6.1 Roles

As discussed in Chapter 2, the object-oriented paradigm views the world as a collection of unique elements (objects, classes, subsystems, and so forth), often referred to as a society of objects, which communicate with one another. Communication from a sender element to a receiver element is used to convey information or request processing. The word "objects" is used in the phrase "society of objects" rather than "classes," "subsystems," or any other word, because objects are the most fundamental, or primitive, parts of the object-oriented paradigm. It is also common to use the phrase "society of elements."

For example, Figure 6-1, a use-case diagram, shows that a project manager may request that the project management system generate a project-status report that is sent to a printer. The Generate Project-Status Report use case involves a Project Manager actor who initiates the generation of the report and a Printer actor through which the report is printed, as discussed in Chapter 4.

Figure 6-1. Generating a project-status report

The Generate Project-Status Report use case may have the following description:

The project manager enters the name of the project of interest.
The system generates the report.
The system sends the report to the printer.
The printer prints the report.

Notice how all the actors and the system are involved in this description. The description indicates that the system will generate the report, but it does not indicate how.

To provide the functionality for the report, a society of objects must work together in much the same way the classes in Figure 6-2 do. Figure 6-2 shows that an organization may execute any number of projects. An object of the Organization class may contain zero or more objects of the Project class, and each object of the Project class must be contained by no more than one object of the Organization class, as discussed in Chapter 3. Therefore, these classes and others could be used to form a society of objects that work together to provide the functionality of the Generate Project-Status Report use case.

Figure 6-2. Organizations and projects

As the classes in Figure 6-2 may interact and collaborate with other classes to provide the functionality in Figure 6-1, each class or association may play a role within that interaction and collaboration. A role is a placeholder for an object or link to play in an interaction and collaboration. Elements that play such roles are known as participating elements. For example, the participating elements in the Generate Project-Status Report use case may play the following roles:

Initiator: Played by a project manager; responsible for initiating the use case
Output device: Played by a printer; responsible for printing the generated report
Project organization: Played by an object of the Organization class; responsible for containing the project that is the subject of the report
Project reference: Played by a link of the Execute association; responsible for referring to the project that is the subject of the report
Project: Played by an object of the Project class; subject of the report

To generate the report in an interaction and collaboration, I can use an object of the Project class to access information about the project directly, or I can use a link of the Execute association as a means to access an object of the Project class. Because I want to demonstrate how to use classes and objects as well as associations and links in interactions and collaborations, I will focus on the Organization class and Execute association rather than the Organization class and Project class.

In the UML, the use of an interaction and collaboration is depicted as a dashed ellipse containing the name of the interaction and collaboration. The interaction and collaboration may be attached to the use case that it realizes using a realization relationship, as discussed in Chapter 3. Figure 6-3, a use-case diagram, shows that the Generate Project-Status Report interaction and collaboration realizes the Generate Project-Status Report use case. It is quite common to use the same name for the interaction and collaboration as the use case it realizes.

Figure 6-3. Interaction and collaboration realizing a use case

The Generate Project-Status Report interaction and collaboration may have the following description using the initiator, output device, project organization, and project reference roles just described:

The initiator role enters the name of the project for which the report is to be generated and submits this information to the system.
The request to generate the project-status report is received by a handler element that is responsible for managing the overall generation of the report.
The handler element creates a report element that is an abstraction of the generated report. That is, the report element represents within the system the report that is generated.
The handler element retrieves information about the organization from the project organization role and about the project using the project reference role, formats the information, and outputs the formatted information to the report element.
If the project is a newly created or unpopulated project that does not have any information (that is, no workers, units of work, and no work products are yet defined), the handler element requests that the project organization element indicates this on the report element.
If the project does have some information, the handler element requests that the project organization element continue generating information for the report element.
1. The project organization role retrieves all the workers of the project and requests that the project output the contact information for each worker using the project reference role. The project organization role does not do any formatting of this output.
2. For each worker, the project organization role retrieves the worker's units of work and work products using the project reference role, formats this information, and outputs the formatted information to the report element. The project organization role does all the formatting of this output.
The handler element prints the report element.
The handler element destroys the report element. That is, it destroys the element that represents within the system the report that is generated.
The output device role prints the report received from the system.

Notice how all the roles and other elements are involved in this description. The description elaborates on the Generate Project-Status Report use case and indicates how the system will generate the report:

Step 1 of the use case description is equivalent to step 1 of the interaction and collaboration description.
Steps 2 and 3 of the use case description are elaborated into steps 2 through 8 of the interaction and collaboration description.
Step 4 of the use case description is equivalent to step 9 of the interaction and collaboration description.

The roles in the Generate Project-Status Report interaction and collaboration can be classified as class roles and association roles.

6.1.1 Classes and Objects

A class role defines a specific use of an object of a class; objects of the class are said to conform to the class role. For example, a project manager plays the role of an initiator, a printer plays the role of an output device, and an organization plays the role of an organization that contains the project that is the subject of a project-status report.

To show that objects of a class conform to a class role, a dashed line is drawn from the interaction and collaboration symbol to the class and labeled with the role objects of the class play in the interaction and collaboration. For example, Figure 6-4, a diagram that uses elements from a use-case diagram and a class diagram, combines Figures Figure 6-1 through Figure 6-3 to show the initiator, outputDevice, and projectOrganization class roles.

Figure 6-4. Interaction and collaboration using classes

In addition, Figure 6-4 demonstrates two methods for identifying a role. One method involves labeling the dashed line to indicate that objects of a class conform to a role. You may also show a class using the notation for classes as defined in Chapter 3, but with the class name preceded by a forward slash followed by the name of the role followed by a colon—for example, /projectOrganization:Organization. The forward slash indicates that the name to follow, projectOrganization, is a rolename. That is followed by a colon and, finally, the class name, which in this case is Organization. Usually, you will want to use just one of these methods consistently.

To show that a specific object of a class conforms to a class role, a dashed line is drawn from the interaction and collaboration symbol to the object and labeled with the role the specific object of the class plays in the interaction and collaboration. As with a class, rather than label the dashed line, you may show an object using the notation for objects as defined in Chapter 3, but the object name is followed by a forward slash followed by the name of the role followed by a colon followed by the class name, all fully underlined.

For example, Figure 6-5, which is a diagram that uses the elements from a use-case diagram and an object diagram, shows that Andy plays the role of an initiator, Andy's printer plays the role of an output device, and Andy's organization plays the role of an organization that contains the project that is the subject of the report.

Figure 6-5. Interaction and collaboration using objects

6.1.2 Associations and Links

An association role defines a specific use of a link of an association. Links of the association are said to conform to the association role. For example, an execute relationship plays the role of a reference to the project that is the subject of a project-status report.

To show that links of an association conform to an association role, a dashed line is drawn from the interaction and collaboration symbol to the association and labeled with the role links of the association play in the interaction and collaboration. Also, rather than label the dashed line, you may show the association using notation similar to that used for classes in Figure 6-4: the association name is preceded by a forward slash followed by the name of the role followed by a colon.

For example, Figure 6-6 updates Figure 6-4 and shows the projectReference association role.

Figure 6-6. Interaction and collaboration using associations

To show that a specific link of an association conforms to an association role, a dashed line is drawn from the interaction and collaboration symbol to the link and labeled with the role the specific link of the association plays in the interaction and collaboration. Also, rather than label the dashed line, you may show the link using notation similar to that used for objects in Figure 6-5: the association name is preceded by a forward slash followed by the role followed by a colon. Recall from Chapter 3 that a link may have its association name shown near the path fully underlined, but links do not have instance names.

For example, Figure 6-7 updates Figure 6-5 and shows the projectReference link role.

Figure 6-7. Interaction and collaboration using links