Software Design
Patterns
Email: Walaagad@cis.asu.edu.eg
Lecture 1
Design Patterns
Prof. Walaa Khaled
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Textbooks
• Eric Freeman; Elisabeth Freeman, O'Reilly, Head First Design Patterns: your
brain on design patterns,
• Design Patterns: Elements of Reusable Object-Oriented Software, Erich
Gamma, Ralph Johnson, John Vlissides, Richard Helm
Course Work and Marking Scheme
• Midterm: 15%
•Class work: 15%
•Project 20%
•Final: 50%
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Course Content
• Creational Patterns
–Factory
–Abstract
–Singleton
–Builder
• Structural Patterns
–Decorator
–Adapter
–Façade
–Proxy
–Bridge
• Behavioral Patterns
–Strategy
–Observer
–Command
–Template
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Perquisites to understand Design Patterns
• Classes
• Encapsulation
• Inheritance
• Polymorphism
• Interfaces
• Abstraction
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What are Design Patterns
“Best practice to solve common software problems”
“Solutions in the form of templates that may be
applied to real-world problems
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Single Responsibility Principle (SRP)
The class should solve only one problem. It should
have a single reason to change
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Open/Closed Principle (OCP)
The class should be open for extension and closed for
modifications.
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Liskov Substitution Principle
If you substitute any type with one of its subtypes,
the behavior should not change.
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Interface Segregation Principle
Avoid making a general interface containing all
methods.
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Dependency Inversion Principle
Higher Level Classes should not know the
implementation of low-level classes but depends on
abstraction.
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UML (Unified Model Language)
• UML is a general-purpose, developmental, modeling
language in the field of software engineering that is
intended to provide a standard way to visualize the
design of a system
• Its primary purpose during analysis workflow: to
create a vocabulary that is used by both the analyst
and users
• It describes the structure of the system in terms of
classes and objects
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What is Not represented in a UML class
diagram?
• Details of how the classes interact with each other.
• Algorithmic details; how a particular behavior is
implemented
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Class representation
The picture of a class contains the following 3 boxes:
• Class name in top box.
– Use italics for an abstract class name.
– Attributes (optional) in middle box.
– Should include all fields of the object.
• Operations / Methods (optional) in bottom box.
– May omit trivial (get/set) methods.
– Should not include inherited methods.
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Class Attributes
Syntax of defining class attributes in a class diagram is:
visibility name : type [count] = default_value
• visibility.
– “ + “ public “ # “ protected “ – “ private “ / “ derived
• Notes:
–Underline static attributes.
–Derived attributes are not stored, but can be computed
from other attribute values.
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Class methods
Syntax of defining class methods in a class diagram is:
visibility name (parameters) :return type
• visibility.
– “ + “ public “ # “ protected “ – “ private “ / “ derived
• Notes:
–Underline static methods.
–Omit return_type on constructors and when return type
is void
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Relationship between classes
There are three kinds of relationships in UML:
•Generalizations
• an inheritance relationship.
• Inheritance between classes.
• Aggregations.
•
whole --> superclass
part --> supclass
Composition.
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Generalization Relationship
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Aggregation Relationship
• Aggregation is a strong form of association in which
an aggregate objects made of constituent parts.
C
B A
• Aggregation is a transitive relation:
–if A is a part of B and B is a part of C then A is also a part of C
• Aggregation is an antisymmetric relation:
B
A
–If A is a part of B then B is not a part of A.
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Aggregation Relationship
• Aggregation adds semantic connotations:
• An aggregation specifies a whole-part relationship
between an aggregate (a whole) and a constituent
part, where the part can exist independently from
the aggregate.
• Aggregations are denoted by a hollow-diamond
adornment on the association.
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Composition Relationship
• Composition is a form of aggregation with additional
constraints:
• A constituent part can belong to at most one
assembly (whole).
• it has a coincident lifetime with the assembly.
• Deletion of an assembly object triggers
automatically a deletion of all constituent objects
via composition.
• They live and die as a whole.
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Composition Relationship
• Compositions are denoted by a filled-diamond
adornment on the association.
solid
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What are Design Patterns
• Design Patterns are evolved from the features of
object oriented programming as reusable solutions
to the problems that we encounter every day of
programming.
• They are generally targeted at solving the problems
of object generation and integration.
• These generalized patterns act as templates that can
applied to the real world problems
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What are Design Patterns
• Implementing design patterns in the applications are
proven and tested.
• Writing the code aligning with design patterns will
make your application reliable, scalable and
maintainable.
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Types of Design Patterns
Important
There are three types of Design Patterns
• Creational
–Deals with the object creation and initialization.
–Ex. Singleton, Factory, Abstract Factory,…
• Structural
–Deals with class and object composition.
–Adapter , Bridge, …
• Behavioral
–It characterizes the way in which classes or objects interact
and distribute responsibility.
–Interpreter, chain of responsibilities, Command, …
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23 Pattern
Design Patterns Space
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