Design Patterns Story

DESIGN PATTERNS STORY
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~ Arunkumar A
6-Mar-2015

CHRISTOPHER ALEXANDER
The Timeless Way of Building, NewYork:
Oxford University Press, 1979.
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Courtesy:Wikipedia
…is the father of the Pattern
Language movement in
Computer Science

DESIGN – GOOD OR BAD?
If a person were going to design an entryway for a
house,

How would he/she know that the design was good?

Can we know good design?

Is there an objective basis for such a judgment?

A basis for describing our common consensus?
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DESIGN – GOOD … [CONTD.,]
What is present in a good quality design that is not
present in a poor quality design?
What is present in a poor quality design that is not
present in a good quality design?
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WHAT IS A PATTERN?
Christopher’s definition,
A pattern is "a solution to a problem in a context":
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
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DESCRIPTION OF A PATTERN

The name of the pattern

The purpose of the pattern, the problem it solves

How we could accomplish this

The constraints and forces we have to consider in order to
accomplish it
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COURTYARD
Courtyard (plural
courtyards)
an area, open to the
sky, partially or wholly
surrounded by walls or
buildings
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Courtesy:Wikipedia

EXCERPTS FROM THE TIMELESS WAY OF
BUILDING
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“In the same way, a courtyard, which is properly formed, helps people come to life in it “
A pattern always has a name and has a purpose. Here, the pattern's name is Courtyard,
and its purpose is to help energize people, to help them feel alive.
“Consider the forces at work in a courtyard. Most fundamental of all, people seek some
kind of private outdoor space, where they can sit under the sky, see the stars, enjoy the
sun, perhaps plant flowers. This is obvious. “
It is important to state explicitly the problem being solved, which is the reason for having
the pattern in the first place. This is what Alexander does here for Courtyard.
“But there are more subtle forces too. For instance, when a courtyard is too tightly
enclosed, has no view out, people feel uncomfortable, and tend to stay away ...they need
to see out into some larger and more distant space.”
He points out a difficulty with the simplified solution and then gives us a way to solve the
problem that he has just pointed out. Familiarity sometimes keeps us from seeing the
obvious. The value of a pattern is that those with less experience can take advantage of
what others have learned before them: both what must be included to have a good design,
and what must be avoided to keep from a poor design.

SOFTWARE DESIGN PATTERNS
Early 1990s,

Were there problems in software that occur over and over
again that could be solved in somewhat the same manner?

Was it possible to design software in terms of patterns,
creating specific solutions based on these patterns only after
the patterns had been identified?
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GANG OF FOUR (GOF)
Gamma, E., Helm, R., Johnson,
R.,Vlissides, J.,
Design Patterns: Elements of
Reusable Object-Oriented
Software, Boston:Addison-
Wesley, 1995.
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PATTERN
A Pattern has
• Name
• Intent
• Problem
• Solution
• Participants & Collaborators
• Consequences
• Implementation
• Generic diagram
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23 GOF DESIGN PATTERNS16
Courtesy:flickr.com

WHY STUDY DESIGN PATTERNS?
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Reuse solutions

get a head start on problems and avoid gotchas.

get the benefit of learning from the experience of others.

do not have to reinvent solutions for commonly recurring
problems.
Establish common terminology

Communication and teamwork require a common base of
vocabulary and a common viewpoint of the problem.

WHY …[CONTD.,]
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Consider two carpenters discussing how to build the drawers
for some cabinets.
Carpenter 1: How do you think we should build these
drawers?
Carpenter 2: Well, I think we should make the joint by cutting
straight down into the wood, and then cut back up 45
degrees, and then going straight back down, and then back
up the other way 45 degrees, and then going straight back
down, and then…

WHY …[CONTD.,]
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Consider a programmer describing,
“And then, I use a WHILE LOOP here to do…followed by a series
of IF statements to do…and here I use a SWITCH to handle…”

WHY …[CONTD.,]
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Carpenter 1: “Should we use a dovetail joint or a miter joint?”
Courtesy:Wikipedia

WHY …[CONTD.,]
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Carpenter 1: Should we use a dovetail joint or a miter joint?
The real question is
Should we use a joint that is expensive to make but is both
beautiful and durable, or should we just make a quick and
dirty joint that will last at least until the check clears?
Courtesy:Wikipedia

WHY …[CONTD.,]
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Who is more efficient?
Who would you rather work with?

WHY …[CONTD.,]
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
Improve team communications and individual learning

Improved modifiability and maintainability of code

Design patterns illustrate basic object-oriented principles

Adoption of improved strategies even when patterns aren't
present

PATTERNS HELP
US SEE
THE FOREST &
THETREES
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Design Patterns Explained : A New Perspective on
Object-Oriented Design, Second Edition
By Alan Shalloway, James R. Trott
Courtesy:Wikipedia
REFERENCES
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Design Patterns Story

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