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Week 3

• Lab 1 due now

• Finding Classes in your System

• CRC Cards

• Collaboration Diagrams

• Lab 2

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%

Need Meeting Space?

• If you are having trouble finding a quiet place for group work, the library offers study rooms you can book for up to two hours…

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Imagine Cup

• Student programmer competition sponsored by Microsoft

• Very generous prizes

• Great opportunity to meet industry professionals and experience reality of software development

• http://imaginecup.com/

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What Classes does the system have?• Finding the classes in your system is the first

step in analysis and design• These classes will encapsulate your business

logic and will live in the business layer(s)• Some people think of it as looking at all the

“stuff” and sorting it into classes• Others look at it as making up classes and

then thinking of examples of them

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Techniques for finding classes

• Thinking about classifications

• Looking through use cases for nouns

• Considering common patterns

• Class responsibilities collaboration (CRC) analysis

• Collaboration diagram

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Grady Booch says…

... Intelligent classification is intellectually hard work, and it best comes about through an incremental and iterative process

... There is no such thing as the perfect class structure, nor the right set of objects. As in any engineering discipline, our design choice is compromisingly shaped by many competing factors.

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Classification Theory

• Classification is the process of checking to see if an object belongs to a category or a class

• Most humans find classification irresistible

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Objects and Classes

• A class is a specification of structure, behavior, and the description of an object. – An object is an instance of a class

• Classification is more concerned with identifying classes than identifying the individual objects in a system.

• Intelligent classification is intellectually hard work and may seem rather arbitrary.

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Use-case Driven Approach

• Describe the system

• Look at the words in the description or steps of each scenario

• Classes and objects needed for the scenario will present themselves to you

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Use case example - Banking

Customer Withdraws Money

A. Client provides Teller amount of withdrawal and account

B. Teller checks balance in account and status of account

C. Teller reduces balance of account by withdrawal amount

D. Teller gives money to customer

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Banking use case cont.

At B, if balance is lower than amount to be withdrawn, teller refuses transaction. Customer may return to A with a lower amount, or use case may end

At B, if account is frozen, teller refuses transaction. Use case ends.

At D, if customer leaves without money, teller reverses transaction and returns money to drawer

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Bank Nouns

Client

Teller

Amount

Account

Balance

Status

Withdrawal

Money

Transaction

Customer

Hold

Drawer

Implied Data• Look for verbs in the use case that imply storage

and retrieval of data– The clerk looks up the reservation– The receptionist records the appointment on the

calendar

• The nouns in these sentences are almost certainly classes

• This is the information that passes between use cases

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Syntactic Lexical Analysis• Read use cases, interviews, and requirements,

looking for nouns and noun phrases– Mark verbs too; you’ll need them later

• Make a list– Change all plurals to singular – Drop irrelevant nouns– Combine synonyms– Establish if a noun is a class or an attribute

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Drop Irrelevant Classes

• Scrap candidate classes that have nothing to do with the system under study– Out of scope– Represent an actor

• But be aware of info about an actor

– Nouns not related to the system

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Redundant Classes• Do not keep two classes that express the

same information.

• If more than one word is being used to describe the same idea, select the one that is the most meaningful in the context of the system.– Customer / Client / Member / Purchaser– Reservation / Booking / Prearranged Stay

• Remember to use the language of the user

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Adjective Classes• Adult Membership, Youth Membership

• Checking Account, Savings Account

• Does the adjective change the behaviour of the object represented by the noun?– If the use of the adjective signals that the behaviour of the object

is different, then make a new class.

– If not, keep a single class and add the type as an attribute

• It is difficult for an object to change its class (you make a whole new object and copy values) but it’s easy to change an attribute– Can you convert a savings account to a checking account? If not,

it’s more likely they are different classes

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Attribute or Class?

• Candidate classes which are used only as values should be defined or restated as attributes and not as a class.

• Example: age, salary, description.

• If it doesn't do anything, just holds information, it's usually not a class

The Power of Words

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Statement of Purpose• Each class must have a purpose, must do

something, not just hold information • You must be able to formulate a statement of

purpose for each candidate class– The Account class holds and manages information

about a customer's money

– The EmployeeInformation class holds salary and contact information and performs payroll calculations

• No statement of purpose? Not a class

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Common Class Patterns

• Tangible Things and Devices

• Concepts

• Events

• People

• Places

• Organization

• Communicating with Actors

• Information about Actors

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CRC Cards

• CRC stands for Class, Responsibilities and Collaborators developed by Cunningham, Wilkerson and Beck.

• CRC analysis helps you to – identify classes you need– Work out which responsibilities each class will

have– Test your analysis

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CRC Process

Iterate

Identify Classes responsibility

Assign responsibility

Identify Collaboration

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Collaborations• An object can accomplish either a certain

responsibility itself, or it may require the assistance of other objects. – Does it know all it needs to know to do it?

• If it requires the assistance of other objects, it must collaborate with those objects to fulfill its responsibility.

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Collaborations

• Invoice asks customer to print itself

• Invoice asks invoice items to print themselves

• Schedule asks task for its effort

• Schedule sets task’s start date

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CRC Cards• CRC cards are 4" x 6" index cards. All the

information for an object is written on a card.

ClassName

Collaborators...

...Responsibilities

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Manages balance

Processes withdrawal

Processes deposit

Transaction log

Transaction log

Savings Account

Adds Transaction

Deletes Transaction

Knows list of Transactions

Transaction Log

One way

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Calculates total

Prints paper invoice

Updates inventory

Customer, Line Item

Inventory, Line Item

InvoiceLine Item

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How do you create them?

• Start with one or two obvious cards. • If the situation calls for a responsibility not already covered

by one of the classes: – Add a responsibility to a class, or– Create a new class to address that responsibility.

• Information held is a responsibility: “knows” or “manages”• Don’t create an exhaustive list of attributes

– Just say “manages contact info” for now

• Keep going until you can cover all the behaviour you listed in the use cases

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Collaboration diagram

• One box for each class, all on one diagram

• One line between boxes for each collaboration

• Arrow shows direction

Savings account

Transaction log

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Two kinds of collaboration diagram

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Overview collaboration diagram

Testing your CRC Cards

• Go through the use cases• Map sentences in the use cases to responsibilities

on your cards– “Clerk records start and end date”

• Yes, we have a class responsible for keeping that

– “Clerk calculates invoice total”• Yes, we have a class responsible for calculating that

– etc

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Naming Classes

• The class should describe a single object, so it should be the singular form of the noun.– If you think you need Employee and Employees, try

Employee and ListOfEmployees for now

• Use names that the users are comfortable with. • The name of a class should reflect its intrinsic

nature.– Not a CS concept like hash table, heap, stack– Queue and list are ok in their informal meanings

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Summary• Finding classes is not easy. You get better

with practice.• There is no such thing as one “right set of

classes.”• Finding classes is an incremental and

iterative process.• Filling out the contents of proposed

classes helps you to find more, orto gain confidence in the ones you have.

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Summary

• Unless you are starting with a lot of domain knowledge, you are probably missing more classes than you will eliminate.

• Naming a class is important.

• The class should describe a single object, so it should be a singular noun or an adjective and a noun.

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Lab 2

• You are given a list of nouns• Reject those that are not classes• Add more if you feel some are missing• Write a statement of purpose for all you keep• Make a CRC card for all you keep• Make an overview collaboration diagram that

summarizes all the collaborations at once

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Next Time

• Read up to where we are

• Lab 2 will be due at the start of class– I will try to have Lab 1 marked for you

• If you want to read ahead, next lecture we start class diagrams