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6.5 Collaboration Diagrams
A collaboration diagram shows elements as they interact over
time and how they are related. That is, it shows a collaboration or
collaboration instance. While sequence diagrams are time-oriented and
emphasize the overall flow of an interaction, collaboration diagrams
are time- and space-oriented and emphasize the overall interaction,
the elements involved, and their relationships. Sequence diagrams are
especially useful for complex interactions, because you read them
from top to bottom. Collaboration diagrams are especially useful for
visualizing the impact of an interaction on the various elements,
because you can place an element on a diagram and immediately see all
the other elements with which it interacts.
6.5.1 Elements
Collaboration diagrams are made up of a number of elements, including
class roles, specific objects, association roles, and specific links.
All of these are described in the following subsections.
6.5.1.1 Class and association roles
In a collaboration diagram, class
roles and other classes are shown
using the same notation as in sequence diagrams. An association role
is shown using the notation for associations as defined in Chapter 3, but the association name is preceded by a
forward slash followed by the name of the role followed by a colon to
which links must conform to participate within the role. Other
associations may also be shown as necessary using the notation for
associations as defined in Chapter 3.Class roles, association roles, other classes, and other associations
are used for specification-level collaborations communicated using
collaboration diagrams. Figure 6-20 shows the class
roles of Figure 6-8 with the
projectReference association role, the
Report and Section classes, and
the Has association.
Figure 6-20. Association roles
6.5.1.2 Specific objects and links
In a collaboration diagram, specific
objects
conforming to class roles and other objects are shown using the same
notation as in sequence diagrams. A specific link of an association
conforming to an association role is shown using the notation for
links as defined in Chapter 3, but the association
name is preceded by a forward slash followed by the name of the role
followed by a colon followed by the regular association name, all
fully underlined. Recall from Chapter 3 that a
link may have its association name shown near the path and is fully
underlined, but that links do not have instance names. Other links
may also be shown as necessary using the notation for links as
defined in Chapter 3.Specific objects conforming to class roles, specific links conforming
to association roles, specific objects, and specific links are used
for instance-level collaborations communicated using collaboration
diagrams. Figure 6-9 with the projectReference
link, which is an anonymous Report object with
Introduction, Body, and
Conclusion objects, and their three
Has links.
Figure 6-21. Links conforming to association roles
6.5.2 Communication
In a collaboration diagram, a
communication,
message, or stimulus is shown as an arrow attached to a relationship
pointing from the sender toward the receiver. A communication is
labeled using the same notation as in sequence diagrams, but a
sequence number is required and expressed using a special
numbering scheme known as dot notation in which
a dot or decimal point is used between different levels of
communication. For example, between element A and B, communication 1
is followed by communication 2; between element B and C,
communication 1.1 is followed by 1.2; and the overall sequence of
communication is 1, 1.1, 1.2, and 2.Figure 6-12, but uses a collaboration diagram to show the
communication occurring between the
GenerateProject-StatusReportHandler class (which
is responsible for managing the overall generation of the report) and
the projectOrganization class role.
Figure 6-22. Collaboration diagram communications
6.5.2.1 Reflexive communication
Figure 6-13
but using a collaboration diagram, shows
a reflexive communication for step 6 of the Generate
Project-Status Report interaction and collaboration
description in which the
GenerateProject-StatusReportHandler class formats
the organization and project information. Notice that a reflexive
communication is shown using a reflexive relationship.
Figure 6-23. Collaboration diagram reflexive communications
6.5.2.2 Element creation and destruction
Figure 6-14
but
using
a collaboration diagram, shows a communication for step 3 of the
Generate Project-Status Report interaction and
collaboration description in which the
GenerateProject-StatusReportHandler class creates
a report. This figure also shows a communication for step 8 of the
Generate Project-Status
Report interaction and collaboration description
in which the GenerateProject-StatusReportHandler
class destroys the report. Notice that a communication that creates
an element and a communication that destroys an element are simply
shown like any other communication.
Figure 6-24. Collaboration diagram creation and destruction communications
6.5.3 Repetition
In a collaboration
diagram, repetition (which involves
repeating a set of messages or stimuli) within a generic-form
interaction is shown as a property. Properties are discussed in Chapter 2.An iteration expression indicating the number of times the
communications occur may be enclosed in a pair of braces ({}) and
attached to the communications to which it applies using dashed
lines. Figure 6-15 but using a collaboration diagram, shows step
6b of the Generate Project-Status Report
interaction and collaboration description. The figure uses an
iteration expression that causes the
GenerateProject-StatusReportHandler class to
retrieve each worker's units of work and list of
work products, to format this information, and to output the
formatted information to the report element.
Figure 6-25. Collaboration diagram repetition using an iteration expression within a generic-form interaction
Alternatively, a guard expression indicating the condition that must
be satisfied to terminate the repetition may be enclosed in a pair of
braces ({}) and attached to the communications to which it applies
using dashed lines. Figure 6-16 but using a collaboration diagram, uses a
guard expression to show the same information as in Figure 6-25.
Figure 6-26. Collaboration diagram repetition using a guard expression within a generic-form interaction
Repetition within an instance-form interaction involves showing the
actual set of messages or stimuli that are repeated. Figure 6-17 but
using a collaboration diagram, shows the same information as in
Figures Figure 6-25 and Figure 6-26
for a project that contains exactly three workers, each with two
units of work and one work product.Notice that Figure 6-27 may be more difficult to
read than Figure 6-17. For more complex
interactions, sequence diagrams are often preferable over
collaboration diagrams, because sequence diagrams are read from top
to bottom. Collaboration diagrams are often preferable over sequence
diagrams when the desire is to show the relationships that allow the
elements to communicate.
Figure 6-27. Collaboration diagram repetition within an instance-form interaction
6.5.4 Conditionality
In a collaboration
diagram,
conditionality which involves
communicating one set of messages or stimuli rather than another set
of messages or stimuli within a generic-form interaction is
shown using the dot notation where the communication at a specific
level indicates the guard expression that must be satisfied for the
next level of communications to occur. For example, between elements
A and B, communication 1 may be labeled with a guard expression and
is followed by communication 2. Communication 1 from A to B may
trigger communications 1.1 and 1.2 between B and C. Because of the
guard condition on communication 1, the following two scenarios are
possible:
The guard condition is satisfied
Communication 1 occurs between A and B, triggering 1.1 and 1.2
between B and C. The overall sequence of communications is then 1,
1.1, 1.2, followed by 2. Communication 2 comes last, because
everything associated with communication 1 must precede it.
The guard condition is not satisfied
Communication 1 does not occur, and thus neither do communications
1.1 and 1.2. However, communication 2, does occur because it is not
protected by the same guard condition as communication 1.
Figure 6-18
but using a collaboration diagram, shows steps 5 and 6 of the
Generate Project-Status Report interaction and
collaboration description where the
GenerateProject-StatusReportHandler class requests
that the projectOrganization class role indicate
that the project is empty if the project is newly created or
unpopulated; and the
GenerateProject-StatusReportHandler class requests
that the projectOrganization class role continue
generating information for the report element if the project is not a
newly created or populated project. In this figure, only the first
communication is shown for actually generating the report.
Figure 6-28. Collaboration diagram conditionality within a generic-form interaction
Conditionality within an instance-form interaction involves the set
of messages or stimuli that are communicated for a specific
condition. Figure 6-19 but using a collaboration diagram, shows Figure 6-28 for a project that is populated.
Figure 6-29. Sequence diagram conditionality within an instance-form interaction
