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Design. Dan Fleck CS 421 George Mason University. What is the design phase?. Analysis phase describes what the system should do

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Dan Fleck

CS 421

George Mason University

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.









Data Structures




the design spec
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.
the design spec1
The Design Spec

But really, how do I do it?

Find examples and use what you think is helpful from them!

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?

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?
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
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
    • Any object contained in O

Purpose: Reduce Coupling

principle of least knowledge aka law of demeter1
Principle of Least Knowledge (aka Law of Demeter)


  • 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

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!
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.
remove cyclic dependencies
Remove Cyclic Dependencies
  • Do not have cyclic dependencies in your packages
  • Decomposition into independent modules
  • Why?






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
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.

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;



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.
factory gof951



CreateEncryption(Key): Encryption


Factory (GoF95)



instance:IEncryptFactorycipher: Encryption

Encrypts/Decrypts with








CreateEncryption(Key): Encryption


command gof95

Abstract Command


Command (GoF95)
  • Encapsulate commands in objects, so we can queue them, undo them or make macros.

+ manager:CmdMgr


Concrete Command



- data





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
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
visitor gof951
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.
visitor diagram
Visitor Diagram

Object with










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
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
design summary
Design Summary
  • The design phase is when you plan HOW you implement your analysis
  • Use
    • Design Principles
    • Design Patterns
    • Design Metrics
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
  • Luc Berthouze, University of Sussex,
  • Robert Martin, Principles and Patterns,
  • Bob Waters, Georgia Tech, CS2340 Slides,