A hands on introduction for developers : Design and UML Class Diagrams

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Design and UML Class Diagrams
Suggested reading:
Practical UML: A hands on introduction for developers
http://dn.codegear.com/article/31863
UML Distilled Ch. 3, by M. Fowler

How do people
draw / write down
software architectures?

Example architectures
person
UW student
CSE 403
student
VerizonWireless
GPS satellite
Cell phone
sea agent
lake agent
amphibious
agent

Big questions
• What is UML?
– Why should I bother? Do people really use UML?
• What is a UML class diagram?
– What kind of information goes into it?
– How do I create it?
– When should I create it?

Design phase
• design: specifying the structure of how a software
system will be written and function, without actually
writing the complete implementation
• a transition from "what" the system must do, to
"how" the system will do it
– What classes will we need to implement a system that
meets our requirements?
– What fields and methods will each class have?
– How will the classes interact with each other?

How do we design classes?
• class identification from project spec / requirements
– nouns are potential classes, objects, fields
– verbs are potential methods or responsibilities of a class
• CRC card exercises
– write down classes' names on index cards
– next to each class, list the following:
• responsibilities: problems to be solved; short verb phrases
• collaborators: other classes that are sent messages by this class
(asymmetric)
• UML diagrams
– class diagrams (today)
– sequence diagrams
– ...

What is UML?
• UML: pictures of an OO system
– programming languages are not abstract enough for OO
design
– UML is an open standard; lots of companies use it
• What is legal UML?
– a descriptive language: rigid formal syntax (like
programming)
– a prescriptive language: shaped by usage and convention
– it's okay to omit things from UML diagrams if they aren't
needed by team/supervisor/instructor

Uses for UML
• as a sketch: to communicate aspects of system
– forward design: doing UML before coding
– backward design: doing UML after coding as documentation
– often done on whiteboard or paper
– used to get rough selective ideas
• as a blueprint: a complete design to be implemented
– sometimes done with CASE (Computer-Aided Software
Engineering) tools
• as a programming language: with the right tools, code can
be auto-generated and executed from UML
– only good if this is faster than coding in a "real" language