Design

1. Design Dan Fleck CS 421 George Mason University Coming up: What is the design phase?

2. 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

3. Analysis --> Design Coming up: Analysis --> Design

4. Analysis --> Design Coming up: Analysis --> Design

5. Analysis --> Design Coming up: Analysis --> Design

6. Analysis --> Design Coming up: Oversimplification

7. Oversimplification Analysis Classes Attributes Operations Relationships Behavior Design Objects Data Structures Algorithms Messaging Control Coming up: The Design Spec

8. The Design Spec Architecture 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 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

9. 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

10. The goal of design is to think with your brain, not your hands! - Dan Fleck Coming up: Applied Design

11. 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

12. 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

13. 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: Example: Paperboy and the Wallet

14. Example: Paperboy and the Wallet Customer getFirstName() getLastName() getWallet() 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 } Wallet addMoney(int a) subtractMoney(int a) countMoney() What is wrong with this? Coming up: Principle of Least Knowledge (aka Law of Demeter)

15. 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)

16. 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

17. Example: Paperboy and the Wallet Customer 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

18. Example: Paperboy and the Wallet Customer getFirstName() getLastName() getPayment(int amt) 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 } Wallet addMoney(int a) subtractMoney(int a) countMoney() 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: Dependency Inversion Principle

19. Dependency Inversion Principle • Depend on abstractions, not concretions • Program to interfaces not implementations • Program to most abstract class possible • Why? Concrete classes may change a lot. Abstract classes/Interfaces generally change very little. • How can we ensure interfaces change very little? See next slide! • Why? Using an interface allows code flexability to changes without affecting other parts. Coming up: Interface Segregation Principle

20. 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: Remove Cyclic Dependencies

21. Remove Cyclic Dependencies • Do not have cyclic dependencies in your packages • Decomposition into independent modules • Why? GUI Logic BusinessLogic UserLogic ErrorHandling Coming up: Design Patterns

22. 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

23. 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)

24. 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)

25. 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 Uses Factory Creates Product Coming up: Factory (GoF95)

26. <<interface>> IEncryptFactory CreateEncryption(Key): Encryption Socket Factory (GoF95) EncryptedSocket Encryption instance:IEncryptFactorycipher: Encryption Encrypts/Decrypts with encryptOut decryptIn EncryptionFactory RSAEncryption Requests Creation DESEncryption CreateEncryption(Key): Encryption Creates How do we add another encryption method? Coming up: Command (GoF95)

27. Abstract Command MacroCommand Command (GoF95) • Encapsulate commands in objects, so we can queue them, undo them or make macros. + manager:CmdMgr * Concrete Command +doIt():bool +undoIt():bool - data +doIt():bool +undoIt():bool +doIt():bool +undoIt():bool Coming up: Flyweight (GoF95)

28. 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)

29. 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)

30. 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

31. 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

32. 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 Example – Different operation to collection of objects

33. Visitor Diagram Object with Structure Bold Spell Check concrete visitor concrete visitor navigates Individual Elements Visitor Coming up: Design Patterns Summary

34. 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. From: http://www.objectmentor.com/resources/articles/dip.pdf

35. 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

36. 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

37. 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

38. 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. • 3 rules of Dependency Inversion Principle • 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

39. 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