Class Diagrams Chapter 3

Classes and Objects Classes are the descriptions –definitions Objects are the things –instances

CSC535 Objects What’s an object? –It’s a “thing” that interacts with other “things” to address the task at hand –It contains attributes (data items) which represent its current state –It contains methods (functions) which are used to manipulate it

CSC535 Objects What’s an object? –An object is a grouping of attributes and behaviors into a single entity

CSC535 Primary and Support Objects What’s a primary object and what’s a support object? –Primary objects Those that are obvious Those that are fundamental to the system Those on which all team members agree must be included in the system (without persuasion) –Support objects Those that you really have to stretch to come up with Those which cause debates among team members Those which you’re just not sure about

CSC535 Primary and Support Objects Identify primary objects early in the design process –This should be relatively easy Leave the support objects for later (maybe even for the implementation phase) Don’t hesitate to change the list of primary objects as the design progresses –Actually, it’s expected to change –Eventually, they’ll all be primary objects since they compose the running system and without them the system will fail [to compile/link/bind at runtime]

CSC535 Tangible and Intangible Objects What’s a tangible object and what’s an intangible object? –Tangible objects generally represent things (nouns) within the system –Intangible objects generally represent calculations (verbs) within the system –It’s not important to specify whether an object is tangible or intangible –It is important to consider both types of object when identifying objects

CSC535 Object Identification How do you identify objects? –Nouns People Places Things –Things with multiple use –Things that store data or state –Things that are composites of other things –Things that are ancestors of other things –Things that perform a useful function

CSC535 Object to Object Relationships How do objects relate to one another? –Inheritance (“is-a” relationship) –Association/Composition (“has-a” relationship) –Application specific (neither of the above)

CSC535 UML Class Symbol Class In the early stages of the design you should not be overly concerned (or concerned at all?) with the attribute and method fields

CSC535 UML Relationship Symbols Inheritance (Generalization) (“is-a”)

CSC535 UML Relationship Symbols Association/Composition (“has-a”) Containing contains 1 Contained Class object An instance of Contained Class may belong to either Containing Class 1 or 2 but not both

UML Relationship Symbols Association/Composition (“has-a”) –There is also something called aggregation which is nothing more than association –Most use the association arrow and not the aggregation diamond

CSC535 UML Relationship Symbols Application Specific (association) –Use this when two objects are related but not by a “has a” or “is a” – you shouldn’t really need to do this but you might

CSC535 UML Relationship Symbols Dependency

CSC535 Class Diagram The result of identifying objects (classes) and their relationships will be a UML Class Diagram The Class Diagram represents the static (unchanging) nature of the system design –It represents compile-time relationships, not run- time usage We will revisit class diagrams later, adding more detail

CSC535 Object Identification Guidelines Eliminate all preconceived notions. Don’t strive for perfection…yet. Don’t get into heated debates over the inclusion or exclusion of a class…yet. If you do debate, realize the difference between a debate and an argument –Debate – good –Argument – bad Don’t get formal…yet. –Remember that UML can be used in “sketch”, “blueprint”, and “programming language” modes – concentrate on “sketch” right now Don’t toss out far fetched ideas too soon. Remember the difference between interface and implementation –Don’t think about the implementation right now. Think about code reuse. Keep in mind that this is an iterative process.

CSC535 Deliverables