Sequence Diagrams

A sequence diagram describes the dynamic aspects of the software system. A sequence diagram, (also known as an object interaction diagram or OID) consists of objects (not classes) and their interactions, which are arranged in time sequence. In particular, it shows the objects participating in the interaction and the sequence of the method calls between them.

As shown in Figure D.8, users (also called actors) interact with the system through sending a message (method call) to Object1, which in turn interacts with other objects in a chain reaction. The vertical dotted lines mark the lifelines that are stemmed from each object. Horizontal arrows represent the messages (method calls) including parameters, if any. A sequence diagram always represents a use case or a scenario of the system.