object oriented design

Object Oriented Design

Sudarsun S., M.TechDirector – R & D

Checktronix India Pvt Ltd

Chennai 600010

OO Design - Sudarsun S 2

Objectives To explain how a software design may be

represented as a set of interacting objects that manage their own state and operations

To introduce various models that describe an object-oriented design

To introduce design patterns


What is an OBJECT

An instance of a CLASS Contains meaningful data Concepts that occupy memory space at

runtime are, according to the definition, objects If not, they are CLASSESFor example: data type vs. double


A little Quiz…

#1 Class or Object?Dog

Scooby-Doo

Dog is a generalization of Scooby-Doo


A little Quiz (cont’d)…

#2 Class or Object?Dog

Scooby-Doo

Animal

The concept of subclass!

Dog is a subclass of the Animal class

Animal is a generalization of Dog


A little Quiz (cont’d)… #3 Class or Object?

Animal

DogBird

The concept of polymorphism!


Characteristics of OOD Objects are abstractions of real-world or system

entities and manage themselves Objects are independent and encapsulate state and

representation information. System functionality is expressed in terms of object

services Shared data areas are eliminated

Objects communicate by message passing Objects may be distributed Objects may execute sequentially or in parallel


Interacting objects

state o3

o3:C3

state o4

o4: C4

state o1

o1: C1

state o6

o6: C1

state o5

o5:C5

state o2

o2: C3

ops1() ops3 () ops4 ()

ops3 () ops1 () ops5 ()


Advantages of OOD Easier maintenance. Objects may be

understood as stand-alone entities Objects are appropriate reusable

components For some systems, there may be an

obvious mapping from real world entities to system objects


Object-oriented development Object-oriented analysis, design and programming

are related but distinct OOA is concerned with developing an object model

of the application domain OOD is concerned with developing an object-

oriented system model to implement requirements OOP is concerned with realising an OOD using an

OO programming language such as Java or C++


Objects and object classes

Objects are entities in a software system which represent instances of real-world and system entities

Object classes are templates for objects Classes may be used to

create objects

Object classes may inherit attributes and services from other object classes

Employee

name: stringaddress: stringdateOfBirth: DateemployeeNo: integersocialSecurityNo: stringdepartment: Deptmanager: Employeesalary: integerstatus: {current, left, retired}taxCode: integer. . .

join ()leave ()retire ()changeDetails ()


Object communication Conceptually, objects communicate by message passing Messages

The name of the service requested by the calling object. Copies of the information required to execute the service

and the name of a holder for the result of the service.

In practice, messages are often implemented by procedure (a.k.a. method) calls Name = method name Information = parameter list Result holder = method return value


Message examples

// Call a method associated with a buffer // object that returns the next value // in the buffer

v = circularBuffer.Get() ;

// Call the method associated with a// thermostat object that sets the // temperature to be maintained

thermostat.setTemp(20) ;


Generalisation and inheritance Objects are members of classes which define attribute

types and operations Classes may be arranged in a class hierarchy where

one class (a super-class) is a generalisation of one or more other classes (sub-classes)

A sub-class inherits the attributes and operations from its super class and may add new methods or attributes of its own

It is a reuse mechanism at both the design and the programming level

Inheritance introduces complexity and this is undesirable, especially in critical systems


A generalisation hierarchyEmployee

Programmer

projectprogLanguage

Manager

ProjectManager

budgetsControlled

dateAppointed

projects

Dept.Manager

StrategicManager

dept responsibilities


Object Relationships Objects and object classes participate in relationships with

other objects and object classes In UML, such a relationship is indicated by an association

Associations may be annotated with information that describes the association

EmployeeDepartment

Manager

is-member-of

is-managed-by

manages


Object identification Identifying objects (or object classes) is the

most difficult part of object oriented design There is no “magic formula” for object

identification It relies on the skill, experience

and domain knowledge of system designers Object identification is an iterative process

You are unlikely to get it right first time


Approaches to identification Use a grammatical approach based on a natural

language description of the system (used in HOOD method)

Base the identification on tangible things in the application domain

Use a behavioural approach and identify objects based on what participates in what behaviour

Use a scenario-based analysis – the objects, attributes and methods in each scenario are identified


Object interface specification Object interfaces have to be specified so that

the objects and other components can be designed in parallel

Designers should avoid designing the interface representation but should hide this in the object itself

Objects may have several interfaces which are viewpoints on the methods provided


Examples of design models Sub-system models that show logical groupings of

objects into coherent subsystems UML package diagrams

Sequence models that show the sequence of object interactions UML sequence diagrams

State machine models that show how individual objects change their state in response to events UML statechart diagrams

Other models include use-case models, aggregation models, generalisation models,etc.


Weather station subsystems«subsystem»

Interface

CommsController

WeatherStation

«subsystem»Data collection

«subsystem»Instruments

Air thermometer

WeatherData

Ground thermometer

Anemometer

WindVane

RainGauge

InstrumentStatus

Barometer


Weather station - data collection sequence

:CommsController

request (report)

acknowledge ()report ()

summarise ()

reply (report)

acknowledge ()

send (report)

:WeatherStation :WeatherData


State charts Object states State transitions triggered by requests to

objects

Shutdown Waiting Testing

Transmitting

Collecting

Summarising

Calibrating

transmission done

calibrate ()

test ()startup ()

shutdown ()

calibration OK

test complete

weather summarycomplete

clock collectiondone

Operation

reportWeather ()


OO Design Process – Access Layer

Create Mirror classes: For every business class identified and created, create one access class.

Simplify classes and their relationships – to eliminate redundant classes and structures

Redundant classes: Do not keep 2 classes that perform similar request and results translation activities. Select one and eliminate the other.

Methods: Revisit the classes that consist of only one or two methods to see if they can be eliminated or combined with existing classes.


Object Oriented Design Process Design the view layer classes

Design the macro & micro level user interface, identifying view layer objects

Test usability and user satisfaction From the UML class diagram, begin to extrapolate the

classes to build and classes to reuse. Also think about the inheritance structure. If we have several classes that seem related but have

specific differences, probably it means an inheritance structure.

All designed components must trace back to the user requirements.


Example: Invoice


Example: Invoice

Classes that come to mind: Invoice, LineItem, and Customer

Good idea to keep a list of candidate classes

Brainstorm, simply put all ideas for classes onto the list

You can cross not useful ones later


Finding Classes Keep the following points in mind:

Class represents set of objects with the same behavior Entities with multiple occurrences in problem

description are good candidates for objects Find out what they have in common Design classes to capture commonalities

Represent some entities as objects, others as primitive types Should we make a class Address or use a String?

Not all classes can be discovered in analysis phase Some classes may already exist


CRC Card CRC Card Describes a class, its responsibilities, and its

collaborators Use an index card for each class Pick the class that should be responsible for each

method (verb) Write the responsibility onto the class card Indicate what other classes are needed to fulfill

responsibility (collaborators)


CRC Card


Self Check

1. Suppose the invoice is to be saved to a file. Name a likely collaborator.

2. Looking at the invoice, what is a likely responsibility of the Customer class?

3. What do you do if a CRC card has ten responsibilities?


Answers

1. FileWriter

2. To produce the shipping address of the customer.

3. Reword the responsibilities so that they are at a higher level, or come up with more classes to handle the responsibilities.


Relationships Between Classes

Inheritance Aggregation Dependency


Inheritance

Is-a relationship Relationship between a more general

class (superclass) anda more specialized class (subclass)

Every savings account is a bank account

Continued…


Inheritance

Every circle is an ellipse (with equal width and height)

It is sometimes abused Should the class Tire be a subclass of a

class Circle? The has-a relationship would be more appropriate


Aggregation Has-a relationship Objects of one class contain references to objects of

another class Use an instance variable

A tire has a circle as its boundary:

Every car has a tire (in fact, it has four)

class Tire{ . . . private String rating; private Circle boundary;}


Example

class Car extends Vehicle{ . . . private Tire[] tires;}


Example

UML Notation for Inheritance and Aggregation


Dependency

Uses relationship Example: many of our applications depend

on the Scanner class to read input Aggregation is a stronger form of

dependency Use aggregation to remember another

object between method calls


UML Relationship SymbolsRelationship Symbol Line Style Arrow TipInheritance Solid Triangle

Interface Implementation

Dotted Triangle

Aggregation Solid Diamond

Dependency Dotted Open


Self Check

4. Consider the Bank and BankAccount classes. How are they related?

5. Consider the BankAccount and SavingsAccount objects. How are they related?

6. Consider the BankAccountTester class. Which classes does it depend on?


Answers

4. Through aggregation. The bank manages bank account objects.

5. Through inheritance.

6. The BankAccount, System, and PrintStream classes.


Attributes and Methods in UML

Attributes and Methods in a Class Diagram


Multiplicities any number (zero or more): * one or more: 1..* zero or one: 0..1 exactly one: 1

An Aggregation Relationship with Multiplicities


Aggregation and Association

Association: more general relationship between classes

Use early in the design phase A class is associated with another if you can

navigate from objects of one class to objects of the other

Given a Bank object, you can navigate to Customer objects

Continued…


Aggregation and Association

An Association Relationship


Five-Part Development Process

Gather requirements Use CRC cards to find classes, responsibilities,

and collaborators Use UML diagrams to record class relationships Use javadoc to document method behavior Implement your program


Printing an Invoice – Requirements Task: print out an invoice Invoice: describes the charges for a set of products in certain

quantities Omit complexities

Dates, taxes, and invoice and customer numbers Print invoice

Billing address, all line items, amount due Line item

Description, unit price, quantity ordered, total price For simplicity, do not provide a user interface Test program: adds line items to the invoice and then prints it

Continued…


Sample Invoice


CRC Cards

Discover classes Nouns are possible classes

InvoiceAddressLineItemProductDescriptionPriceQuantityTotalAmount Due


CRC Cards

Analyze classes

InvoiceAddressLineItem // Records the product and the quantityProductDescription // Field of the Product classPrice // Field of the Product classQuantity // Not an attribute of a ProductTotal // Computed–not stored anywhereAmount Due // Computed–not stored anywhere

Continued…


CRC Cards

Classes after a process of elimination

InvoiceAddressLineItemProduct


CRC Cards for Printing Invoice Invoice and Address must be able to

format themselves:


CRC Cards for Printing Invoice Add collaborators to invoice card:


CRC Cards for Printing Invoice

Product and LineItem CRC cards:


CRC Cards for Printing Invoice Invoice must be populated with products

and quantities:


Printing an Invoice – UML Diagrams

The Relationships Between the Invoice Classes


Method Documentation

Use javadoc documentation to record the behavior of the classes

Leave the body of the methods blank Run javadoc to obtain formatted version of

documentation in HTML format Advantages:

Share HTML documentation with other team members Format is immediately useful: Java source files Supply the comments of the key methods


Method Documentation – Invoice class

/** Describes an invoice for a set of purchased products.*/public class Invoice{ /** Adds a charge for a product to this invoice. @param aProduct the product that the customer ordered @param quantity the quantity of the product */ public void add(Product aProduct, int quantity) { }

/** Formats the invoice. @return the formatted invoice */ public String format() { }}


Method Documentation – LineItem class

/** Describes a quantity of an article to purchase and its price.*/public class LineItem{ /** Computes the total cost of this line item. @return the total price */ public double getTotalPrice() { }

/** Formats this item. @return a formatted string of this line item */ public String format() { }}


Method Documentation – Product class

/** Describes a product with a description and a price.*/public class Product{ /** Gets the product description. @return the description */ public String getDescription() { }

/** Gets the product price. @return the unit price */ public double getPrice() { }}


Method Documentation – Address class

/** Describes a mailing address.*/public class Address{ /** Formats the address. @return the address as a string with three lines */ public String format() { }}


Implementation

Invoice aggregates Address and LineItem

Every invoice has one billing address An invoice can have many line items:

public class Invoice{ . . . private Address billingAddress; private ArrayList<LineItem> items;}


Implementation

A line item needs to store a Product object and quantity:

public class LineItem{ . . . private int quantity; private Product theProduct;}


Implementation The methods themselves are now very easy Example:

getTotalPrice of LineItem gets the unit price of the product and multiplies it with the quantity

/** Computes the total cost of this line item. @return the total price*/public double getTotalPrice(){ return theProduct.getPrice() * quantity;}


Self Check

7. Which class is responsible for computing the amount due? What are its collaborators for this task?

8. Why do the format methods return String objects instead of directly printing to System.out?


Answers

7. The Invoice class is responsible for computing the amount due. It collaborates with the LineItem class.

8. This design decision reduces coupling. It enables us to reuse the classes when we want to show the invoice in a dialog box or on a web page.


Suh’s Axioms of OOD The independence axiom: Maintain

independence of components. Each component must satisfy its requirements without affecting other requirements.

The information axiom: Minimize the information content of the design. It is concerned with simplicity. Rely on Occam’s Razor


Occum’s Razor rule of simplicity

The best designs usually involve the least complex code but not necessarily the fewest number of classes or methods.

Minimizing complexity should be the goal, because that produces the most easily maintained and enhanced application.

In an object-oriented system, the best way to minimize complexity is to use inheritance and the system’s built-in classes and to add as little as possible to what already is there.


Some Corollaries… Uncoupled design Single purpose Large number of simple classes Strong mapping Standardization Design with inheritance


Uncoupled Design Coupling is a measure of the strength of

association established by a connection from one object or software component to another.

Types Interaction coupling

amount and complexity of messages between components. Preferred to be minimal What happens when high and low ?

Inheritance coupling coupling between super and sub classes coupling in terms of attributes and methods Preferred to be high What happens when high and low ? { override all, unused

methods }


Cohesion Cohesion reflects the “single-purposeness” of an

object. Highly cohesion Lower coupling Method cohesion: method carrying one function. A method that carries multiple functions is

undesirable. Class cohesion: All the class’s methods and

attributes are highly cohesive, meaning to be used by internal methods or derived classes’ methods.


Corollary 2: Single Purpose Every class should be clearly defined and

necessary in the context of achieving the system’s goals.

When we document a class, we should be able to explain its purpose in a sentence or two.

If we cannot, then the class should be subdivided into independent pieces.

Each method must provide only one service. Each method should be of moderate size, no

more than a page; half a page is better.


Achieving Multiple Inheritance What is Single inheritance? What is the problem with Multiple Inheritance ? What is the ideal case for MI ? In C++ objects can inherit behavior from

unrelated areas of the class tree. How to determine which behavior to get from

which class, particularly when several ancestors define the same method.

Virtual Inheritance !!??


Class Visibility Purpose of Access Specifiers ? Difference between c++ struct and class ? The class’s protocol or the messages that a class

understands, can be hidden from other objects (private protocol) or made available to other objects (public protocol).

Public protocols define the functionality and external messages of an object.

Private protocols define the implementation of an object. The lack of well-designed protocol can manifest itself as

encapsulation leakage. It happens when details about a class’s internal implementation are disclosed through the interface


Design patterns A design pattern is a way of reusing abstract knowledge

about a problem and its solution Patterns are devices that allow programs to share knowledge

about their design A pattern is a description of the problem and the essence of

its solution Documenting patterns is one way to reuse and share the

information about how it is best to solve a specific design problem A pattern should be sufficiently abstract to be reused in

different settings Patterns often rely on object characteristics such as

inheritance and polymorphism


Pattern elements

Name A meaningful pattern identifier

Problem description Solution description

Not a concrete design but a template for a design solution that can be instantiated in different ways

Consequences The results and trade-offs of applying the pattern


Types of Patterns

Creational Abstract Factory

Structural Adapter Façade Proxy

Behavioral Observer Mediator


Patterns by Example:Multiple displays enabled by Observer

A=10%B=40%C=30%D=20%

Application data

A

BC

D

A DCB

Relative Percentages

Y 10 40 30 20

X 15 35 35 15

Z 10 40 30 20

A B C D

Change notification

Requests, modifications


The Observer pattern Name

Observer

Description Separates the display of object state from the object itself

Problem description Used when multiple displays of state are needed

Solution description See slide with UML description

Consequences Optimisations to enhance display performance are impractical


The Observer pattern

Subject

attach (Observer)

detach (Observer)

Notify ()

Observer

Update()

Concrete Observer

Update()

observerState

Concrete Subject

GetState()

SetState()

subjectState

observers

subject

For all x in observers{ x Update(); }

observerState= subject getState();


The Mediator Pattern Description

Define an object that encapsulates how a set of objects interact Mediator promotes loose coupling by keeping objects from referring to

each other explicitly It lets you vary their interaction independently

Applicability Complex interaction exists You do not want to bury the interaction in the objects Distributed behavior should be customizable without a lot of sub

classing Consequences

Limits sub classing Decouples colleagues Simplifies object protocols Abstracts how objects cooperate Centralizes control


The Mediator Pattern


The Façade Pattern Description

Provide a unified interface to a set of interfaces in a subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

Applicability Need to provide a simple interface to a complex system Need to decouple a subsystem from its clients Need to provide an interface to a software layer

Consequences Shields clients from subsystem components Promotes weak coupling between the subsystem and its clients


Facade Pattern: Problem

Client Classes

Subsystem classes

Need to communicatewith


Facade Pattern: Solution

Client Classes

Subsystem classes

Facade


The Façade Pattern


The Proxy Pattern

Description Provide a surrogate or placeholder for another object to

control access to it.

Applicability Remote proxies can hide the fact that a real object is in

another address space Virtual proxies can create expensive objects on demand. Protection proxies can control access to an object. Smart references can perform additional action above a

simple pointer


The Proxy Pattern


The Adapter Pattern Description

Convert the interface of a class into another interface clients expect

Adapter lets classes work together that could not otherwise because of incompatible interfaces

Applicability Need to use an existing class whose interface does not

match Need to make use of incompatible classes

Consequences Class adapter commits to the concrete Adapter class Class adapter introduces only one object and no pointer

indirection


The Adapter Pattern


The Abstract Factory Pattern Description

Provides an interface for creating families of related or dependent objects without specifying their concrete classes.

Applicability Need to abstract from details of implementation of products Need to have multiple families of products Need to enforce families of products that must be used together Need to hide product implementations and just present interfaces

Consequences Isolates concrete classes Makes exchanging product families easy Promotes consistency among products Supporting new kinds (in each family) of products is difficult


The Abstract Factory PatternAbstractFactory

CreateProductA()

CreateProductB()

ConcreteFactory1

Client

ProductA1ProductA2

AbstractProductA

ProductB2 ProductB1

AbstractProductB

ConcreteFactory2

CreateProductA()

CreateProductB()


Abstract Factory ExampleWidgetFactory

CreateScrollbar()

CreateWindow()

Window

ScrollBarWWidgetFactory

MacWidgetFactory

Client

WWindowMacWindow

MacScrollBar WScrollBar

One for each standard.


What is dependency management? What bearing does DM have on software? What is the result of poor DM? What is the advantage of good DM?

Dependency Management


What is dependency management?

A simple idea - as interdependencies increase, features like reusability, flexibility, and maintainability decrease.

Dependency management is controlling interdependencies.


What bearing does DM have on software?

Coupling and cohesion are the eternal concerns of software development

One can say that OO is just a set of tools and techniques for Dependency Management


What is the penalty for practicing poor DM?

It is rigid It is fragile It is not reusable It has high viscosity

A system with poor dependency structure will typically exhibit these four negative traits:


It is Rigid

The impact of a change cannot be predicted If not predicted, it cannot be estimated Time and cost cannot be quantified Managers become reluctant to authorize change Official Rigidity for “Roach Motel” modules

Rigidity is the inability to be changed


Changes with Rigidity

The System

Officially Rigid Area

Where the change should be made

Where the change must be made now

Are we containing risk, or spreading rot?


It is Fragile

A single change requires a cascade of subsequent changes

New errors appear in areas that seem unconnected to the changed areas

Quality is unpredictable. The development team loses

credibility

Software changes seem to exhibit non-local effects


Increasing RiskDefects v. Cumulative Modifications

Systems tend to become increasingly fragile over time. Intentional, plannedpartial rewrites may be necessary to sustain growth and maintenance.

Changes

Pro

babi

lity

of

intr

oduc

ing

a bu

g

1.0


It is not reusable

Desirable parts of the design are dependent upon undesirable parts

The work and risk of extracting the desirable part may exceed the cost of redeveloping from scratch.


The TrailerThe Trailer


It has high viscosity

When the “right changes” are much more difficult than hacking, the viscosity of the system is high.

Over time, it will become increasingly difficult to continue developing the product.

Viscosity is resistance to fluid motion.


What is the benefit of good DM?Interdependencies are managed, with firewalls

separating aspects that need to vary independently.

More FlexibleLess fragile,

the bugs are boxed in

Easier to reuse Easier to make the right change


What causes “Code Rot”?

A case study “The Copy Routine”

It’s been blamed on stupidity, lack of discipline, and phases of the moon, but...


First VersionAll designs start well

The program is an overnight success!How could it be more simple, elegant, and maintainable?

ReadKeyboard

Copy

W ritePRinter

void copy(void){

int ch;while( (ch=ReadKeyboard()) != EOF)

WritePrinter(ch);}


Second Version

We sometimes want to read from paper tape reader.

We could put a parameter in the call, but we have hundreds of users already!

No big deal, this is just an exception… we can make it work.

Oh, no! Nobody said the requirements might change!


Second Version Design

ReadKeyboard

Copy

WritePrinter

ReadTape

bool GtapeReader = false; // remember to clear

void copy(void){

int ch;while( (ch=GtapeReader ? ReadTape() : ReadKeyboard()) != EOF)

WritePrinter(ch);}


Third VersionHow unexpected! Requirements changed again!

bool GtapeReader = false; Bool GtapePunch = false; // remember to clear

void copy(void){

int ch;while( (ch=GtapeReader ? ReadTape() : ReadKeyboard()) != EOF)

GtapePunch ? WritePunch(ch) : WritePrinter(ch);}

It seems that sometimes we need to write to a paper tape punch. We’ve had this problem before, and just added a flag. Looks like it should work again.


Example of a Good DesignFirst and only version.

void Copy(){

int c;while( (c=getchar()) != EOF)

putchar(c);}

But wait! Aren’t we supposed to be learning OO design? This isn’t OO is it?


…is it?

FILE is an abstraction It represents some kind of byte stream It has many variations

It has methodsRead, Write, getchar, putchar, etcThe methods are *dynamically* bound

It is a small program based on abstractions!

FILE is a class, just implemented differently.


Rephrased in OO

Copy

Reader Writer

KeyboardReader PrinterWriter

«interface» «interface»

interface Reader{ char read(); }

interface Writer{ void write(char c); }

public class Copy{

Copy(Reader r, Writer w){

itsReader = r;itsWriter = w;

}public void copy(){

int c;while( (c==itsReader.read()) != EOF )

itsWriter.write(c);}private Reader itsReader;private Writer itsWriter;

}


Class Design Principles

SRP: The Single Responsibility Principle OCP: The Open/Closed Principle LSP: The Liskov Substitution Principle ISP: The Interface Segregation Principle DIP: The Dependency Inversion Principle

From: Agile Software Development: Principles, Patterns, and Practices.Robert C. Martin, Prentice Hall, 2002.


The Single Responsibility Principle

A class should have one, and only one, reason to change.

+ CalcPay+ ReportHours+ W riteEmployee

EmployeePayroll


Open/Closed Principle

A principle which states that we should add new functionality by adding new code, not by editing old code.

Defines a lot of the value of OO programming Abstraction is the key

“Modules should be open for extension, but closed for modification”-Bertrand Meyer


Abstraction is Key

Client/Server relationships are “open”

Changes to servers cause changes to clients

Abstract servers “close” clients to changes in implementation.

Abstraction is the most important word in OOD

Client Server

ClientAbstractServer

ConcreteServer


The Shape Example Procedural (not closed) implementation OO (closed) implementation


Procedural (open) versionenum ShapeType {circle, square};struct Shape {enum ShapeType itsType;};

struct Circle {

enum ShapeType itsType;double itsRadius;Point itsCenter;

};void DrawCircle(struct Circle*)

struct Square {

enum ShapeType itsType;double itsSide;Point itsTopLeft;

};void DrawSquare(struct Square*)

#include <Shape.h>#include <Circle.h>#include <Square.h>

typedef struct Shape* ShapePtr;

void DrawAllShapes(ShapePtr list[], int n){

int i;for( i=0; i< n, i++ ){

ShapePtr s = list[i];switch ( s->itsType ){case square:

DrawSquare((struct Square*)s);break;

case circle:DrawCircle((struct Circle*)s);break;

}}

}

Shape.h

Circle.h

Square.h

DrawAllShapes.c


What is wrong with the code?

DrawAllShapes is not closed.Switch/case tend to recur in diverse places. If we add a shape, we add to the switch/caseAll switch/case statements must be found and editd.Switch/Case statements are seldom this tidyWhen we add to the enum, we must rebuild

everything The software is both rigid and brittle

It can be demonstrated to work. Isn’t that the important thing?


A Closed Implementation

Class Shape{public:

virtual void Draw() const =0;};

#include <Shape.h>

void DrawAllShapes(Shape* list[],int n){

for(int i=0; i< n; i++)list[i]->draw();

}

Shape.h

DrawAllShapes.cpp

Circle.h

Square.h

Class Square: public Shape{public:

virtual void Draw() const;};

Class Circle: public Shape{public:

virtual void Draw() const;};


Strategic Closure

Closure Against What? Closure is strategic. You have to choose which changes you’ll isolate yourself

against. What if we have to draw all circles first? Now DrawAllShapes must be edited (or we

have to hack something) Opened Where?

Somewhere, someone has to instantiate the individual shapes. It’s best if we can keep the dependencies confined

Use abstraction to gain explicit closure provide class methods which can be dynamically invoked

to determine general policy decisions e.g. draw Squares before Circles

design using abstract ancestor classes Use "Data-Driven" approach to achieve closure

place volatile policy decisions in a separate location e.g. a file or a separate object

minimizes future change locations

No program is 100% closed.


Liskov Substitution Principle

All derived classes must be substitutable for their base classes

This principle guides us in the creation of abstractions.

Derived classes must be usable through the base class interface, without the need for the user to know the difference.


Square/Rectangle

Square

Rectangle

+S etH eight()+S etW idth()

-he ight : rea l-w id th : rea l

A square is-a rectangle, right? So lets consider Square as a subtype of Rectangle.

void Square::SetWidth(double w){

width = w;height = w;

}void Square::SetHeight(double h){

width = h; height = h;}

We can make it work:

Uh, oh. Thisdoesn’t quite seem to fit


Substitution… denied!

It is reasonable for users of a rectangle to expect that height and width may change independently.

These expectations are preconditions and postconditions Bertrand Meyer calls it “Design by Contract” Post condition contract for rectangle is

width = new Width height = old height

Square violates Rectangle’s contract


Liskov Substitution Principle A client of rectangle expects height and

width to be changed independentlyvoid setAspectRatio( Rectange* r, double ratio );

By deriving Square from Rectangle, we are allowing someone to set the aspect ratio of a Square !

We can still make it work if ( typeid(r) == typeid(Rectangle) )Violates Open/Closed Principle !


Liskov Substitution Principle Design by Contract

Bertrand Meyer Pre-conditions, Post-conditions, invariants

Rectangle's postconditions for setWidth() width = newWidth length = oldLength

Square can require no more of clients, nor promise any less Doesn't maintain invariant of length Violates the contract


Design by Contract Advertised Behavior of an object:

advertised Requirements (Preconditions) advertised Promises (Postconditions)

When redefining a method in a derivate class, you may only replace its precondition by a weaker one, and

its postcondition by a stronger oneB. Meyer, 1988

Derived class services should require no more and promise no less

int Base::f(int x);// REQUIRE: x is odd// PROMISE: return even int

int Derived::f(int x);// REQUIRE: x is int// PROMISE: return 8


LSP is about Semantics and Replacement

The meaning and purpose of every method and class must be clearly documented Lack of user understanding will induce de facto violations of LSP

Replaceability is crucial Whenever any class is referenced by any code in any system, any future or existing subclasses of that class must be

100% replaceable Because, sooner or later, someone will substitute a subclass;

it’s almost inevitable.


Dependency Inversion Principle

Details should depend on abstractions. Abstractions should not depend on details.

Copy

Reader W riter

KeyboardReader PrinterW riter

ReadKeyboard

Copy

W ritePRinter

V.


Dependency Inversion PrincipleI. High-level modules should not depend on low-level modules.

Both should depend on abstractions.

II. Abstractions should not depend on details.

Details should depend on abstractionsR. Martin, 1996

OCP states the goal; DIP states the mechanism A base class in an inheritance hierarchy should not know

any of its subclasses Modules with detailed implementations are not depended

upon, but depend themselves upon abstractions


Procedural vs. OO Architecture

Procedural Architecture

Object-Oriented Architecture


DIP Applied on ExampleCopy

Reader Writer

KeyboardReader

PrinterWriter

DiskWriter

class Reader { public: virtual int read()=0;};

class Writer { public: virtual void write(int)=0;};

void Copy(Reader& r, Writer& w){ int c; while((c = r.read()) != EOF) w.write(c);}


Interface Segregation Principle

Sometimes class methods have various groupings.

These classes are used for different purposes. Not all users rely upon all methods. This lack of cohesion can cause serious

dependency problems These problems can be refactored away.

Helps deal with “fat” or inappropriate interfaces


Interface Pollution by “collection”

Client 3Client 2Client 1

Server

+ C1Function()+ C2Function()+ C3Function()

Distinct clients of our class have distinct interface needs.


A Segregated ExampleClient 3Client 2Client 1

Client 3InterfaceClient 2InterfaceClient 1Interface«interface» «interface»«interface»

Server

+ C1Function()+ C2Function()+ C3Function

+ C1Function() + C2Function() + C3Function()


ATM UI Example

TransferDepositW ithdraw

«interface»ATM UI

+ G etW ithdraw AmountAndAccount()+ G etDepositAmountAndAccount()+ G etTransferAmountAndAccounts()

French ATM UI English ATM UI


A Segregated ATM UI ExampleTransferW ithdrawDeposit

ATM Transfer UIATM W ithdraw UIATM Deposit UI«interface» «interface»«interface»

+ GetDepositAm ountAndAccount() + GetW ithdrawAm ountAndAccount + GetTransferAm ountAndAccounts()

«interface»ATM UI

+ GetW ithdrawAmountAndAccount()+ GetDepositAmountAndAccount()+ GetTransferAmountAndAccounts()

French ATM UI English ATM UI


Logger ExampleLog ControlApplicationCode needing to

log som ething

LogManager

EventLog

«interface»

«interface»

M anagedEventLog

+ Log(s: string*)+ SendLotToFile(nam e)+ LogToMem (size)+ LogToConsole()

+ Log(s: string*)

+ SendLotToFile(name)+ LogToM em(size)+ LogToConsole()


Four Class Design Principles - Review

OCP: Extend function without editing codeLSP: Child instances substitute cleanly for baseDIP: Depend on abstractions instead of detailsISP: Split interfaces to manage dependencies


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