[SOLVED] CS SOFT2201 / COMP9201

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Software Design and

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SOFT2201 / COMP9201
Introduction to Design

School of Computer Science

The University of 2

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The University of 4

Design Patterns
GoF Design Patterns

Creational Deign Patterns
Factory Method Pattern
Builder Pattern

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What is Design Pattern?

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Design Patterns

A pattern is a description of a problem and its solution

Tried and tested ideas for recurring design problem

Not readily coded solution, but rather the solution path to a
common programming problem

Design or implementation structure that achieves a particular

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Essential Components of a Pattern

The pattern name
e.g., Factory Method

The problem
The pattern is designed to solve (i.e., when to apply the pattern)

The solution
The components of the design and how they related to each other

Consequence
The results and trade-offs of applying the pattern
Advantages and disadvantages of using the pattern

The University of 8

Gang of Four Patterns (GoF)

Official design pattern reference
Famous and influential book about design patterns
Recommended for students who wish to become
We will cover the most widely used patterns
from the book
23 patterns in total our unit will focus on 11
GoF Design Patterns Design Patterns as short

The University of 9

Design Patterns Classification based on purpose

Scope Creational Structural Behavioural

Class Factory Method Adapter (class) Interpreter
Template Method

Object Abstract Factory

Chain of Responsibility

The University of 10

Design Patterns Classification based on purpose

Creational patterns
Abstract the instantiation process
Make a system independent of how its objects are created, composed

and represented

Structural patterns
How classes and objects are composed to form larger structures

Behavioral patterns
Concerns with algorithms and the assignment of responsibilities between

The University of 11

Design Patterns Classification based on relationships

Patterns often used together
E.g., Composite often used with Iterator or Visitor

Alternative Patterns
E.g., Prototype often alternative to Abstract Factory

Patterns results in similar designs
E.g., Structure diagram of composite and Decorator are similar

The University of 12

Design Patterns Classification based on relationships

Reponsibility

Interpreter

State Strategy

SingletonFlyweight

Structural

Behavioural

Creational

hysteresis

saving state
of iteration

composites

responsibilities

to objects

traversals

operations

strategies

dependency
management

changing skin
vs internals defining

algorithms
steps often

configure factory
dynamically

operations defining

* Adapted from the GoF book: Design Patterns

The University of 13

Selecting an Appropriate Design Pattern

It is useful to have some guidelines of how to select one. Here are
some thoughts on how you might do that:
Consider the ways in which the design patterns solve problems.

We will go into details on this for several design patterns
Decide on what the intent of each design is.

Without knowing what the motivation of the design pattern is, it will be
hard to know whether it is right for you

Look at the relationships among patterns
It makes sense to use patterns that have a clear relationship, rather than

one that you have manufacture ad hoc

The University of 14

Selecting an Appropriate Design Pattern

Consider patterns with similar purpose
Creational, Structural and Behavioral are quite different purposes so

you should consider which one you need

Look at why redesign might be necessary
Knowing why a redesign might be needed will help you select the right

design to avoid having to redesign later

Why can vary?
Your design should be open to variation where necessary
Choose a design that will not lock you into a particular one, and enable

you to make variations without changing your design

The University of 15

Design Aspects Can be Varied by Design Patterns

Purpose Pattern Aspects that can change

Creational

Abstract Factory
Factory Method

families of product objects
how a composite object gets created
subclass of object that is instantiated
class of object that is instantiated
the sole instance of a class

Structural

interface to an object
implementation of an object
structure and composition of an object
responsibilities of an object without subclassing
interface to a subsystem
storage costs of objects
how an object is accessed; its location

The University of 16

Design Aspects Can be Varied by Design Patterns

Purpose Pattern Aspects that can change

Behavioral

Chain of Responsibility
Interpreter

Template Method

object that can fulfill a request
when and how a request is fulfilled
grammar and interpretation of a language
how an aggregates elements are accessed, traversed
how and which objects interact with each other
what private info. is stored outside an object, & when
number of objects that depend on another object; how the
dependent objects stay up to date
states of an object
an algorithm
steps of an algorithm
operations that can be applied to object(s) without changing
their class(es)

The University of 17

Creational Patterns

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Creational Patterns

Abstract the instantiation process
Make a system independent of how its objects are created, composed and

represented
Class creational pattern uses inheritance to vary the class thats instantiated
Object creational pattern delegates instantiation to another object

Provides flexibility in what gets created, who creates it, how it gets created

The University of 19

Creational Patterns

Pattern Name Description

Abstract Factory Provide an interface for creating families of related or dependent objects
without specifying their concrete classes

a class only has one instance, and provide global point of access to it

Factory Method Define an interface for creating an object, but let sub-class decide which
class to instantiate (class instantiation deferred to subclasses)

Builder Separate the construction of a complex object from its representation so that
the same construction process can create different representations

Prototype Specify the kinds of objects to create using a prototype instance, and create
new objects by copying this prototype

The University of 20

Factory Method

Class Creational Pattern

An interface for creating an object

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Motivated Scenario

Suppose you have a general shape creator that can create a
variety of different shapes.

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Motivated Scenario

Client Perspective:

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Factory Method Pattern

Purpose/Intent
Define an interface for creating an object
Let subclasses decide which class to instantiate.
Let a class defer instantiation to subclasses

Also known as
Virtual Constructor

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Factory Method Structure
Defines the interface of objects the
factory method creates

Implements the Product interface

Declares the factory method, which
returns an object of type Product.

Overrides the factory method to return
an instance of the ConcreteProduct

The University of 25

Factory Method Participants

Defines the interface of objects the factory method creates

ConcreteProduct
Implements the Product interface

Declares the factory method, which returns an object of type Product.

ConcreteCreator
Overrides the factory method to return an instance of the ConcreteProduct

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Revisit the Motivated Example

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Revisit the Motivated Example

Client Perspective

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Factory Method Pattern

Applicability
A class cannot anticipate the class objects it must create
A class wants its subclasses to specify the objects it creates
Classes delegate responsibility to one of several helper subclasses, and you

want to localize the knowledge of which helper subclass is the delegate

Benefits
Flexibility: subclasses get a hook for providing an extension to an

object; connects parallel class hierarchies
Limitations

Can require subclassing just to get an implementation

The University of 29

Two varieties of Factory

Variety 1: the Creator class is abstract
This requires subclasses to be made because there is no reasonable

default value.
On plus side, this avoids the problem of dealing with instantiating

unforeseeable classes

Variety 2: the Creator class is concrete
Creator may also define a default implementation of the factory

method that returns a default ConcreteProduct object
This provides reasonable default behaviors, and enables subclasses to

override the default behaviors where required

The University of 30

Factory Registry Selecting a Factory Method Source

In essence, a mapping from identifier to implementation
Responsibility for choice of Factory Method concentrated in one

public class FactoryRegistry {
private Map factories = new TreeMap<>();
public void registerFactory(Factory factory, Identifier identifier) {}
public Factory getFactory(Identifier identifier) {}

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Object Creational Pattern

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Motivated Scenario

Suppose the design team is going outside to have a coffee
John would like a CoffeeA that contains no milk and no sugar
Xi would like a CoffeeB that contains no milk but full sugar
Sue would like a CoffeeC that contains full milk and full sugar

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Purpose/Intent
Separate the construction of a complex object from its representation so

that the same construction process can create different representations
You have a range of implementations that might expand, so must be

flexible, or you want to create instances of complex objects

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Builder Structure

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Builder Participants

Specifies an abstract interface for creating parts of a Product object

ConcreteBuilder
Constructs and assembles parts of the product by implementing the

Builder interface
defines and keeps track of the representation it creates.
provides an interface (GetResult) for retrieving the product

The University of 36

Builder Participants

Director
Constructs an object using the Builder interface

Represents the complex object under construction.
ConcreteBuilder builds the products internal representation and defines the process

by which its assembled
Includes classes that define the constituent parts, including interfaces for assembling

the parts into the final result

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Revisit the Motivated Example
abstract builder one concrete builder as an example

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Revisit the Motivated Example
director client perspective

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Applicability
The algorithm for creating a complex object should be independent of

the parts that make up the object and how theyre assembled
The construction process must allow different representations for the

object thats constructed

Benefits
Gives flexibility that can be extended by implementing new subclasses

of the Builder, and isolates code of construction from implementation

Limitations
Not completely generic, less useful in situations where variety of

implementations is not high

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Builder Collaboration

Director d = new Director( );
Builder b = new ConcreteBuilder1( );
d.construct(b);
Product p = b.GetResult( );

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Builder Consequences (1)

Varying products internal representation
Because the product is constructed through an abstract interface, all you have to

do to change the products internal representation is define a new kind of

Isolation of code construction and representation
Each ConcreteBuilder contains all the code to create and assemble a particular

kind of product.
Different Directors can reuse it to build Product variants from the same set of

The University of 45

Builder Consequences (2)

Finer control over the construction process
The builder pattern constructs the product step by step under the directors

Only when the product finished does the director retrieve it from the builder
The builder interface reflects the process of constructing the product more than

other creational patterns

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Task for Week 5

Submit weekly exercise on canvas before 23.59pm Saturday
Prepare questions and ask during tutorial this week

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What are we going to learn next week?

Behavioral Design Patterns
Strategy Pattern
State Pattern

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References

,,, and.
1995. Design Patterns: Elements of Reusable Object-Oriented Software.
Addison-Publishing Co., Inc., Boston, MA, USA.

. 2004. Applying UML and Patterns: An Introduction to Object-
Oriented Analysis and Design and Iterative Development (3rd Edition).
PTR, Upper Saddle River, NJ, USA.

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