Activity diagrams tell you what happens, but they do not tell you who does what. In programming, this means that the diagram does not convey which class is responsible for each activity.
In domain modeling, this means that the diagram does not convey which people or departments are responsible for each activity. One way around this is to label each activity with the responsible class or human. This works, but does not offer the same clarity as interaction diagrams (see Chapter 5) for showing communication among objects.
Swimlanes are a way around this.
To use swimlanes, you must arrange your activity diagrams into vertical zones separated by lines. Each zone represents the responsibilities of a particular class or, in the case of Figure 9-5, a particular department.
Swimlanes are good in that they combine the activity diagram's depiction of logic with the interaction diagram's depiction of responsibility. However, they can be difficult to draw on a complex diagram. I have used nonlinear zones on occasion, which is better than nothing. (Sometimes you have to stop trying to say too much in one diagram.)
When they draw an activity diagram, some people make sure to assign activities to objects. Other people are happy to work with the activity diagram first, to get an overall sense of the behavior, and assign the activities to objects later. I've seen people who assign immediately get emotional about those who defer assignment; they make unpleasant accusations of drawing dataflow diagrams and not being object-oriented.
I confess that I often draw an activity diagram without assigning behavior to objects until later. I find it useful to figure out one thing at a time. This is particularly true when I'm doing business modeling and encouraging a domain expert to think of new ways of doing things. That way works for me. Others prefer to assign behavior to objects immediately. You should do whatever you're more comfortable doing. The important thing is to assign activities to classes before you are done. For that, I prefer to use an interaction diagram (see Chapter 5).