Design
Dan Fleck
CS 421
George Mason University
Coming up: What is the design phase?
What is the design
phase?
• Analysis phase describes what the system
should do
• Analysis has provided a collection of classes
and descriptions of the scenarios that the
objects will be involved in. These functions
are clustered in groups with related
behavior.
• The design phase is to work out how the
system should do these things. This is the
goal of the design phase.
Coming up: Analysis --> Design
Analysis --> Design
Coming up: Analysis --> Design
Analysis --> Design
Coming up: Analysis --> Design
Analysis --> Design
Coming up: Analysis --> Design
Analysis --> Design
Coming up: Oversimplification
Architecture Design -
The Design Spec
• Layers of the software (e.g.model, view, controller (MVC))
• Categories of classes (e.g. UI, Business logic, interfaces)
Component design • Description of classes/methods/algorithms
• State machines for classes
• (Think: individual classes)
UI design
• sample screens
• UI guidelines/standards we’re using
• detailed description of how UI components work
Data design • database design
• data structures we’re using.
Coming up: The Design Spec
The Design Spec
But really, how do I do it?
Find examples and use what you think is helpful
from them!
http://www.mhhe.com/engcs/compsci/pressman/graphics/Pressman5sepa/common/cs2/design.pdf
http://www.cmcrossroads.com/bradapp/docs/sdd.html
Coming up: The Design Spec
The goal of design is to think with your
brain, not your hands! - Dan Fleck
Coming up: Applied Design
Applied Design
We know what to do now, but that is just
a set of documents..
How do we create a GOOD design?
Coming up: Good Design
Good Design
• Design Principles
– What should you try to do.
• Design Patterns
– How have people done it before you?
• Design Metrics
– How do you know you have done it well?
Coming up: Single Responsibility Principle
Single Responsibility Principle
• Each class should have a single
overriding responsibility (high
cohesion)
• Each class has only one reason for why
it should change
Coming up: Single Responsability Example
Single Responsability
Example
Student
name
address
grades
fileToSave
calculate GPA
storeStudent
Coming up: Example: Paperboy and the Wallet
Why might this class
definition change?
Customer
Example: Paperboy and
the Wallet
getFirstName()
getLastName()
getWallet()
Wallet
addMoney(int a)
subtractMoney(int a)
countMoney()
PaperBoy’s getPayment method:
payment = 2.00; // “I want my two dollars!”
Wallet theWallet = myCustomer.getWallet();
if (theWallet.getTotalMoney() > payment) {
theWallet.subtractMoney(payment);
} else {
// come back later and get my money
}
What is wrong with this?
Coming up: Principle of Least Knowledge (aka Law of Demeter)
Principle of Least Knowledge
(aka Law of Demeter)
• “Only talk to your immediate friends”
• Object O has a method M.
– M may call other methods in O
– M may call methods of any parameter passed
into the M method
– M may call methods of any object it creates
– M can call methods on any object contained
in O
Purpose: Reduce Coupling
Coming up: Principle of Least Knowledge (aka Law of Demeter)
Principle of Least Knowledge
(aka Law of Demeter)
Simplified:
• I can play by myself
• I can play with toys given to me
• I can play toys I made myself
• I can play with my own toys (but not
take them apart)
Purpose: Reduce Coupling
Coming up: Example: Paperboy and the Wallet
Customer
Example: Paperboy and
the Wallet
getFirstName()
getLastName()
getWallet()
Bad because the paperboy needs
to know about the Wallet, and
also the customer has to hand
the wallet to the paperboy
Wallet
addMoney(int a)
subtractMoney(int a)
countMoney()
What is wrong with this?
Coming up: Example: Paperboy and the Wallet
Customer
Example: Paperboy and
the Wallet
getFirstName()
getLastName()
getPayment(int amt)
Wallet
addMoney(int a)
subtractMoney(int a)
countMoney()
PaperBoy’s getPayment method:
payment = 2.00; // “I want my two dollars!”
int amt= myCustomer.getPayment(payment);
if (amt >= payment) {
// say thanks!
} else {
// come back later and get my money
}
Better – paperboy only accesses what
he needs and models the real world!
This example from: http://www.ccs.neu.edu/research/demeter/demeter-method/LawOfDemeter/paperboy/demeter.pdf
Coming up: Dependency Inversion Principle
Remove Cyclic Dependencies
• Do not have cyclic dependencies in
your packages
• Decomposition into independent
modules
GUI
• Why?
Logic
BusinessLogic
UserLogic
ErrorHandling
Coming up: Design Patterns
Design Patterns
• Proven solutions to common problems
• Capture design expertise
• Aid in meeting quality metrics
• Core patterns are
from the
“Gang of Four (GoF)”
OOPSLA - 1994
Coming up: Singleton Pattern
Singleton Pattern
• Problem: I want to limit the
application to only one instance of a
particular class, but need global access
to that class.
• Normally used to control access to key
resources.
• Solution?
override new, make static accessor method.
Coming up: Singleton Pattern (in Java)
Singleton Pattern (in Java)
public class MySingleton {
private static MySingleton instance;
private MySingleton() {
// do anything you need to do
}
public static MySingleton getInstance() {
if (instance == null) instance = new MySingleton();
return instance;
}
}
Coming up: Factory (GoF95)
Factory (GoF95)
• Define an interface for a group of objects
• Create a Factory to decide which specific object needs to be
instantiated
• The goal: decouple knowledge of the object instantiation from
the Class that needs the object.
• Can also be used when a complex initialization of objects is
necessary, for instance when aggregation is heavily used.
• Can also be used to take advantage of memory-optimization
like object pools, cached objects, etc.
Client
Coming up: Factory (GoF95)
Uses
Factory
Creates
Product
Socket
Factory (GoF95)
EncryptedSocket
Product
Encryption
encryptOut
decryptIn
Encrypts/Decrypts with
instance:IEncryptFactory
cipher: Encryption
Client
DESEncryption RSAEncryption
Creates
EncryptionFactory
CreateEncryption(Key): Encryption
Factory
<<interface>>
IEncryptFactory
CreateEncryption(Key): Encryption
Coming up: Factory (GoF95)
Requests
Creation
Socket
Factory (GoF95)
EncryptedSocket
Product
Encryption
encryptOut
decryptIn
Encrypts/Decrypts with
instance:IEncryptFactory
cipher: Encryption
Client
DESEncryption RSAEncryption
Creates
EncryptionFactory
CreateEncryption(Key): Encryption
Factory
How do we add
another encryption
method?
Coming up: Command (GoF95)
<<interface>>
IEncryptFactory
CreateEncryption(Key): Encryption
Requests
Creation
Command (GoF95)
• Encapsulate commands in objects, so we can
queue them, undo them or make macros.
Abstract Command
+ manager:CmdMgr
*
Concrete Command
+doIt():bool
+undoIt():bool
MacroCommand
- data
+doIt():bool
+undoIt():bool
Coming up: Flyweight (GoF95)
+doIt():bool
+undoIt():bool
What makes a design “bad”
• Rigidity: It is hard to change because every
change affects too many other parts of the
system.
• Fragility: When you make a change,
unexpected parts of the system break.
• Immobility: It is hard to reuse in another
application because it cannot be disentangled
from the current application.
Coming up: Design Patterns Summary
From: http://www.objectmentor.com/resources/articles/dip.pdf
Design Patterns Summary
• Many design patterns exist
• Implementations are usually available
in every language
• Use them as guides where appropriate
and make sure you understand the
tradeoffs for each one. They aren’t
always good for YOUR situation
Coming up: Design Metrics
Design Metrics
• Class Size
• Methods per class
• Lack of Cohesion (count of methods
with dissimilar purpose)
• Coupling Between Classes (count of
other classes that this class refers to)
• Depth of Inheritance Tree
• Method Complexity - tools can do this
Coming up: Design Summary
Design Summary
• The design phase is when you plan
HOW you implement your analysis
• Use
– Design Principles
– Design Patterns
– Design Metrics
Coming up: What should you know
What should you know
•
•
•
•
Analysis = what the system should do
Design = how it should do it
Meaning of the parts of the design spec
Design Principles:
– Single Responsibility Principle - write it
– Law of Demeter. Describe it and state why it is good.
– Why you need to remove cyclic dependencies
• Metrics
– Definition of cohesion and coupling - not how to calculate it, but
what it means!
• Be able to describe patterns - singleton, factory, command
Coming up: References
References
•
Luc Berthouze, University of Sussex,
http://www.informatics.sussex.ac.uk/users/lb203/se/SE08.pdf
•
Robert Martin, Principles and Patterns,
http://www.objectmentor.com/resources/articles/Principles_and_Patterns.pdf
•
Bob Waters, Georgia Tech, CS2340 Slides,
http://www.cc.gatech.edu/classes/AY2007/cs2340_summer/
•
http://www.surfscranton.com/architecture/VisitorPattern.htm
End of presentation