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Chapter 6:
Introduction to Design Patterns
King Fahd University of Petroleum & Minerals
SWE 316: Software Design & Architecture
Semester: 072
Objectives
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To introduce design patterns and their types,
characteristics, and forms
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Chapter 6: Introduction to Design Patterns
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What is a Pattern?
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"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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In general, a pattern has four essential elements:
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Name
Problem - When to apply the pattern
Solution - Elements that make up the pattern, their
relationships, responsibilities, and collaborations.
Consequences - Results and trade-offs of applying the pattern
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Chapter 6: Introduction to Design Patterns
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What is a Design Pattern?
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Design patterns are class combinations and
accompanying algorithms that fulfill common design
purposes.
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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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Chapter 6: Introduction to Design Patterns
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Types of Design Patterns
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Creational patterns (Chapter 7)
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Structural patterns (Chapter 8)
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Creating a collection of objects in flexible ways.
Representing a collection of related objects.
Behavioral patterns (Chapter 9)
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Capturing behavior among a collection of objects.
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Chapter 6: Introduction to Design Patterns
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Characteristics of Design Patterns
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Viewpoints – ways to describe patterns
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Levels – decomposition of patterns
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Static: class model (building blocks)
Dynamic: sequence or state diagram (operation)
Abstract level describes the core of the pattern
Concrete (= non abstract) level describes the particulars of
this case
Roles – the “players” in pattern usage
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Application of the design pattern itself
Clients of the design pattern application
Setup code initializes and controls
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Chapter 6: Introduction to Design Patterns
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Characteristics of Design Patterns (Cont’d)
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Exercise:
Characteristics of Design Patterns
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Consider the Observer design pattern class model shown below.
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Group the classes to show abstract and concrete levels.
Group the classes to show client, setup and application roles.
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Chapter 6: Introduction to Design Patterns
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Design Pattern Forms
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Delegation
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An implementation mechanism in which an object forwards
(delegates) a request to another object.
The delegate carries out the request on behalf of the original
object.
Recursion
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Part of the pattern essentially uses itself
Example: constructing graphical user interfaces that allow for
windows within window within window …
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Chapter 6: Introduction to Design Patterns
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The Delegation Design Pattern Form
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Basic idea of delegation
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The Delegation Design Pattern Form
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Chapter 6: Introduction to Design Patterns
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The Recursion Design Pattern Form
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Chapter 6: Introduction to Design Patterns
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Example: The Recursion Form Applied
to an Organization Chart
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Summary
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Design Patterns are recurring designs satisfying recurring design
purposes
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Described by Static and Dynamic Viewpoints
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Typically class models and sequence diagrams respectively
Use of a pattern application is a Client Role
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Client interface carefully controlled
“Setup,” typically initialization, a separate role
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Design patterns Forms usually Delegation or Recursion
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Classified as Creational, Structural, or Behavioral
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Chapter 6: Introduction to Design Patterns
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