Chapter 9 Behavioral Design Patterns o Interpreter o Iterator o Mediator o Observer o State o Chain...

Chapter 9Behavioral Design Patterns

Interpreter Iterator Mediator Observer State Chain of Responsibility Command Template

Interpreter Design Pattern

1 of 8

Design Purpose

Interpret expressions written in a formal grammar.

Design Pattern Summary

Represent the grammar using a

recursive design pattern form: Pass

interpretation to aggregated objects.

Interpreter

Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

AbstractExpressioninterpret()

Client

TerminalExpressioninterpret()

NonTerminalExpressioninterpret()

interpret()

Interpreter Sequence Diagram

TerminalExpression

:NonterminalExpression:Client AbstractExpression

NonterminalExpression

create & interpret()

Example Interpreter Application: Network Assembly

260Mhz & 64MB

400Mhz & 128MB

260Mhz & 32MB

assemble ….

Graphics reproduced with permission from Corel.Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Input For Network Assembly Example

Componentassemble()

NetSystemassemble()

component1

Client

Application of Interpreter Design Pattern

Componentassemble()

Computerassemble()

NetSystemassemble()

component1

component2

RAMdescribe()

CPUdescribe()

Client

SetupgetInputFromUser()

parse()

<<interface>>

component ::= net_system | computernet_system ::= { component } { component } | { component }computer ::= {cpu ram }cpu ::= integerram ::= integer

Source code example

interface Component{

class Computer implements Component{

class NetSystem implements Component{

class CPU{

class RAM{

class Setup{ main()

class Client{. . .Component networkOrder;. . .networkOrder.assemble();. . .}

Key Concept: Interpreter Design Pattern

-- a form for parsing and a means of processing expressions.

Iterator Design Pattern

2 of 8

Design Purpose (Gamma et al)

Provide a way to access the elements of an

aggregate object sequentially without

exposing its underlying representation.

Encapsulate the iteration in a class pointing

(in effect) to an element of the aggregate.

Iterator

- given a collection of objects

o the videos in a video store

o a directory

- having specified ways to progress through them

o “list in alphabetical order”

o “list all videos currently on loan”

... encapsulate each of these ways

Aggregate object

Purpose of Iterator

iterator2

iterator7

Iterator Class Model

Aggregate

Client

IteratorsetToFirst()increment()

isDone()getCurrentItem()

ConcreteIterator ConcreteAggregateAggregatedElement 1…n1

Using Iterator Functions

To perform desiredOperation() on elements of

the aggregate according to the iteration (order) i:

for( i.setToFirst(); !i.isDone(); i.increment() )

desiredOperation( i.getCurrentElement() );

Functions for Iterator

// Iterator "points" to first element:

void setToFirst();

// true if iterator "points" past the last element:

boolean isDone();

// Causes the iterator to point to its next element:

void increment();

// Return the element pointed to by the iterator:

C getCurrentElement(); Adapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Iterator Operations

Set to beginning Increment Get current

elementCheck not done yet

The Iterator

Index (integer) i on

array myArrayi = 0 ++i myArray[ i ]

i < myArray.length

Index (integer) j on

VectormyVector

j = 0 ++jmyVector

.get( j )j < myVector

.size()

Iterator (object)myIterator

myIterator.setToFirst()

myIterator.increment()

myIterator.getCurrent Element()

! myIterator.isDone()

Iterator in Arrays, Vector, and in General

Imagining Iterator

iterator:Iterator

element:Element

Aggregate of Elementobjects

Key: Intended sequence of Element objects

After first() executes, iterator references this object.

After increment() executes, iterator references this object.

Before increment() executes, iterator references this object.

Iterator Example Setup Code

// Suppose that we have iterators for forward and

// backward order: we can re-use print_employees()

List employees = new List();

ForwardListIterator forward // to go from front to back

= new ForwardListIterator ( employees );

ReverseListIterator backward // to go from back to front

= new ReverseListIterator ( employees );

client.print_employees( forward ); // print from front to back

client.print_employees( backward ); // print from back to front

An Organizational Chart Example

Vanna Presleyvice president, 4 years

Sue Millersvce. mgr., 7 years

Sam Markhamsvce. mgr., 5 years

Sal Monahansvce. mgr., 1 year

Iolanthe Carpind. contrib., 8

Inge Carlsonind. contrib., 12

Inez Clappind. contrib., 11

Inky Conwayind. contrib., 6

Iterating by Years of Service Over an Organization Chart

Design Goal At Work: Flexibility ,

Correctness

Separate the “visiting” procedure from the processing of individual employees.

Class Structure for Iterator Example

ServiceIterator

OrgSeniorityIterator

OrgChartDP OrgChartIteratorDP

Teller

Employeedisplay()

Supervisor

Client Setup

OrgChartIteratorfirst()next()isDone()currentItem()

Iterator in the Java API

CollectionIterator iterator()

Iteratornext()hasNext()remove()

ListIterator List AbstractCollection

AbstractListListIterator listIterator()

Iter *

ListItr *

* IBM implementationArrayList Vector

<<interface>>

Address Book Application using Java Iterator

CollectionIterator iterator()

Iteratornext()hasNext()remove()

ListIterator List

AbstractListListIterator listIterator()

ListItr *

* IBM implementation

ArrayList

<<interface>>

AddressBookNameAddressEntry 0..n

Key Concept: Iterator Design Pattern

-- to access the elements of a collection.

Mediator Design Pattern

3 of 8

Design Purpose

Avoid references between dependent objects

Capture mutual behavior in a separate class

Mediator

The Mediator Class Model

Mediator Colleague

ConcreteMediator

ConcreteColleague1 ConcreteColleague2

<<abstract>> <<abstract>>

Design Goal At Work: Reusability and

Robustness

Avoid hard-coded dependencies among the game’s GUI classes, enabling their use in other contexts.

Solicitation of Customer Information (1 of 2)

Name and Location

Basic information

Solicitation of Customer Information (2 of 2)

Account Information

Additional information

Customer ID

Total business

Amount due

Application of Mediator To Customer Information Application

CustomerDisplay

BasicInfoGUI

CustomerGUI

CustTypeDisplay CustInfoDisplay

Concretemediator

The Mediator Class Model in the Java API

ComponentListeneractionPerformed()

ComponentaddComponentListener()

MyEventListener

MyComponent1 MyComponent2

Key Concept: Mediator Design Pattern

-- to capture mutual behavior without direct dependency.

Observer Design Pattern

4 of 8

Design Purpose

Arrange for a set of objects to

be affected by a single object

The single object aggregates the set, calling a

method with a fixed name on each member

Observer

Sourcenotify()

Observerupdate()

for all Observer’s o:o.update();

Client1

Server part Client part

Observable

Sourcenotify()

Observerupdate()

ConcreteSourcestate

ConcreteObserverobserverState

update()

for all Observer’s o: o.update();

Client

Observer Applied to International Hamburger Co.

Sourcenotify()

Observerupdate()

Headquartersdemand

SeniorManagementforecastupdate()

Client1..n

MarketingmarketingDemand

update()doDisplay()if( abs( hq.demand - marketingDemand ) > 0.01 )

{ marketingDemand = hq.getDemand();doDisplay();

Design Goal At Work: Flexibility

Allow mutual funds objects to easily acquire or divest of stocks.

ObservablenotifyObservers()

Observerupdate( Observable, Object )

AwesomeIncprice

LongTermMutualFund….

update(…)

Observer Example:

Mutual Funds

Key:Developer Class

Java API Class

MediumTermMutualFund….

update(…)HiGrowthMutualFund

….update(…)

MutualFundvalue

numAwesomeShares

Client

Source

ObservablenotifyObservers()

Observerupdate( Observable, Object )

MyObservableMyConcreteObserver

observerStateupdate(…)

Observer in the Java API

Key: Developer ClassJava API Class

Model-View-Controller pattern : (Observable, Observer, (Client & Setup) )

data GUIs

Key Concept: Observer Design Pattern

-- to keep a set of objects up to date with the state of a designated object.

State Design Pattern

5 of 8

Design Purpose

Cause an object to behave in a

manner determined by its state

Aggregate a state object

and delegate behavior to it

TargetStateBhandleRequest()

TargetdoRequest()

State Design Pattern Structure: doRequest() behaves according to state of Target

targetState TargetStatehandleRequest()

Client

TargetStateAhandleRequest()

{ targetState.handleRequest(); }

. . . . . .

GUI For a Role-Playing Video Game

Set Characteristics

Courtesy Tom VanCourt and CorelAdapted from Software Design: From Programming to Architecture by Eric J. Braude (Wiley 2003), with permission.

Design Goal At Work: Correctness and

Reusability

Separate the generic code for handling button clicks from the actions which depend on the game’s status at the time.

State Design Pattern Applied to Role-Playing Game

MyGamesetCharacteristics()

MyGameStatehandleClick()

{ state.handleClick(); }

EngaginghandleClick()

SettingUphandleClick()

WaitinghandleClick()

SettingCharacteristicshandleClick()

{ showWindow(); …. // more }

{ // already responding … // display message }

{ // do nothing … // display message}

Key Concept: State Design Pattern

-- to cause a object’s functions to behave according to the state it’s in.

Chain of Responsibility Design Pattern

6 of 8

Design Purpose

Allow a set of objects to service a request.

It presents clients with a simple interface

Link the objects in a chain via aggregation,

allowing each to perform some of the

responsibility, passing the request along

Chain of Responsibility

Chain of Responsibility: Object Model

entryHandler:HandlerSubclassX

(next element): HandlerSubclassY

(next element): HandlerSubclassZ

successor successor

HandlerSubclassAhandleRequest()respondA()

Chain of Responsibility Class Model

Client

HandlerSubclassBhandleRequest() respondB()

successor

Handler

handleRequest()

respondA(); // handle some of the required functionality successor.handleRequest(); // pass along remaining responsibility

. . . .

GUI For Customer Information Application

XML Output for Customer Information Application

Design Goal At Work: Flexibility

Isolate the responsibilities of each part of the input form to generate its XML.

Class Model For User Information Collection

CustomerInfoElement

CustomerName CompanyNameCustomerInfo

CustomerPersonal CustomerProfessionalCustomerAddress Company

container

TextFieldListener«client»

CustomerInfoApp«setup»

Each of these classes supports handleClick()

Object Model Fragment for Customer Information Example

: CompanyName

:Company

:CustomerProfessional

container

:CustomerInfo

handleClick()

container

handleClick()

container

handleClick()

Key Concept: Chain of Responsibility Design Pattern

-- to distribute functional responsibility among a collection of objects.

Command Design Pattern

7 of 8

Design Purpose

Increase flexibility in calling for a service

e.g., allow undo-able operations

Capture operations as classes

Command

Target1action1()

The Command Design Pattern

Action2Commandexecute()

Target2action2()

old approachCommand

execute()

Action1Commandexecute()

Client

<<abstract>>

new approach

Source Code Example

Command myCommand = Command.getCommand(. . .);

myCommand.execute();

The Command Design Pattern: Example

MenuItemhandleClick()

Commandexecute()

CutCommandexecute()

Documentcut()

copy()

document

command

CopyCommandexecute()

document

document.cut()

document.copy()

command.execute()

client

Design Goal At Work: Flexibility and

Robustness

Isolate the responsibilities of the Word Processor commands, making them save-able and reversible.

Undo Example Class Model

WordProcessorCommand

execute()undo()

CutCommandtextCut: String

offset: int execute()

undo()

Documenttext: String

commandHistory

PasteCommand offset: intlength: intexecute()

undo()

QuitCommandexecute()

UndoCommandexecute()

document

DocumentCommandgetInputFromUser()document

Key Concept: Command Design Pattern

-- to avoid calling a method directly (e.g., so as to record or intercept it).

Template Design Pattern

8 of 8

Design Purpose

Allow runtime variants on an algorithm

Express the basic algorithm in a base class,

using method calls where variation is required

Template

• Required to solve equations of the form

ax2 + bx + c = 0.

• Must be able to handle all input possibilities for a, b, and c.

• This is a tutorial application that must provide full explanations to users about the solutions for all values for a, b, and c.

Example of Template Motivation

1. Report progress2. Display number of solutions3. Display first solution, if any 4. Display second solution, if any

A Basic Quadratic Algorithm

Class Model for Template

TemplateAlgorithmhandleRequest()

nonInheritedMethod()calledMethod1()calledMethod2()

Algorithm1calledMethod1()calledMethod2()

algorithm

Client

{ algorithm.handleRequest(); }

SubjectdoRequest()

Design Goal At Work: Flexibility and

Robustness

Isolate the main algorithm for quadratic solution display. Isolate the variants that depend on the coefficients.

NoSolutionsdisplayNumSolutions()displayFirstSolution()

displaySecondSolution()

OneSolutiondisplayNumSolutions()displayFirstSolution()

Class Model for Quadratic Problem

QuadraticEquationfloat a, b, c

solveQuadratic()

QuadraticAlgorithmdisplayWhatsHappening()

displaySolution()inputMakesSense()

displayNumSolutions();displayFirstSolution()

myQuadraticAlgorithm

TwoSolutionsdisplayNumSolutions()displayFirstSolution()

Client

{ myQuadraticAlgorithm.displaySolution(); }

SetupgetABC()

InfinitelyManySolutionsdisplayNumSolutions()displayFirstSolution()

<<abstract>>

Source Code Example

QuadraticEquation myQuadraticEquation;

myQuadraticEquation.solve();

abstract class QuadraticAlgorithm{

class OneSolution extends QuadraticAlgorithm{

class QuadraticEquation{ QuadraticAlgorithm myQuadraticAlgorithm;}

Key Concept: Template Design Pattern

-- to capture a basic algorithm and its variants.

Behavioral Design Patterns capture behavior among objects

Interpreter handles expressions in grammars

Iterator visits members of a collection in a sequential fashion

Mediator captures behavior among peer objects without building a

dependency between the objects

Observer updates objects affected by a single object

State allows method behavior to depend on current status

Chain of Responsibility allows a set of objects to provide

functionality collectively

Command captures function flexibly (e.g. undo-able)

Template captures basic algorithms, allowing variability

Summary of Behavioral Design Patterns

Chapter 9 Behavioral Design Patterns o Interpreter o Iterator o Mediator o Observer o State o Chain...

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