Chapter 8: Interactions

This chapter discusses the various aspects of interactions. The focus is on the messages that pass back and forth between objects during system execution.

Interactions, Lifelines, and Messages

An interaction is a behavior that focuses on the observable exchange of information between objects.

A lifeline represents the participation of a given object in a particular interaction. A lifeline appears on a sequence diagram (discussed in the section "Sequence Diagrams," later in this chapter) as a dashed line extending from the bottom of the object to which it belongs. Figure 8-1 shows two examples of lifelines.

Figure 8-1: Lifelines

Objects communicate via messages between lifelines. The notation for a message is always an arrow, but the nature of the arrow and the arrowhead varies based on the type of message, as follows :

• A synchronous call or signal message appears as a solid line with a solid arrowhead.

• An asynchronous call or signal message appears as a solid line with a half-arrowhead.

• A reply message appears as a dashed line with a solid arrowhead.

• An object creation message appears as a dashed line with a feathered arrowhead.

• A "lost" message (one in which the sender is known but the receiver is not) has a small black circle next to the arrowhead.

• A "found" message (one in which the receiver is known but the sender is not) has a small black circle next to the start of the arrow.