Æliens

Object-oriented design -- decomposition into objects

• application/system/class oriented

Identifying objects -- responsibilities

• data/procedure oriented

Layers of abstraction

• components, subsystems, frameworks

Objects -- crisp entities

• object = an entity that suffers and requires actions

• $[1]$ Identify the objects and their attributes
• $[2]$ Identify operations suffered and required
• $[3,4]$ Establish visibility/interface

• model of reality -- balance nouns (objects) and verbs (operations)

• directed action -- drives, instructs
• communication -- talks-to, tells, instructs
• ownership -- has, part-of
• resemblance -- like, is-a
• others -- works-for, married-to

ATM

• account -- represents the customer's account in the banks database
• atm -- performs financial services for a customer
• cardreader -- reads and validates a customer's bankcard
• cashdispenser -- gives cash to the customer
• screen -- presents text and visual information
• keypad -- the keys a customer can press
• pin -- the authorization code
• transaction -- performs financial services and updates the database

Eliminating spurious classes

• vague -- system, security-provision, record-keeping
• attribute -- account-data, receipt, cash
• redundant -- user
• irrelevant -- cost
• implementation -- transaction-log, access, communication

• our candidate classes

CRC

• Immerse the reader in the {\em object-ness} of the material
• Give up global knowledge of control
• Rely on the local knowledge of objects

OO design with CRC cards

• Class, Responsibility, Collaborators

Object roles

• actor -- operates (suffers no operations)
• server -- suffers operations
• agent -- suffers and operates ( actor & server)