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9.3.3 Collaboration


   A collaboration describes a structure of collaborating elements (roles), each performing a specialized function, which collectively accomplish some desired functionality. Its primary purpose is to explain how a system works and, therefore, it typically only incorporates those aspects of reality that are deemed relevant to the explanation. Thus, details, such as the identity or precise class of the actual participating instances are suppressed.



    BehavioredClassifier (from BasicBehaviors , Communications ) on page 451


    StructuredClassifier (from InternalStructures ) on page 193


   A collaboration is represented as a kind of classifier and defines a set of cooperating entities to be played by instances (its roles), as well as a set of connectors that define communication paths between the participating instances. The cooperating entities are the properties of the collaboration (see Property (from InternalStructures ) on page 190).

   A collaboration specifies a view (or projection) of a set of cooperating classifiers. It describes the required links between instances that play the roles of the collaboration, as well as the features required of the classifiers that specify the participating instances. Several collaborations may describe different projections of the same set of classifiers.


   No additional attributes


collaborationRole: ConnectableElement [*] References connectable elements (possibly owned by other classifiers), which represent roles that instances may play in this collaboration. (Subsets StructuredClassifier .role)


   No additional constraints


   Collaborations are generally used to explain how a collection of cooperating instances achieve a joint task or set of tasks. Therefore, a collaboration typically incorporates only those aspects that are necessary for its explanation and suppresses everything else. Thus, a given object may be simultaneously playing roles in multiple different collaborations, but each collaboration would only represent those aspects of that object that are relevant to its purpose.

   A collaboration defines a set of cooperating participants that are needed for a given task. The roles of a collaboration will be played by instances when interacting with each other. Their relationships relevant for the given task are shown as connectors between the roles. Roles of collaborations define a usage of instances, while the classifiers typing these roles specify all required properties of these instances. Thus, a collaboration specifies what properties instances must have to be able to participate in the collaboration. A role specifies (through its type) the required set of features a participating instance must have. The connectors between the roles specify what communication paths must exist between the participating instances.

   Neither all features nor all contents of the participating instances nor all links between these instances are always required in a particular collaboration. Therefore, a collaboration is often defined in terms of roles typed by interfaces (see Interface (from Interfaces ) on page 87). An interface is a description of a set of properties (externally observable features) required or provided by an instance. An interface can be viewed as a projection of the externally observable features of a classifier realizing the interface. Instances of different classifiers can play a role defined by a given interface, as long as these classifiers realize the interface (i.e., have all the required properties). Several interfaces may be realized by the same classifier, even in the same context, but their features may be different subsets of the features of the realizing classifier.

   Collaborations may be specialized from other collaborations. If a role is extended in the specialization, the type of a role in the specialized collaboration must conform to the type of the role in the general collaboration. The specialization of the types of the roles does not imply corresponding specialization of the classifiers that realize those roles. It is sufficient that they conform to the constraints defined by those roles.

   A collaboration may be attached to an operation or a classifier through a CollaborationUse . A collaboration used in this way describes how this operation or this classifier is realized by a set of cooperating instances. The connectors defined within the collaboration specify links between the instances when they perform the behavior specified in the classifier. The collaboration specifies the context in which behavior is performed. Such a collaboration may constrain the set of valid interactions that may occur between the instances that are connected by a link.

   A collaboration is not directly instantiable. Instead, the cooperation defined by the collaboration comes about as a consequence of the actual cooperation between the instances that play the roles defined in the collaboration (the collaboration is a selective view of that situation).


   A collaboration is shown as a dashed ellipse icon containing the name of the collaboration. The internal structure of a collaboration as comprised by roles and connectors may be shown in a compartment within the dashed ellipse icon. Alternatively, a composite structure diagram can be used.


Figure 9.11 - The internal structure of the Observer collaboration shown inside the collaboration icon (a connection is shown between the Subject and the Observer role).

   Using an alternative notation for properties, a line may be drawn from the collaboration icon to each of the symbols denoting classifiers that are the types of properties of the collaboration. Each line is labeled by the name of the property. In this manner, a collaboration icon can show the use of a collaboration together with the actual classifiers that occur in

    that particular use of the collaboration (see Figure 9.12).


   queue: List of Call source: Object waitAlarm: Alarm capacity: Integer


Figure 9.12 - In the Observer collaboration two roles, a Subject and an Observer, collaborate to produce the desired behavior. Any instance playing the Subject role must possess the properties specified by CallQueue, and similarly for the Observer role.




   reading: Real color: Color range: Interval

   The primary purpose of collaborations is to explain how a system of communicating entities collectively accomplish a specific task or set of tasks without necessarily having to incorporate detail that is irrelevant to the explanation. It is particularly useful as a means for capturing standard design patterns.

*Changes from previous UML

   The contents of a collaboration is specified as its internal structure relying on roles and connectors; the concepts of ClassifierRole, AssociationRole, and AssociationEndRole have been superseded. A collaboration in UML 2.0 is a kind of classifier, and can have any kind of behavioral descriptions associated. There is no loss in modeling capabilities.