Kate Gregory

Week 5

• Static Object Model– Attributes– Methods – Associations– Visibility

• Lab 3 available– Rating your group

See Me

• Send me email if you did not hand in Lab 1 or 2– Include your full name– Tell me what group you have joined

• Hands up now– if you are not in a group– if you are in a group of 1 or 2

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Date Week Topic Hand Out Due Back Test

6-Sep-13 1 Administrivia / Overview / Motivation, benefits of OO

13-Sep-13 2 Use Cases Lab 1: Use cases

20-Sep-13 3 CRC Cards, collab graphs Lab 2: CRC cards lab 1 5%

27-Sep-134 start class diag lab 2 5%

4-Oct-13 5 Finish class diag, Associations Lab 3: Class Diag    

11-Oct-13 6 Inh & Polymorphism / midterm review lab 3 5%

18-Oct-13 7 midterm Midterm 25%

25-Oct-13 Reading Break

1-Nov-13 8 Interaction diag / Design Patterns Lab 4: Interaction Diag    

8-Nov-13 9 Good Design / Modules & Packages / Deployment and component diagrams /Metrics / SOLID

Lab 5: Critiques lab 4 5%

15-Nov-13 10 State diagrams / Activity diagrams / Summary and Conclusion / The Future

22-Nov-13 11 Critiques critique lab (before class) 15%

29-Nov-13 12 Critiques

TBD Final Exam Final 40%

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Current car: Car

Objects are not Attributes

• On a class diagram, do not list attributes that are objects.

• Every person has a car, but not:

Person

Date of birth: Date

travel ()

Current car: Car

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Associations

• A reference from one class to another is an association.

• Most relationships between classes are associations.– A person has a car.– An employee reports to a supervisor– An invoice is filled with items

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Objects are not Attributes

Person

Date of birth: Date

travel ()

Car

Mileage: inthas

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Guidelines For Identifying Associations

• Association often appears as a verb in a problem statement and represents relationships between classes. – A pilot flies a plane.– A manager approves a transaction

• Other phrases include part of, next to, works for, contained in, etc.

• Verbs help to qualify an association, links for passing messages

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Common Association Patterns

• Location Association: next To, part of, contained in, ingredient of etc. – each employee is part of a department.

• Communication association: talk to, order to. – a customer places an order with an operator.

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Binary Association Notation• On a class diagram, binary association is a line

connecting two classes• Both ends may be connected to the same class

Company PersonworksFor

employer employee

Person

marriedTo

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Links to pass Messages

Listing

Description

MLS

Agent

Phone No

writeInfo() writeInfo()

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Multiplicity• Multiplicity specifies the number of instances of associated

classes for each instance of this class. • It is given for roles within associations, parts within

compositions, repetitions, and other purposes. • A number or range of numbers is written on each end of the

line– lower bound .. upper bound. (0..1, 2..5 etc)– Single number (0, 1, etc)– * to indicate any number including 0– 1..* to indicate 1 and up

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Single Multiplicity

• Exactly one province for each city.

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Unlimited Multiplicity

• Sometimes there is no limit on the number of objects in the association.

• Any number of cities in a province

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Fixed Multiplicity

• Sometimes a object must be associated with a fixed number of other objects

• obvious or business rules state limited number (2 teams, 4 agents)

Car W heels4

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Range of Multiplicity

• The most common range is 0 to 1: a single optional value

• The person either has an employer or does not: in this model no-one has more than one employer

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Range of Multiplicity

• Another popular range is 1 or more, represented as 1..*

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Range of Multiplicity

• Other than 0 to 1 and 1 or more, ranges are unusual. – A car has 2 to 5 doors… OK, but is “car door” likely to

be an object?– A person has 0 to 5 credit cards… why can’t someone

have 6?

• Push back when customers tell you ranges for relationships; make sure there’s a real business reason for the restriction and not some leftover from legacy software that couldn’t model reality

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Qualified Associations

• Sometimes multiplicity alone can’t describe the restrictions on a relationship– How many bank accounts does a customer have

with each bank?– Can two of a customer’s accounts have the

same number?• As long as they’re with different banks

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Qualifier

Bank

Person

account#

*

.

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OR Association• An OR association indicates a situation in which

only one of several potential associations may be in effect at one time for any single object.

Person

Company

Car {or}

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Some Attributes Are Hard To Place

• Consider Company and Person– Person works for company– Where does the salary attribute go?

• Company?– Everyone makes the same salary

• Person?– All my jobs pay the same

• Salary is an attribute of the relationship

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Association Class• An association class holds properties of the

association

• An association class is shown as a class symbol attached by a dashed line to an association path.

WorksFor

Personemployer employee

Company

salary

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Association Class• Association class is like a normal class with

attributes and methods

• Adds extra information to the link

House for Sale Agent

ContractExpirydate

commission

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Eliminate Unnecessary Associations

• Derived associations can be defined in terms of other associations. They are redundant; avoid.

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Eliminate Unnecessary Associations

• Implementation association. Defer implementation-specific associations to the design phase. – For example, if inventory is implemented by using an Array

class, don’t show that relationship. Only show business relationships as determined by your analysis

• Ternary associations. Ternary or n-ary association is an association among more than two classes. Avoid.– Replace with a new class

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N-Ary Association• Association among more than two classes. • Difficult to understand

GradeBook

StudentCourse

gradeexamlab

Year

semester

*

*

*

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Convert N-Ary Associations• Create a class to represent the relationship

GradeBook

StudentCourse

labgradeexam

Year

Session

*

*

* *

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Association Class Replaces N-Ary

House for Sale AgentContractContractExpirydate

commision

Client

**

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• Aggregation is a form of association representing a “part of” relationship. For example, players are part of a team.

• A hollow diamond is attached to the end of the path to indicate aggregation.

• “there is no single accepted definition of the differences between aggregation and association”, Fowler, UML Distilled, 1997

Aggregation

P la y e rc la ss

T e a m1 *c o n sis tO f

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Composition• Composition is a form of aggregation with strong

ownership to represent a component of a complex object. • The “part” may belong to only one whole.

– Compare a picture pasted into a Word document (composition) to a picture on a web page (aggregation)

• Cascading Destruction – can the part exist without the whole ?– A customer has a credit card. – A customer has a sales rep. – A window on the screen has a title, a scroll bar, menus, frame and

so on.

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Composition Notation

• The UML notation for composition is a solid diamond at the “container” end of a path.

Window

Title Scroll bar Menu Frame

0..21

1 *

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Composition

House for Sale picture

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Classes without Associations

• Sometimes a class is sitting all alone on the diagram• Check the CRC cards

– Collaborations generally can’t happen if objects can’t find each other– Those connections are associations– Sometimes connections are temporary, so don’t create an association– Think about the relationships between the real things these classes

represent• Look for words like “has”, “uses”, “owns”, “contains”• A class without associations does not always need to be

dropped– Another layer (e.g. user interface) may use it– Ask yourself if it’s really something in your system

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Cohesion and Coupling• Only certain classes need to know about other classes

-- this is coupling. The lower the coupling (number of connecting lines between classes) the better.

• Certain information should reside in the same location (class) – this is cohesion and makes classes self-sufficient. The higher cohesion, the better

• More on these in our metrics section later. Start to think about whether the classes you are shown exhibit high or low cohesion, and are tightly or loosely coupled.

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Remember what class diagrams are for

• Reference– Names of classes– Names of methods– Parameters to methods– Names of attributes

• Excellent summary of decisions

• Visual understanding of the system

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Next Week

• Inheritance

• Lab 3 Due– Think about concurrency issues– Rate your group members

• Read ahead if you like