J2EE Design Patterns

Interaction Diagrams

Class diagrams are not well suited to describing program flow. While modelers frequently use class diagrams to model aspects of systems that are oriented towards process instead of data, seeing the internal class relations does not always translate into a clear view of the underlying logic. All modeling systems have some mechanism for indicating the flow of control between multiple objects in the system. In the UML, these diagrams are referred to as interaction diagrams . There are two kinds of interaction diagrams: sequence diagrams and collaboration diagrams.

Sequence Diagrams

Sequence diagrams , which show the sequential interactions of a set of objects, are the most common kind of interaction diagram. They can be used for understanding the flow of control within an application (or more often, given the scale of the average enterprise system, within a component of the system).

Sequence diagrams consist of a row of boxes representing objects, generally placed from left to right in order of their appearance within the flow of control. These are the same boxes used for object diagrams with the labels underlined to indicate that they are objects rather than classes. A lifeline stretches down from each object. The lifeline begins when the object is created and ends when the object is removed. If the object is never removed, the lifeline continues to the bottom of the page.

Each lifeline contains one or more activation boxes. Activation boxes show that the object ...