Software Engineering, Page 018

B. Interaction diagrams

Interaction diagrams help to show how objects interact with each other during particular scenerios (or use cases). There are two types of such diagrams: sequence diagrams and collaboration diagrams.

1. Sequence diagrams

Sequence diagrams put the accent on viewing the temporal flow of object interactions, by lining objects up horizontally and then regarding the vertical direction as a "time line". Here are some points worth noting, some, but not all of which, are seen in the next example:

A thin box along the dashed vertical line for an object represents an activation of the object. In the absence of concurrent propgramming, an activation starts when the object is given control by means of a method call and ends with the return from this call.
When one object activates another, an arrow is drawn from the "lifeline" (vertical line) of the first object to that of the second object.
Solid arrow heads are used for sequential activations, i.e. for method calls. In a concurrent programming situation, other types of arrow heads are used, as for example, when one object activates another, but the first object does not need to wait on the second object to do something.
It's nice to arrange the objects so that the arrows point left to right as much as possible.
Arrows can be labeled using method call notation or plain english to indicate the nature of the activation.
If a method call has a return value that needs to be saved and used by the caller, then use notation like value := method(...).
A dashed arrow can be used to represent a return from a method call (but these tend to clutter up the diagram when over used).
An asterisk attached to an arrow represents that the arrow might be used repeatedly (see example).
Guards (enclosed in brackets) of the sort seen earlier in activity diagrams, can also be used here (and in collaboration diagrams). Multiple arrows can leave from the same point as long as they are labeled with guards that are mutually exclusive. In this case, the arrow used during a particular scenerio depends on which guard is true. (See Figure 2-30 on page 51 of Bruegge-Dutoit.)
A "X" marks the end of an object's life.

Let's look at one possible sequence diagram for one possible selection process scenerio in the jukebox example.