Workshop for CS-AP Teachers
Chapter 4
Object-Oriented Design
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Learning Goals
• Understand the basics of object-oriented
design
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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
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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
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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 Example
Name: Placing an Order
Actors: customer (initiates), salesperson, Frequency: common,
Priority: Essential
1. 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 number
3. The salesperson verifies the customer name and catalog
name and asks for the first item
4. For each item the customer gives the item number
5. For each item the salesperson gives the item name, price and
asks the customer for the options and quantity
6. For each item the customer gives the quantity and the
salesperson reserves this quantity for the customer
7. When there are no more items in the order the salesperson
gives the shipping choices, length of time, and cost
8. The customer picks a shipping method
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Detailed Use Case Example
9.
10.
11.
12.
13.
14.
15.
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.
The salesperson gives the order confirmation number (order
number) and tells the customer when to expect the order.
The order status is set to pending and after the credit card
approval occurs it changes to approved
The shipping clerk uses the Waiting to be Shipped page to see
the items needed for the order.
The shipping clerk uses the Item Locator page to find where
the items are in the warehouse.
The shipping clerk prints off an invoice, mailing label, and
packs the order. The credit card is charged for the order.
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 Example
Use Case: Withdraw from ATM
Actors: Customer, ATM (Customer Initiates)
Frequency: Common
Priority: Essential
Description:
1. Customer inserts card.
3. Customer types in PIN.
2. System prompts for PIN.
4. System validates the PIN.
And so on
And so on
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Vending Machine Exercise
Objective:
Duration:
Exercise:
Practice Object-Oriented design
30 minutes
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 Case
Name: Get item from machine
Actors(s): Customer (initiates)
Frequency: Common
Priority: Essential
Description:
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 dispense
6. 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 item
10. The item holder tells the money holder to dispense the change
11. 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
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