Collaboration Diagram has been renamed to
Communication Diagram. Refer to Differences Between UML 1.x and UML 2.0 for more information.
Communication diagrams are used to show how objects interact to perform the behavior of a particular use case, or a
part of a use case. Along with sequence diagrams, communication diagrams are used by designers to define and clarify
the roles of the objects that perform a particular flow of events of a use case. They are the primary source of
information used to determining class responsibilities and interfaces.
Unlike a sequence diagram, a communication diagram shows the relationships among the objects. Sequence diagrams and
communication diagrams express similar information, but show it in different ways. Communication diagrams show the
relationships among objects and are better for understanding all the effects on a given object and for procedural
Because of the format of the communication diagram, they tend to better suited for analysis tasks (see Task: Use-Case Analysis). Specifically, they tend to be better suited
to depicting simpler interactions of smaller numbers of objects. As the number of objects and messages grows, the
diagram becomes increasingly hard to read. In addition, it is difficult to show additional descriptive information such
as timing, decision points, or other unstructured information that can be easily added to the notes in a sequence
Contents of Communication Diagrams
You can have objects and actor instances in communication diagrams, together with links and messages describing how
they are related and how they interact. The diagram describes what takes place in the participating objects, in terms
of how the objects communicate by sending messages to one another. You can make a communication diagram for each
variant of a use case's flow of events.
A communication diagram that describes part of the flow of events of the use case Receive Deposit Item in the
An object is represented by an object symbol showing the name of the object and its class underlined, separated by a
objectname : classname
You can use objects in communication diagrams in the following ways:
An object's class can be unspecified. Normally you create a communication diagram with objects first and specify
their classes later.
The objects can be unnamed, but you should name them if you want to discriminate different objects of the same
An object's class can itself be represented in a communication diagram, if it actively participates in the
Normally an actor instance occurs in the communication diagram, as the invoker of the interaction. If you have several
actor instances in the same diagram, try keeping them in the periphery of the diagram.
Links are defined as follows:
A link is a relationship among objects across which messages can be sent. In communication diagrams, a link is
shown as a solid line between two objects.
An object interacts with, or navigates to, other objects through its links to these objects.
A link can be an instance of an association, or it can be anonymous, meaning that its association is
Message flows are attached to links, see Messages.
A message is a communication between objects that conveys information with the expectation that activity will ensue. In
communication diagrams, a message is shown as a labeled arrow placed near a link. This means that the link is used to
transport, or otherwise implement the delivery of the message to the target object. The arrow points along the link in
the direction of the target object (the one that receives the message). The arrow is labeled with the name of the
message, and its parameters. The arrow may also be labeled with a sequence number to show the sequence of the message
in the overall interaction. Sequence numbers are often used in communication diagrams, because they are the only way of
describing the relative sequencing of messages.
A message can be unassigned, meaning that its name is a temporary string that describes the overall meaning of the
message. You can later assign the message by specifying the operation of the message's destination object. The
specified operation will then replace the name of the message.