SE 03 OO Concepts [Read-Only]

7/28/2019 SE 03 OO Concepts [Read-Only]

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History

Meta-model of Objects

Objects & Messages

Classes Inheritance


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Background

Meta-Model


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- 3 - Prof. Dr. H. Lichter

History of Object Orientation

Simula: Ole-Johan Dahl and Krysten Nygaard, Norway,

1967 A language for simulation (i.e. modeling)


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Why objects?

Finally I would like to try to answer the following fair

question: How could it happen that a team of twoworking in the periphery of Europe could hit onprogramming principles of lasting importance?

No doubt a bit of good luck was involved. We weredesigning a language for simulation modeling, and

such models are most easily conceived of in terms ofcooperating objects.

Our approach, however, was general enough to beapplicable to many aspects of system development.

Ole-Joh an Dah l: s dm Con ferenc e Soft ware Pio neers , Bo nn , 2001


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OO Languages - Milestones


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Object based

Objects

CLU

(Ada)

(Modula-2)

Ada

Modula-2

Simula

Smalltalk-80Eiffel

C++

Java

Modula-3

Aspects of Object Oriented Languages

Class based

Objects

Classes

Object oriented

Objects

Classes

Inheritance

Objects are

encapsulated

data, described one

by one

Classes describe

types of objects

Data, objects can

be derived from

classes.

Inheritance defines

a specialization-

hierarchy between

classes


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A Metamodel of Objects 1

A metamodel of objects

defines elements and relationships which can be used for modeling

Basic elements are

object and

class

Relationships are

uses and

inherits

Additionally, rules fo r modeling are defined

Different programming languages may define differentmetamodels


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Ab stract Cla ss

Method

Ab stract Ob je ct

Method

Object AttributeClass Attribute

Class Method

0..*

1..*

0..*

1..*

accesses

Ab stract Cla ss

Object Method

Ab stract Me th od

1..*1..*

specifies

Attr ibute

0..*1..* 0..*1..*

accesses

Class

Program Module

0..*

0..*

+Server0..*

uses

+Client

0..*

0..*

0..*+Super

0..*

inherits+Sub

0..*

0..*0..*

implements

0..*0..*

defines /

redefines

Object

is_instance_of

Method

0..*0..*

redefines

Message

sends

receives

triggers execution of

A Metamodel of Objects 2


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Objects

Messages


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Objects in a Domain Perspective

Objects are models of relevant things

e.g. my account, your car, his house

Things are characterized through their behavior andhow they can be dealt with

Two basic questions:

Which information can be derived? Which actions can be triggered?

Objects

encapsulate related information and behavior

define an interface through these can be accessed


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Objects in a Programming Perspective

A program is made up of objects

Each activity is performed by objects

Objects

have a lifetime (from creation to destruction)

have an identity (ensured by the system)

are active

change through their lifetime

send and receive messages


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Inside an object

An object consists of two parts

Data - which represent the state of the object

Methods - which can change the data (and only these!)

Objects behave by executing their methods

If an object receives a message the corresponding method willbe executed

The state may change through the execution

Each object is associated with a type

the type defines the methods

usually the type is set by the class of the object


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Object Interfaces 1

Each method defines a signature, consisting of

a name,

the argument objects

a result object

An interface of an object is defined by all his publicsignatures

Method 1

Method 1

Method n

arguments

resu

lts

Signature

Interface


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Object Interfaces 2

The interface of an object defines the offered services

Server/Client-Relationship

An object offers services which a client-object uses

An object may be a client of one object and a server for anotherobject at the same time

Sometimes offered services are implemented by using servicesof other (associated) objects

( Call-in/call-out interface)


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Server/Client-Relationship

Client Server

Service

Service

uses

Service

Client

Server

Associated

Object

uses


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Methods

moveTo (p: Point)center : Point

area : Integer

Data

origin = (10, 10)

corner = (100, 400)

E.g. Objects

a rectangle

a d o t

Methods

+ (p : Point) : Point

putX (i : Integer)

putY (i : Integer)

Data

x= 10

y = 10


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E.g. Objects in Eiffel

class Root-- declaration of objects

featureb : Boolean; -- a basic typei : Integer -- a basic typer : Rectangle; -- a classp, q : Point; -- a cass

Create isdo

p.Create; -- create a dot pp.putX (10); -- send p a messagep.putY (10);r.Create; -- create a rectangle r

...

b := r.contains (p);r.moveTo(p);

p.Forget -- destroy dot p

end;


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receiver message name parameter

r.contains (p)

Messages

Objects communicate by sending and receivingmessages

Objects react to messages by executing a method

The receiving object is responsible for

understanding the message

execution of the associated method (and its results)


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Classes

Inheritance


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Classes in a Domain Perspective

Different things may have a common behavior

my account, your account

These similarities are expressed by a term orconcept

Concepts can be generalized orspecialized we gethierarchies

Concepts (or terms) make up the language of a problem domain

giro account

a c c o u n t

fixed deposit


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Classes in a Programming Perspective

Concepts (or terms) are implemented through classes

A class is characterized by a name

an inheritance-relationship to superclasses

data (attributes, variables)

methods (operations, routines),

methods can be public or private (ie for internal use only)

Only the name and the public methods are visible

From a class an unlimited number of objects can bederived (instantiated)


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From Classes to Objects

Classes

A class defines how their instances will be created (andinitialized)

Classes have a behavior for creating object

Objects of a class

perform the same set of methods

have different states

Basically

a class is an Abstract Data Type

an object is an Data Capsule

Object oriented concepts extend the principle of InformationHiding


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Inheritance 1

Inheritance

A basic feature of object oriented languages not found in otherparadigms

Inheritance allows hierarchical class structures

Technical implications

All aspects of a superclass are valid for its subclasses also

the unit of inheritance is the class Subclasses may specialize orextend aspects of their superclass

The superclasses relationships are statically defined, theserelationships are typically unchanged during runtime.


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Common features of different classes

are collected in a own superclass

are now inher i t ed from this superclass

Rule

Class A inherits from class B if and only if A is a specialization ofB

Rectangle Circle

Geo_Object

Inheritance 2

E.g.

both have

common aspects,

E.g. area, extent


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Types of Inheritance

means ofmodification

no of super-classes one / none unlimited

extension /

definition

extension /redefinition /

definition

strictly singleinheritance

non-strictly singleinheritance

strictly multiple

inheritance

non-strictly multiple

inheritance


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Examples 1

class Rectangle

inherit

Geo_Object

define moveTo, contains

feature

origin, corner : Point;...

end -- class Rectangle

class Geo_Object

featuremoveTo: (p : Point) is

deferredend;

contains (p : Point) : Boolean isdeferredend;

end -- class Geo_Object

class Circle

inherit

Geo_Object


feature

center : Point;

radius : Integer:

...

end -- class Circle

Abstract superclass

for specification

Simple inheritance


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class Rectangle

inheritGeo_Object


featureorigin, corner : Point;...

end -- class Rectangle

class DisplayableRectangle

inheritRectangle

inheritDispayableObject

define psDescription

feature...

end -- classDisplayableRectangle

class DisplayableObject

featurepsDescription : String is

deferredend;

end -- class DisplayableObject

class Geo_Object

featuremoveTo: (p : Point) isdeferredend;

contains (p : Point) : Boolean isdeferredend;

end -- class Geo_Object

Examples 2

Multiple inheritance


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Examples 3

giro account

deposit

paytransferinterest

0815 0816

a c c o u n t

fixed depositcurrency

beginend

personal drawing creditcredit card

transfer

4711 4712

owner: Horst Lichternumber: 0815balance: $1000

interest rare: 0,5 %personal drawing credit: $1000credit card: visa 3249-3458-...

owner

numberbalance

interest rate

Simple Inheritance


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Examples 4

depositpaytransferinterest

0815 0816

a c c o u n t

fixed depositcurrencybeginend

personal drawing creditcredit card

transfer

4711 4712

ownernumberbalance

interest rate

Multiple Inheritance

te lebanking O bject code log inchange code

giro account


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Abstract classes

A class is not necessarily a complete implementation

All methods ofconcrete classes are executable

Some of the methods ofabstract (virtual) classes are not yetimplemented (only specified)

Concrete classes

can have unlimited instances (by receiving a create-message)

Abstract classes

must not have any instances


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Abstract Classes Methods

Abstract methods

Abstract methods are not yet implemented This has to be done by the subclasses.

Abstract methods define an interface for all subclasses

If a method only implements a standard behavior, there is a slotfor the subclasses to extend this behavior (hook)

Template methods

Template methods implement a complete algorithm byabstractmethods

It can only run when the concrete methods are implemented insubclasses

Basic methods

Basic methods are fully executable methods of a abstract class(i.e. completely implemented)


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public String toString() {

/* Returns a string representation of the object. */

return getClass().getName()+'@'+integer.toHexString(hashCode());

/* Standard implementation returns name of the class of which the

object is an instance and the hexadecimal representation of thehash code */}

public String toString() {return x + " , " + y ;

}

Example Hook Method

Each object can receive the message toString() (asimplemented in the abstract class Object)

Redefinition in the Class Point


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Example - Template Method

Each instance ofAbstractCollection (and istsubclasses) can receive the message clear()

public void clear() {

/* Removes all of the elements from this collection.This collection will be empty after this methodreturns unless it throws an exception. */

Iterator iter = this.iterator();

while (iter.hasNext()) {iter.next();iter.remove();

}

}


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Superclasses vs. subclasses

Inherited aspects can be modified by the subclass inthree ways:

Extension

add new aspects (data or methods)

Redefinition overwrite existing methods

Definition

implement abstract methods


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Example - Extension

class Geo_Object


deferredend;

contains (p : Point) : Booleanis

deferred

end;end -- class Geo_Object

class Rectangle

inheritGeo_Object


featureorigin, corner : Point;

feature

height : Integer isdo ...end;

width : Integer isdo ...end;

...end -- class Rectangle

extending with

additional aspectsheight and width


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Example - Definition

class Geo_Object


deferredend;

contains (p : Point) : Booleanis

deferred

end;end -- class Geo_Object

class Rectangle

inheritGeo_Object


featureorigin, corner : Point;


doorigin := origin + p;corner := corner + p;

end;contains (p : Point) : Boolean

isdo

...end;

...end -- class Rectangle

definition of the

proposed aspectsmoveTo and contains


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Example - Redefinition

class DisplayableObjectfeature

psDescription: String isdeferredend;

end -- class DisplayableObject

class Rectangle...feature

initialize is doorigin.Create; corner.Create;origin.initialize;corner.initialize;

end...

end -- class Rectangleclass DisplayableRectangle

inheritRectanglerename initialize as initRectredefine initialize

...feature

color : Color;

featureinitialize is do

current.initRect;color.create; color.black;

end;...

end -- class DisplayableRectangle

the featureinitialize doesnt fit

anymore so it must be

redefined


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Design Classes

Relationships between classesuse

inherit

for specificationfor implementation

Code(Static)Classes

(partially) implemented abstract data types

Run-time(Dynamic)

ObjectsInstances of the classes

Relationships between objects

Send and receive messages

Aspects of OO Architectures

Classes are part of the

design andthe code


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Summary

Central modeling concepts

objects

classes

inheritance

Classes model concepts of the domain

Inheritance is used to model generalization /specialization

This leads to abstract classes

Subclasses can extent, redefine, define methods ofsuperclasses

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