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Workshop for CS-AP Teachers

Chapter 4

Object-Oriented Design

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Learning Goals

• Understand the basics of object-oriented design– Purpose of Design– Goals of Object-Oriented Design– Principles of Good Object-Oriented Design– How to transition from analysis to design– The design process

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Object-Oriented Design• The purpose of design is to build an abstract,

high-level, description of HOW the system will work

• In Structured Design the emphasis is on:– Top-down design– Successive refinement of functions

• In OO Design the emphasis is on:– Refinement of the classes and the relationships

between them– Mapping classes to the implementation

environment

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Goals of Object-Oriented Design• Decrease coupling: the

degree to which two components depend on each other’s implementations (minimize the effect of changes)

• Increase cohesion: the degree to which the responsibilities of a class are related (maximize the ability to combine objects)

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Good Object-Oriented Design• Is based on real world artifacts

– That makes the design reusable

• Information access is enough – Objects don't need information that they can't get to. – Objects can get to the information they need

• directly or indirectly

• Responsibility, control, and communication is distributed – No one object does everything

• Attributes and operations are factored out as high in the class hierarchy as possible– Using inheritance

• There should be little or no redundancy– Code should appear only once

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Good Design Exercise

• Double click on battle.html and run the battleship game

• Open the class battle.java in the folder exercises/battleship– What do you think of this design?– How easy or hard would it be to change this

game?• adding 2 players• adding more ships• changing the size of the game board

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Getting Started on Design

• Design can be partitioned into– Internal Design:

• Converting conceptual models into specification models • Refinement of classes and the relationships between

classes

– Interface Design: Define the external interface to• Users • Persistent data• Other programs

• OOD does not usually require– Pseudo-code for class operations– Class designs to be “frozen” before implementation

commences

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Design Process

• Review the analysis class diagram– Check that the names are correct and

understandable

• Create detailed use cases– Include user interface details and interface

prototypes

• Modify the class diagram to add attributes, operations, and new classes – Found in the detailed use cases– Decide which class will track an association

• One direction or bi-directional

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Review the Class Diagram

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Example Class Diagram

• This model is just one feasible static object model• At design time, we can refine the model

– Is a Deck the same object as the Stock pile?

– How do we handle moving piles of cards?

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Review the Class Diagram

• Review classes and remove– vague classes

• system

– redundant classes• a class should appear only once in the diagram

– irrelevant classes • outside the domain

• Review the class names• Review attributes• Review operations• Review the relationships between classes

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Review the Class Names

• Do the names reflect the domain?• Does the name reflect the purpose of the

class?– Is a connector between shapes a line or a

connector?

• Are the names pronounceable?• Does each word in a class name start with a

capital letter: ShapeCanvas?• Are the names clear and unambiguous?• Are the names singular?

– Course not Courses

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Review Attributes• Add types to attributes

– Using types in the chosen language (if needed)

• Some attributes need to be expanded– A person’s name

• Some attributes should be classes– An address

• Some attributes can be grouped into a class– A date range

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Review Operations

• Check the names of the operations– Are they easy to read and clear?

• Add parameters and return types• Check that the operations are on the correct

class– The class you expect would be doing the action

• Check if several classes have similar operations that can be put in a parent class– Even if the names of the operations are different

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Review Inheritance• Use inheritance only

when the child class is a “type of” the parent class– Use association if this

isn’t true

• Modify multiple inheritance to use interface realization– if you are using a

language that doesn’t support multiple inheritance

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Add Classes to Clarify Relations

• Doctor, Nurse, and Patient are all “a kind of” Person

• But Doctor and Nurse are both Medical Personnel too– Introduce a new class Medical Person

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Delegation instead of Inheritance• Use delegation to

simply use (buy) another class– An object handles a

message by sending messages to other objects

• User gets the person name from person

– Allows an object to invoke other objects’ operations without adding the overhead of inherited operations

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Review Association• Use association when

an object of a class “has a” object of another class– A catalog has items

• Add navigability to show which side of an association cares about the relationship– A catalog will track the

items in it but the items won’t care what catalog they are in

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Create Detailed Use Cases

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Create Detailed Use Cases

• Expand the high-level use cases from analysis– Go into the details including the user interface

• Elicit rare or minor use cases– People remember the “standard” things.

• Consider future use cases– What things might we want the system to do in the

near future?

• Assign use cases to iterations– When will you implement the use case?

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A Detailed Use Case

• Defines how as well as what– Names the user interface elements– Names parts of the system– Go into the details of how

• Describes the success scenario– Sequence of steps when everything goes well

• Describes alternative scenarios as well– Times when things go wrong or– Times when things are handled differently

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Scenarios versus Use Cases

• A scenario is a sequence of interactions between a user and a system– About a user goal – Example for the goal of making a bulleted list:

• User selects some text and then clicks on the icon to make the text a bulleted list. The system modifies the text to make it a bulleted list.

• A use case is a set of scenarios that have the same user goal– Includes the success scenario “happy day” scenario– Includes failures and alternative success scenarios

as well

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Detailed Use Case Format

• There is no standard format• Two typical types

– Use case with alternative scenarios included in the body of the success scenario

• Indenting used to show alternatives

– Use case with uninterrupted success scenario• Alternatives are listed after the success scenario

• Numbering– Some use simple numbering 1, 2, 3, …– Others use decimal numbering 1, 1.1 1.2, 1.2.1,

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Detailed Use Case ExampleName: Placing an OrderActors: customer (initiates), salesperson, Frequency: common,

Priority: Essential1. A customer calls in an order. The salesperson brings up an

Order Entry page. The User Interface (UI) prompts the salesperson for the number on the catalog that identifies the catalog and customer.

2. The customer gives the number3. The salesperson verifies the customer name and catalog

name and asks for the first item4. For each item the customer gives the item number5. For each item the salesperson gives the item name, price and

asks the customer for the options and quantity6. For each item the customer gives the quantity and the

salesperson reserves this quantity for the customer7. When there are no more items in the order the salesperson

gives the shipping choices, length of time, and cost8. The customer picks a shipping method

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Detailed Use Case Example9. The salesperson gives the total cost of the order and gets or

verifies the credit card information. A check is made to make sure the credit card will allow the charge.

10. The salesperson gives the order confirmation number (order number) and tells the customer when to expect the order.

11. The order status is set to pending and after the credit card approval occurs it changes to approved

12. The shipping clerk uses the Waiting to be Shipped page to see the items needed for the order.

13. The shipping clerk uses the Item Locator page to find where the items are in the warehouse.

14. The shipping clerk prints off an invoice, mailing label, and packs the order. The credit card is charged for the order.

15. The shipping company picks up the order and delivers it.

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Use Case Alternatives• Alternative: New Customer

– At step 1 this may be a new customer so use the use case of creating a new customer

• Alternative: Expired Catalog– At step 1 this may be an expired catalog and if so

notify the user that the prices may be out of date

• Alternative: Backordered Item– At step 6 the item may indicate that it is on

backorder. If so the salesperson will notify the customer and ask if they want to continue

• Alternative: Cancel– At any point the customer can cancel the order

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Detailed Use Case ExampleUse Case: Withdraw from ATM

Actors: Customer, ATM (Customer Initiates)

Frequency: Common

Priority: Essential

Description:

1. Customer inserts card. 2. System prompts for PIN.3. Customer types in PIN. 4. System validates the PIN.

And so on And so on

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Vending Machine ExerciseObjective: Practice Object-Oriented design

Duration:30 minutes

Exercise:Modify a class diagram for the following requirements. Then write a detailed

use case describing getting an item from the vending machine.

Create the next generation vending machine that can tell when a part of it is broken and can place a service call so that a repairman can come and fix it.

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Vending Machine Class Diagram

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Revised Vending Machine

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Detailed Vending Use CaseName: Get item from machineActors(s): Customer (initiates)Frequency: CommonPriority: EssentialDescription:

1. The customer starts a transaction by putting money in the vending machine

2. The money holder validates the money entered and if valid adds it to the amount entered

3. The money holder updates the display to show the amount entered.4. The customer pushes an item selector (sends a select to the item

selector)5. The item selector tells the item holder to dispense6. The item holder gets the price from the item.7. The item holder tells the display to display the item price.8. The item holder checks if the money holder can make the required

change. 9. The item holder dispenses the item10. The item holder tells the money holder to dispense the change11. The item holder tells item to decrement the quantity in stock 12. Item will decrement the quantity in stock and check if it is at the restock

level and if so send a message to the supplier

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Detailed Vending Use Case - Alts• Alternative – Validate money fails

– Step 2. If the validate money fails the money is returned to the user and the display tells the user

• Alternative – Can’t make change– Step 8. If the money holder can’t make change then the

display tells the user so the user can cancel the transaction

• Alternative – Can’t dispense item– Step 9. If the item isn’t dispensed the display is updated to

tell the user so the user can cancel the transaction

• Alternative – Can’t reach supplier– Step 12. If the supplier can’t be reached try again every few

hours and notify the vending machine company

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Revise the Class Diagram

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Adding Operations and Attributes

• Creating detailed use cases will likely uncover attributes and operations that were not thought of in analysis.

• Determining where the new attributes and operations should go depends on the class responsibilities.

• The assignment of responsibilities is a crucial skill in object-oriented design.

• Often new classes need to be created to refine the responsibilities.

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Responsibility Assignment

• Where to put an attribute?– Assign attributes to the class that it belongs to or

who you would expect to be able to ask about it.

• Where to put an operation?– Assign an operation to the class that has the data

needed for the operation or assign the operation to an intermediate class to reduce coupling (indirection).

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Responsibility Assignment

• Who should create an object of a class?– A should create B if A contains B.– A should create B if A has the initializing data for

B.

• Who should handle an event?– The object that takes action based on the event or

the container that holds the object that where the event occurred.

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Summary

• The purpose of design is to build a high-level description of how the system will work

• The goals of design are to reduce coupling and increase cohesion

• During design we should – Build a class diagram of the solution

• Add types and method parameters

– Create detailed use cases– Create user interface prototypes