The UML notation introduced a second type of interaction diagram, called a collaboration diagram, as an alternative to sequence diagrams; both types of diagram present more or less the same information, but portrayed in a different manner.
In a collaboration diagram, we eliminate the lifelines used to portray objects and actors. Rather, we lay out instance icons representing objects and stick figures representing actors in whatever configuration is most visually appealing. We then use lines and arrows to represent the flow of messages and responses back and forth between these objects/actors. Because we lose the top-to-bottom chronological sense of message flow that we had with the sequence diagrams, we compensate by numbering the arrows in the order that they would occur during execution of a particular scenario.
The collaboration diagram in Figure 11-12 is equivalent to the sequence diagram that we produced for Scenario #1.
Again, from Fred's vantage point, he observes only a few of these actions, as shown in Figure 11-13.
Because the sequence and collaboration diagrams reflect essentially the same information, many object modeling software tools automatically enable us to produce one diagram from the other with the push of a button.