Introduction to Design Patterns · Software” book written by Gamma, Helm, Johnson, and Vlissides...

!" 2003 - 2007 DevelopIntelligence

Introduction to Design Patterns

What is a Pattern?

5

! " There are many definitions for a pattern:

! " A model or design used as a guide

! " A set of instructions to be followed when implementing something

! " An example for others to follow

! " A decorative design

! " Where do patterns exist?

! " In arts and crafts (think quilts)

! " In buildings and architecture

! " In manufacturing (think models and production)

! " In leadership (think of followers modeling a leader)

! " In software


History of Design Patterns

! " Concept of design patterns originated as an architectural

concept by Christopher Alexander in 1977

! " Introduced into the programming community in 1987 by Ward Cunningham and Kent Beck

! " Focused on apply design pattern concept to software

! " Applied the concept using the SmallTalk language

! " Released a paper at OOPSLA 87 called “Using pattern languages

for object-oriented programmers”

! " Gained popularity in the 90s

! " Result of “Design Patterns: Elements of Reusable Object-Oriented

Software” book written by Gamma, Helm, Johnson, and Vlissides

! " AKA: Gang of Four (GoF) book

! " Have grown in popularity over time


What is a Design Pattern?

! " Term comes from work performed by architect

Christopher Alexander

! " Used to describe architectural solutions to reoccurring problems

! " Specifically, "Each pattern describes a problem which occurs over

and over again in our environment, and then describes the core of

the solution to that problem, in such a way that you can use this

solution a million times over, without ever doing it the same way

twice”

! " Other definitions:

! " A identified and proven solution to a reoccurring problem in a

specific context

! " A proven recipe describing how to solve a problem

! " A blue-print of how to solve a problem


What is a Design Pattern? [Redux]

Let’s think in OO terms

! " A class is a blueprint describing an object

! " An object is an instance of that blueprint

! " A design pattern is a blueprint describing a design

! " An implementation is an instance of that design within

your software


Purpose of Design Patterns

! " Think about motivations for OO

! " Reusable components

! " Modular software

! " Flexible and extensible solutions

! " It’s hard to get that right


Purpose of Design Patterns [cont.]

! " Think about basic object oriented concepts

! " Abstraction – details are left ambiguous

! " Encapsulation – details are hidden

! " Inheritence – reusability through specialization; “is-a”

! " Polymorphism – many forms

! " Composition – collection of objects; “has-a”

! " High-cohesion – self-contained

! " Loose-coupling – flexible

! " It’s harder to get that right


Purpose of Design Patterns [Redux]

Specifically – design patterns solve design problems:

! " Find appropriate objects – help you identify less-obvious

abstractions and objects that capture them

! " Determine object granularity – patterns can help simplify

the process of composition

! " Specify object interfaces – help harden your interfaces,

what’s included and what’s not included in the interface

! " Describe object implementations – help you understand the implications of certain implementations

! " Encourage reusability – don’t have to think about best

approach

! " Support extensibility – change and flexibility built right in


Types of Design Patterns

! " Architectural patterns

! " Specify schema / structure of a software solution

! " Example: 3-tier architecture

! " General object oriented patterns

! " Typically technology agnostic; can be applied in any OO system

! " Focus on good OO design

! " Example: Model-View-Controller

! " Technology specific patterns

! " Typically idioms or blueprints specific to a language, technology,

or infrastructure

! " Focus on solving technology specific problems [possibly due to

underlying technology design problems

! " Example: Java EE Service Locator Pattern


Issues with Design Patterns

! " Patterns aren’t the panacea of good OO; they can be

misused

! " Over-engineering

! " Slow and cumbersome

! " Confusing to maintain

! " Antipatterns – “it seemed like a good idea at the time”

! " You many end up nailing in a screw

! " First introduced by Andrew Koenigh in 1995

! " Standard work is AntiPatterns by Brown, Malveau, McCormick,

and Mowbray


AntiPatterns

! " AntiPatterns are described in terms of the:

! " Cause – how did we get here

! " Symptoms – how did we recognized there’s a problem

! " Consequences – what’s the looming disaster

! " Solution – a plan to get out of the mess

! " Common AntiPatterns

! " Functional Decomposition

! " Programmer still stuck in procedural programming mindset

! " Creates classes that act like “functions”

! " Too much decomposition

! " The Blob

! " Programmer stuck in procedural mindset

! " Creates single class that contains everything

! " Too little decomposition


Summary

! " Design Patterns conceptually grew from architectural

works

! " Three main types of patterns: Architectural, Design, and Idioms

! " Watch out for AntiPatterns

Applying Design Patterns

16


Adopting Design Patterns

! " Adopting patterns should be easy, but be aware of

! " Cultural barriers

! " Organizational barriers

! " Implementation barriers

! " Like most new things start small

! " Pick an easy pattern to implement

! " Experiment with the pattern and how it effects your system

! " Document outcomes and move forward

! " Familiarize yourself with pattern catalogues

! " Define a process


Pattern Catalogues

Each pattern in the catalogue should be defined by:

! " Pattern name – easy to convey a concept once its part of your

vocabulary

! " Classification – where it “fits” into the overall design of things

! " Intent – answers the question “What does the pattern do?”

! " Also known as – different names of the pattern

! " Motivation – a scenario that helps illustrate the program

! " Applicability – describes situations when the pattern applies

! " Structure and Implementation – guidelines on how to implement

! " Participants and Collaborations – entities involved in the

implementation

! " Consequences – side effects of applying the pattern

! " Related patterns – other patterns closely related


Adopting Design Patterns

Define a selection and adoption process

! " Determine need – what’s driving the adoption of a pattern

! " Determine the classification needed

! " Take an inventory of available patterns

! " Examine the intent of the pattern

! " Consider how the pattern solves the problem

! " Determine the patterns implementation constraints

! " Review any “like” patterns

! " Determine level of effort and weigh it against need

! " Implement


Implementing Design Patterns

! " Two implementation strategies:

! " Design-time application

! " Refactoring application

! " Once you have identified a pattern, you’ll need to

implement it:

! " Familiarize yourself with pattern

! " Examine the structure, participants, collaboration, etc.

! " Look at some sample code or example implementations

! " Choose implementation appropriate names

! " Define the classes

! " Define the application specific names for operations

! " Implement the operations


Design-Time Application

! " Design-time application occurs during system design ! " Might be part of class design

! " Might be the result of a system review

! " Occurs before any code has been written

! " Design-time application gives you the most ! " Flexibility

! " Creativity

! " Choice

! " Choosing the wrong design pattern during design is easy to fix

! " But there are down-sides ! " May over-engineer

! " Design something that is hard to implement

! " Etc.


What is Refactoring?

! " Purist definition: “modifying a piece of software without

changing its external behavior or interface”

! " General definition: “modifying a piece of software to make the implementation more robust, flexible, maintainable,

reusable, etc.”

! " As the number of artifacts in a software project grows,

refactoring to design patterns is a common way to “clean

up an implementation”


The Refactoring Process

! " Refactoring is commonly attributed to the test-driven

development and Agile process software engineering

methodologies

! " The basic TDD refactoring process

! " Identify high-level considerations for the refactoring

! " Reconcile any issues that may exist prior to refactoring

! " Simplify the scope

! " Extract / inline a method

! " Extract / push-down an interface

! " Test

! " Repeat


Refactoring Application

! " Refactoring application occurs during system evolution ! " Might be part of initial development

! " May occur late in the applications lifecycle

! " Changes made to existing software

! " Refactoring application can create ! " Create extensibility, reusability, maintainability, etc.

! " Limit over-engineering

! " But there are down-sides ! " May introduce a large number of bugs and potentially other design

issues

! " Choosing the wrong design pattern during refactoring is hard to fix


Refactoring Design Patterns

! " Refactored always refactoring…

! " Rapid application development is dynamic

! " Decisions made today may not make sense tomorrow

! " A design pattern applied today may not work tomorrow

! " Like regular code, design patterns can be refactored

! " Could become compositions of design patterns

! " Or replacements

! " Using a formalized design pattern selection process should help minimize this churn


Summary

! " Beware of challenges when adopting design patterns

! " Examine the application during initial design

! " Refactor only when you need to

About DevelopIntelligence

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! "Provides outsourced services to learning organizations in area of software development

! "Represents over 35 years of combined experience, enabling software development community through educational and performance services

! "Represents over 50 years of combined software development experience

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! 2003 - 2007 DevelopIntelligence

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Introduction to Design Patterns · Software” book written by Gamma, Helm, Johnson, and Vlissides...

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