6.3 Interactions and Collaborations

Formally, a collection of classes, their messages, and their associations is known as a collaboration. A collection of specific objects playing the roles of the classes, their stimuli, and their specific links playing the roles of the associations, is known as a collaboration instance. A collection of messages is known as an interaction and a collection of stimuli is known as an interaction instance. An interaction is said to occur within the context of a collaboration because an interaction is a subset of the contents of a collaboration; and an interaction instance is said to occur within the context of a collaboration instance because an interaction instance is a subset of the contents of a collaboration instance.

You may show an interaction using one of two forms:

Generic-form interaction

Shows two or more possible sequences of message or stimuli exchanges using repetition and conditionality.

Repetition

Involves repeating a set of messages or stimuli — for example, repeating the set of messages or stimuli in steps 6a and 6b of the interaction and collaboration description to generate the project-status report.

Conditionality

Involves communicating one set of messages or stimuli rather than another set of messages or stimuli — for example, conditionally performing step 5 or 6 of the interaction and collaboration description to generate the project-status report.

Instance-form interaction

Shows one actual sequence of message or stimuli exchanges without any repetition or conditionality. For example, generating a specific project-status report that contains exactly three workers, each with two units of work and one work product. Rather than showing repetition and conditionality as in a generic-form interaction, instance-form interactions show the actual set of messages or stimuli that are repeated and the set of messages or stimuli that are communicated for specific conditions.

You may show a collaboration using one of two levels:

Specification-level collaboration

Shows class roles, association roles, and their messages. For example, you might use class roles, association roles, and their messages to communicate a generic-form or instance-form interaction.

Instance-level Collaboration

Shows specific objects, links, and their stimuli. For example, you might use objects, links, and their stimuli to communicate a generic-form or instance-form interaction.

In the UML, sequence and collaboration diagrams, rather than textual descriptions, are used to capture the details of interactions, collaborations, interaction instances, and collaboration instances. The interaction forms and collaboration levels just described make the UML very expressive.