Dan Fleck CS 421 George Mason University 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

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

The Design Spec Architecture Design - Component design - UI 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

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 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 Design Patterns Design Metrics 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: Example: Paperboy and the Wallet

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)

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

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 doesn’t model the real world and lets paperboy have WAY to much access! addMoney(int a) subtractMoney(int a) countMoney() What is wrong with this? Coming up: Example: Paperboy and the Wallet

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

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! Answer: Add interfaces instead of modifying them. Coming up: Interface Segregation Principle

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

Remove Cyclic Dependencies Do not have cyclic dependencies in your packages Decomposition into independent modules Why? GUI - Makes testing a much bigger problem 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 Think of a multi-document application framework. An application object may know when an object needs to be created, but not which object. How do we create the correct object when needed? 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)

Factory (GoF95) Socket EncryptedSocket Encryption instance:IEncryptFactory cipher: Encryption encryptOut decryptIn Encrypts/Decrypts with EncryptionFactory DESEncryption RSAEncryption Requests Creation Creates CreateEncryption(Key): Encryption <<interface>> IEncryptFactory CreateEncryption(Key): Encryption Coming up: Command (GoF95)

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

Flyweight (GoF95) I have a bunch of classes, but I need to minimize the number of objects 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 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. Coming up: Visitor Diagram

Visitor Diagram Object with Structure concrete concrete visitor navigates Individual Elements Visitor Coming up: Design Patterns Summary

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. 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

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/ End of presentation