A distinguishing feature of a Collaboration diagram is that it shows the objects and their association with other objects in the system apart from how they interact with each other. The association between objects is not represented in a Sequence diagram.

A Collaboration diagram consists of the following elements:

Object: The objects interacting with each other in the system. Depicted by a rectangle with the name of the object in it, preceded by a colon and underlined.

Links/Association: The connecting lines drawn between objects in a collaboration diagram are links. These links are what set collaboration diagrams apart from sequence diagrams. They enable us to see the relationships between objects. Each link represents a relationship between objects and symbolizes the ability of objects to send messages to each other. A single link can support one or more messages sent between objects. This is different from sequence diagrams, where the lines drawn between objects represent messages sent from one object to another

A link is assumed to represent an association between objects. However, following adornments for links to indicate how objects are associated:

• Global (the object is visible as a global variable)
• Local (the object is visible as a local variable)
• Parameters (the object is visible as a parameter)
• Self (represents the ability of an object to send a message to itself)

Messages: An arrow pointing from the commencing object to the destination object shows the interaction between the objects. The number represents the order/sequence of this interaction

The numbers next to the messages are called sequence numbers. As the name suggests, they show the sequence of the messages as they are passed between the objects. There are many acceptable sequence numbering schemes in UML. A simple 1, 2, 3... format can be used, as the example below shows.

Collaboration diagrams have a distinct advantage over sequence diagrams in that they allow you to show more complex branching as well as multiple concurrent flows of control. In contrast, the format and nature of sequence diagrams really only allow you to show simple branching