What is Communication Diagram? First of all, I would like to make clear that it is also known as a Collaboration Diagram. So these two diagram communication and collaboration diagrams are same. Communication Diagram is used to show how objects interact to perform the behavior of a particular use case or a part of a use case.
After the expanded use case description of the major use cases, we start the communication diagram. So that you should know what is and how to write expanded use case description. Expanded use case description is a two-way interaction step written between the actor and the system how that particular use case will execute. Its like the script of the movie which is written, step by step execution. As we are drawing the design in an object-oriented way, a collaboration diagram describes a pattern of interaction among objects. It shows the objects participating in the interaction by their links to each other and the messages that they send to each other.
Actually Collaboration diagrams are used by designers to define and clarify the roles of the objects that perform a particular flow of events of use cases. They are the primary source of information used for determining class responsibilities and interfaces.
After the collaboration diagram, we will also get the numbers of classes and their names which need to be defined in the application and its respective objects as well. Not only that, also we will know how these objects of different classes will interact and flow messages with each other in stepwise. All showing this kind of information in the diagram is the Collaboration diagram. Its main purpose is to visualize the interactive behavior of the system.
We all know that, objects of the classes communicate by sending and receiving messages.