use cases need the behavior to search for and then update information about students, indicating the potential need for an “Update Student Record” use case included by the other use cases. As you would expect, the use case “Enroll in University” should list “UC17 Enroll in Seminar” in its “Includes” list. Why should you bother maintaining an “Includes” and an “Extends” list in your use cases? The answer is simple: your use cases should stand on their own, you shouldn’t expect people to have your use-case diagram in front of them. Yes, it would be nice if everyone has access to the use-case diagram because it also contains this information, but the reality is that sometimes you use different tools to document each part of your model. For example, your diagrams could be drawn using a drawing package and your use cases documented in a word processor. Some of your project stakeholders may have access to the word processor you are using, but not the drawing package. The main disadvantage of this approach is you need to maintain these two lists in parallel with the diagram, the danger being they may become unsynchronized.

Inheritance Between Use Cases

Use cases can inherit from other use cases, offering a third opportunity to indicate potential reuse. Figure 1 depicts an example of this, showing that “Enroll Family Member in University” inherits from the “Enroll In University” use case. Inheritance between use cases is not as common as either the use of extend or include dependencies, but it is still possible. The inheriting use case would completely replace one or more of the courses of action of the inherited use case. In this case, the basic course of action is completely rewritten to reflect that new business rules are applied when the family member of a professor is enrolling at the university. Family members are allowed to enroll in the school, regardless of the marks they earned in high school, they don’t have to pay any enrollment fees, and they are given top priority for enrollment in the university.

Inheritance between use cases should be applied whenever a single condition, in this case, the student is a family member of a professor, would result in the definition of several alternate courses. Without the option to define an inheriting use case, you need to introduce an alternate course to rework the check of the student’s high-school marks, the charging of enrollment feeds, and for prioritization of who is allowed to enroll in the given semester.

The inheriting use case is much simpler than the use case from which it inherits. It should have a name, description, and identifier, and it should also indicate from which use case it inherits in the “Inherits From” section. This includes any section that is replaced, particularly the pre-conditions and post-conditions as well as any courses of action. If something is not replaced, then leave that section blank, assuming it is inherited from the parent use case (you might want to put text, such as “see parent use case,” in the section).

Inheritance Between Actors

The fourth opportunity for indicating potential reuse within use-case models occurs between actors: an actor on a use-case diagram can inherit from another actor. An example of this is shown in Figure 1 , the “International Student” actor inherits from “Student.” An international student is a student, the only difference being is he or she is subject to different rules and policies (the international student pays more in tuition and is restricted from taking several seminars because of political reasons). The standard UML notation for inheritance, the open-headed arrow, is used and the advice presented about the appropriate use of inheritance still applies: it should make sense to say the inheriting actor is or is like the inherited actor.

Source

This article is excerpted from Chapter 5 of the book The Object Primer 3/e.

Recommended Resources 

The Object Primer 3rd Edition: Agile Model Driven Development with UML 2 is an important reference book for agile modelers, describing how to develop 35 types of agile models including all 13 UML 2 diagrams.  Furthermore, this book describes the techniques of the Full Lifecycle Object Oriented Testing (FLOOT) methodology to give you the fundamental testing skills which you require to succeed at agile software development.  The book also shows how to move from your agile models to source code (Java examples are provided) as well as how to succeed at implementation techniques such as refactoring and test-driven development (TDD).  The Object Primer also includes a chapter overviewing the critical database development techniques (database refactoring, object/relational mapping, legacy analysis, and database access coding) from my award-winning Agile Database Techniques book. Agile Modeling: Effective Practices for Extreme Programming and the Unified Process is the seminal book describing how agile software developers approach modeling and documentation.  It describes principles and practices which you can tailor into your existing software process, such as XP, the Rational Unified Process (RUP), or the Agile Unified Process (AUP), to streamline your modeling and documentation efforts.  Modeling and documentation are important aspects of any software project, including agile projects, and this book describes in detail how to elicit requirements, architect, and then design your system in an agile manner. The Elements of UML 2.0 Style describes a collection of standards, conventions, and guidelines for creating effective UML diagrams. They are based on sound, proven software engineering principles that lead to diagrams that are easier to understand and work with.  These conventions exist as a collection of simple, concise guidelines that if applied consistently, represent an important first step in increasing your productivity as a modeler.  This book is oriented towards intermediate to advanced UML modelers, although there are numerous examples throughout the book it would not be a good way to learn the UML (instead, consider The Object Primer).  The book is a brief 188 pages long and is conveniently pocket-sized so it's easy to carry around.

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