ITPEC FE Morning October 2022 Question 46

Sequence diagram — it lays out the messages between objects in the order they happen in time.

Each object participating in the interaction gets a vertical lifeline, and time runs from top to bottom. Messages are horizontal arrows from one lifeline to another, so reading downwards gives the exact order of calls and replies. It is the UML diagram to use when the question stresses "interactions" and "time series" together.

Answer (c)

Why not others:

• •(a) Activity diagram — shows the flow of control from one action to the next, like a flowchart, with no lifelines or messages between objects
• •(b) Component diagram — a structure diagram of the software components and their interfaces and dependencies; it shows no behaviour over time
• •(d) State transition diagram — follows the states of a single object and the events that move it between them, not the interaction among several objects

Key rule: Among the UML interaction diagrams, a sequence diagram emphasises time order and a communication diagram emphasises the links between objects; structure diagrams (class, component) show no time at all.

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