Coming up: What is the design phase? Design (Ch 8 and Ch 12) Dan Fleck CS 421 George Mason...
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Transcript of Coming up: What is the design phase? Design (Ch 8 and Ch 12) Dan Fleck CS 421 George Mason...
Coming up: What is the design phase?Coming up: What is the design phase?
Design(Ch 8 and Ch 12)
Dan FleckCS 421
George Mason University
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
Oversimplification
Analysis
ClassesAttributesOperationsRelationshipsBehavior
Design
ObjectsData StructsAlgorithmsMessagingControl
Coming up: The Design Spec
The Design SpecArchitecture Design -
• 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 create a design spec?
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 Responsibility
Example
Coming up: Example: Paperboy and the Wallet
StudentnameaddressgradesfileToSavecalculate GPAstoreStudent
Why might this class definition change?
Why might this class definition change?
student namestudent name
student addressstudent address
gradesgrades
which file we store the information inwhich file we store the information in
Example: Paperboy and the Wallet
CustomergetFirstName()getLastName()getWallet()
WalletaddMoney(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
} Coming up: Principle of Least Knowledge (aka Law of Demeter)
What is wrong with this? What is wrong with this?
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 object passed
into the method– M may call methods of any object it
creates– M can call methods on any object
contained in O
Purpose: Reduce CouplingComing up: Principle of Least Knowledge (aka Law of Demeter)
But
not
take
them
ap
art
But
not
take
them
ap
art
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 CouplingComing up: Example: Paperboy and the Wallet
Example: Paperboy and the Wallet
CustomergetFirstName()getLastName()getWallet()
WalletaddMoney(int a)subtractMoney(int
a)countMoney()
Bad because the paperboy needs to know about the Wallet (violation of principle of least knowledge), and also the customer has to hand the wallet to the paperboy (unrealistic)
Bad because the paperboy needs to know about the Wallet (violation of principle of least knowledge), and also the customer has to hand the wallet to the paperboy (unrealistic)
Coming up: Example: Paperboy and the Wallet
What is wrong with this? What is wrong with this?
Example: Paperboy and the WalletCustomer
getFirstName()getLastName()getPayment(int
amt)
WalletaddMoney(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!
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/paper-boy/demeter.pdf
Coming up: Interface Segregation Principle
Make
Interface Segregation Principle
• Don’t make large multipurpose interfaces – instead use several small focused ones.
• Don’t make clients depend on interfaces they don’t use.
• Class should depend on each other through the smallest possible interface.
• Why? When I change something I want to minimize changes for everyone else.
Coming up: Interface Segregation Principle
Interface Segregation Principle
public interface Worker {public void eat();
public void work();}
public class OfficeWorker implements Worker{public void work() {// ....working}public void eat() {// .... eating in lunch break}
}
Coming up: Interface Segregation Principle – Fixed!
How to add a robot?
public class RobotWorker implements Worker {
public void work() { // Do work }
public void eat() {throw new NotImplementedException();
}}
public class RobotWorker implements Worker {
public void work() { // Do work }
public void eat() {throw new NotImplementedException();
}}
Interface Segregation Principle – Fixed!
public interface Worker {public void work();
}
public interface Eater{public void eat();
}
public class OfficeWorker implements Worker, Eater{ … }
public class RobotWorker implements Worker{ …}
Coming up: Remove Cyclic Dependencies
Now each interface has one purpose
Now each interface has one purpose
Remove Cyclic Dependencies
• Do not have cyclic dependencies in your packages
• Decomposition into independent modules
• Why?GUI
Logic
UserLogic BusinessLogic
ErrorHandlingComing 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.
Coming up: Factory (GoF95)
ClientClient FactoryFactory ProductProductUsesUses Creates
Creates
Factory (GoF95)
• Example:– http://www.devdaily.com/java/java-
factory-pattern-example
Factory (GoF95)Encryption
encryptOutdecryptIn
DESEncryption RSAEncryption
Socket
EncryptedSocket
instance:IEncryptFactorycipher: Encryption
<<interface>>IEncryptFactory
CreateEncryption(Key): Encryption
RequestsCreation
EncryptionFactory
CreateEncryption(Key): EncryptionCreates
Encrypts/Decrypts with
Coming up: Factory (GoF95)
Client
Product
Factory
Factory (GoF95)Encryption
encryptOutdecryptIn
DESEncryption RSAEncryption
Socket
EncryptedSocket
instance:IEncryptFactorycipher: Encryption
<<interface>>IEncryptFactory
CreateEncryption(Key): Encryption
RequestsCreation
EncryptionFactory
CreateEncryption(Key): EncryptionCreates
Encrypts/Decrypts with
Coming up: Command (GoF95)
How do we add another encryption method?
How do we add another encryption method?
Client
Product
Factory
Concrete Command
Command (GoF95)• Encapsulate commands in objects, so
we can queue them, undo them or make macros.
• http://twit88.com/blog/2008/01/26/design-pattern-in-java-101-command-pattern-behavioral-pattern/Abstract Command
+doIt():bool+undoIt():bool MacroCommand
+doIt():bool+undoIt():bool +doIt():bool
+undoIt():bool
- data
*+ manager:CmdMgr
Coming up: Design Patterns Summary
Flyweight (GoF95)• I have a bunch of classes, each takes up a lot
of memory, so I need to minimize the number of them I am using.
• Instances of the objects contain the same information and can be used interchangeably
• Avoid the expense of multiple instances.
• Example: DocChar class used to hold characters in a line of text. Picture is stored once, location is stored for every instance.
Coming up: Visitor (GoF95)
Visitor (GoF95)
• If you need to perform an operation in multiple objects in a complex structure you could create the logic in each class.
• OR…the visitor pattern creates a single class that implements the logic and knows how to “visit” each object in your complex structure
Coming up: Visitor (GoF95)
Visitor (GoF95)• I need to apply different operations
to a collection of objects. • I want to centralize these operations • I want to reduce coupling • For example in a word processor,
grammar check, spell check, table of contents builder, outliner all need to traverse the document.– Solution: Create a Visitor to visit the
whole document and apply the operation given
Coming up: Visitor Traversal Example
Visitor Traversal Example
• root.accept(theVisitor);• accept(Visitor visitor )
visitor.visit( this ); // Do operation for each child of mine
child.accept( visitor ) // Visit children
• All children are visited, but the caller doesn’t need to know “how”
• Supports multiple class types also…
Coming up: Visitor Traversal Example
Visitor Traversal Example
• visit(Document node); • visit(Sentence node);• visit(Word w);
• The correct version is called based on the runtime type of the child! (Hello Polymorphism!)
Coming up: Visitor Diagram
Visitor Diagram
Object withStructure
Individual ElementsVisitor
navigates
concretevisitor
concretevisitor
Coming up: Visitor Example – Different operation to collection of objects
Spell CheckBold
Visitor Example – Different operation to collection of
objectspublic interface Visitor{
// Go through all datapublic void visitCollection(Collection collection);
// Handle specific types public void visitString(String string);
public void visitFloat(Float float);}
public interface Visitable{ public void accept(Visitor visitor);
}
Coming up: Visitor Example – Different operation to collection of objects
Visitor Example – Different operation to collection of objects
public class VisitableString implements Visitable { private String value; public VisitableString(String string) {
value = string; }
public void accept(Visitor visitor) { visitor.visitString(this); // Call correct method for this
object }}
// Do the same for other types (Float, etc…)
Coming up: What should you know
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 usually need to be modified for YOUR situation.
Coming up: What makes a design “bad”
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 MetricsFrom: http://www.objectmentor.com/resources/articles/dip.pdfFrom: http://www.objectmentor.com/resources/articles/dip.pdf
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: Question
Design Summary
• The design phase is when you plan HOW you implement your analysis
• Use – Design Principles– Design Patterns– Design Metrics
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
• http://www.oodesign.com/interface-segregation-principle.html
Coming up: Dependency Inversion Principle
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 - what it means!
• Be able to describe patterns - singleton, factory, command
End of presentation