Communication diagrams emphasize the participants in collaborations but model their timing a bit awkwardly. A sequence diagram helps model the timing of collaborations more clearly. Figure 22.25 shows a sequence diagram modeling the sequence of interactions that occur when a
Withdrawal executes. The dotted line extending down from an object’s rectangle is that object’s lifeline, which represents the progression of time. Actions typically occur along an object’s lifeline in chronological order from top to bottom—an action near the top typically happens before one near the bottom.