Class 09: The UML (3): Processes and Computation

Held: Wednesday, 15 September 2004

Summary: Today we continue our exploration of the UML by visiting the ways in which the UML can represent action within a system.

Related Pages:

• EBoard.

Overview:

• Representing code in action.
• Sequence diagrams.
• Use cases.
• Finite state amchines.

Action in UML

• Class/object diagrams represent the static relationships of a system.
• However, it is also useful/necessary to represent the system in action.
• UML provides basic techniques for representing the system in action
  • Sometimes other techniques are better.
• What kinds of action do we want to represent?
  • General (Messages): What messages what objects send to each other.
  • Internal (Sequence Diagrams): What steps are involved in some computation?
  • Examples (Use Cases): What does the system do in particular situations?
  • Single Object (State Diagrams): What "states" can an object be in and how does it transition between those states?

Messages

• Our first notation is non-normative.
• We can enhance a normal class diagram by indicating what messages each object may send to another.
  • Traditionally, CRC cards help us figure such things out.

Sequence Diagrams

• Represenent the internal data for a sequence of actions (often relating to some problem we're solving or the implementation of some method).
• Time runs vertically (top to bottom)
• Objects distributed horizontally.
• Some implicit relationships (by name)
• Solid line, solid head for message send.
• Dashed line, open head for return.
• Why bother? Won't code suffice?

Use Cases

• Use cases are a helpful design tool. You walk through the steps from the user's perspective (with some generalities about the internals).
• The UML acts primarily to show the relationship between individual use cases, and not the details of each use case.

State Machine Diagrams

• State machine diagrams describe the possible states of an object over time and how it transitions between those states.
• State machine diagrams can be particularly useful to describe things like modern watches, in which the same button has different functions that depend on the current state of the watch.

Questions

• What questions do you have?
• My question for you: What aspects of OOP haven't we covered yet?