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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.
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.
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 colonfor 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
