Introduction to Design Pattern

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Presented by:
- Nozha KANNOUF
- Safaâ ELOUAHABI
- Sanae BEKKAR
Supervised by:
Mr. ABDALAOUI
Introduction to DesignIntroduction to Design
PatternPattern
Oujda Med First University
National College of Applied Sciences
Department of Computer Engineering

PlanPlan
Why design patterns ?
What is a design patterns ?
The patterns elements
Types of design patterns
Describing design patterns
The catalog of design patterns
Examples of design patterns
Conclusion
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WhyWhy Design Patterns?Design Patterns?
The DP satisfy this need for good ,simple and reusable solutions.
DP catalog common interactions between objects that
programmers have often found useful.
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Difficulties of ObjectDifficulties of Object--Oriented DesignOriented Design
Oriented-Object Design
find pertinent objects,
factor them into classes at the right granularity,
define class interfaces and inheritance hierarchies,
and establish key relationships among them.
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Difficulties of ObjectDifficulties of Object--Oriented DesignOriented Design
A reusable and flexible design is difficult if not
impossible to get “right” the first time.
Experienced OO designers do make good designs.
Whereas , New designers are overwhelmed by the
options available and tend to fall back on non-OO
techniques they’ve used before.
What’s their difference?
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PatternsPatterns
When they find a good solution, they use it
again and again – pattern.
A designer who is familiar with such patterns
can apply them immediately to design
problems without having to rediscover them.
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What is a design pattern?What is a design pattern?
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Design patternsDesign patterns
"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"
[Christopher Alexander]
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Design PatternDesign Pattern
Design patterns are class combinations and
accompanying algorithms that fulfill common design
purposes.
Design patterns are descriptions of communicating
objects and classes that are customized to solve a general
design problem in a particular context.
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In software engineering, a design pattern is a general
reusable solution to a commonly occurring problem in
software design. A design pattern is not a finished design
that can be transformed directly into code. It is a
description or template for how to solve a problem that
can be used in many different situations. Object-oriented
design patterns typically show relationships and
interactions between classes or objects, without specifying
the final application classes or objects that are involved

The essential elements of a patternThe essential elements of a pattern
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Pattern elementsPattern elements
Name
A meaningful pattern identifier
Problem description
When to apply the pattern
Solution description
Elements that make up the pattern, their relationships,
responsibilities, and collaborations.
Consequences
The results and trade-offs of applying the pattern
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Types of design patternTypes of design pattern
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Types of Design PatternsTypes of Design Patterns
Creational patterns
Creating a collection of objects in flexible ways.
Structural patterns
Representing a collection of related objects.
Behavioral patterns
Capturing behavior among a collection of objects.
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Describing Design PatternsDescribing Design Patterns
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Intent
short description of the pattern & its purpose
Also Known As
Any aliases this pattern is known by
Motivation
motivating scenario demonstrating pattern’s use
Applicability
circumstances in which pattern applies
Structure
graphical representation of the pattern using modified UML notation
Participants
participating classes and/or objects & their responsibilities
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Collaborations
how participants cooperate to carry out their responsibilities
Consequences
the results of application, benefits, liabilities
Implementation
pitfalls, hints, techniques, plus language-dependent issues
Sample Code
sample implementations in C++, Java, C#, Smalltalk, C, etc.
Known Uses
examples drawn from existing systems
Related Patterns
discussion of other patterns that relate to this one
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The Catalog of Design Patterns
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Two criteria
Purpose: what a pattern does
Creational: concern the process of object creation
Structural: the composition of classes or objects
Behavioral: characterize the ways in which classes or objects
interact and distribute responsibility
Scope: whether the pattern applies primarily to
classes or to objects
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Examples of design patternsExamples of design patterns
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Intent
create families of related objects without specifying subclass names
Applicability
when clients cannot anticipate groups of classes to instantiate
ABSTRACT FACTORY object creational

Intent
"compose" objects into tree structures to represent part-whole
hierarchies.
Applicability
when clients should ignore the difference between
compositions of objects and individual objects
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COMPOSITE object structural

Intent
encapsulate the request for a service
Applicability
to parameterize objects with an action to perform
to specify, file, & execute requests at different times.
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Cammand/1 (object behavioral)

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1.Do not lead to direct code reuse
2. Complex in nature
3. They are deceptively simple
4. They are validated by experience and discussion
5. They consume more memory because of generalized
format they are written, to store any kind of data
6. Design patterns may increase or decrease the
understandability of a design or implementation

References and related resource
[Knuth] Computer Programming as an Art.
http://fresh.homeunix.net/~luke/misc/knuth-turingaward.pdf
[Columbia] Code convention.
http://nsl.cs.columbia.edu/projects/projects.html
Design pattern catalog.
http://www.dofactory.com/Patterns/Patterns.aspx#list
Introduction to Software Engineering
http://sunset.usc.edu/classes/cs377_2004/

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