Objects and Components

The adaptive organization

The competitive environment of businesses continuously changing , and the pace of that change is increasing at an accelerating rate.

Where it was once possible for a company to stake out its marketing turf and defend its position for years, static positioning is now viable only in a few isolated industries.

For most companies today, the only constant is change.

Object Orientation

Basic building blocks of a system are the OBJECTSblack boxes encapsulating data and proceduresmessages can be send and received by objectsobject libraries and templates

Importance reusability maintainability flexibility

But OO languages needed not enough object libraries available training

Why objects ?

Grady Booch:

simply because there appears to be no other way to economically produce an enduring and resilient programming system.

David A. Taylor

objects are the enabling technology for adaptive business systems

Planning !!!

The waterfall model

projectdefinition

system study

design

programming

Installation

Post Imple-mentation

Project proposal report

Functional specifications

Feasibility report

design specifications

program specscode

tests of system performance

audit , feed-back

- intermediate reports

- go/nogo intervals

Number of problems

time

System reliability evolution

Why OO ? Good reasons

With the increasing complexity of the systems,the traditional techniques suffer from two

illusions:

The analyst knows everything and understands the problem completely before implementation starts

The users read the system analysis report and approve it

Project management

Iterative styledevelop a series of solutions to a problem ,each of them closer to satisfying the requirements( also called : evolutionary development )

Incremental styleBuilds system functionality a little at a time.The results are not entire solutions.

Matthew Pittman proposes iterative analysis and design combined with incremental development

OO - Life cycle

Facts:System requirements are not fully known at the startknowledge of the system grows during developmentbetter develop a system incrementallystart with some core functions

object modeling

analysis

design

construction

fullsystemdefinition

coordination and reuse

OMGLife Cycle

Database Evolution

File System Database OO - system

What is an object ?

An object is a software package that contains a collection of related procedures ( methods ) and data ( variables )

An object is a data abstraction with a state, a behavior and an identity

whereby operations are encapsulated together with the data structures on which they are defined

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The object paradigm

Five principles:

Abstraction

Encapsulation

Object communication , message passing

Inheritance

Polymorphism

Abstraction

an abstraction is a simplified view of some part of reality, focusing on some aspects , suppressing others

good abstraction emphasizes on those details that are important for the actual observer

the use of abstraction makes it possible to postpone decisions regarding details

an object is an abstraction of both data and functionality, with a focus on the outside view , separating the implementation of the object from the essential

Abstraction focuses upon the essential characteristics of some object, relative to the perspective of the user

From: Grady BoochAbstraction

EncapsulationFrom: Grady Booch

Encapsulation hides the details of the implementation of an object

Encapsulation

Encapsulation is the mechanism by which an object is made to look like a black box to an external observer

It is the packaging of a set of ideas into one logical unit, which can be referred to as a single unit

Good encapsulation hides design decisions .

An external observer sees what the object does, not how it does it.

It is the basis for reusability

Classes

A Class is a software template that defines the methods and variables to be included in a particular kind of object.

objects of the same structure and behavior belong to the same class

an object belongs to a class or is an instance of a class

objects only exist at run-time classes are set up at design time

a class can contain rules that all instances must satisfy

Messages passing

A message starts an operation on an object

A message is simply the name of an object, followed by the name of the method the object knows how to execute, and eventually followed by parameters

Objects can pass messages to other objects

Signature: The message signature contains the stipulation of the message format

a message consists of the name of a method and the required arguments

the set of messages to which an object responds is the behavior of the object

Message passing

The task of software development is to engineer the illusion of simplicity.

Message Interface

Message interface

The set of messages an object commits to respond to.

A class must specify the messages that objects of this type will make available to other objects

Interfaces

protect objects from being corrupted by other objects

protect other objects from depending on its internal structure

Interfaces may be segmented by combining them into composite interfaces

Anatomy of a message

A message consists of three parts

1. A receiver object2. A method the receiver knows how to execute3. An ordered set of parameters that this method

requires to carry out this function (optional)

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receiver method parameter

Messages conform to signatures

Inheritance

the ability of an instance of a class C to use not only the methods and variables defined for C but also those designed for an ancestor of C

Inheritance allows new classes to be build incrementally on existing classes

Message interfaces are also inherited students and staff members

are sub-classes of person

Person

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student

grade

Staff member

pay

Composite objects

Objects can contain other objects(Composite objects)

Collection class is a special kind of class

Benefits of Composition

even a deeply nested structure can be treated as a single, unified object

this helps manage complexity

Delegation

when an object assigns a task to another object

Method overriding

it is possible to override methods

Polygon

Give area

HexagonTriangle

Give area

Rectangle

Give area

Attention

Person

Head Leg

This is not inheritance (but Aggregation)

Polymorphism

The ability of different objects to respond differently to the same message is called POLYMORPHISM

e.g. DRAW NEW

A variable can point to an object whose class is not absolutely known

Number of problems

time

OO-System Reliability

With OO development techniquesA system is never replaced entirelycontinuous evolution lower implementation risk

More about objects

Components

Connecting objects through interfacesOMG: Object Management Group IDL: Interface Definition LanguageCORBA: Common Object Request Broker Architecture

The IDL specification allows objects from different vendors, written in different languages to interact without any knowledge of one another's classes.

The use of interfaces can eliminate most class dependencies.Exception made for inheritance where class references remain unavoidable.

Microsoft COM technology for ActiveX objects does not includes inheritance.

Distributing Objects

ORB

Message

Object Request Broker channel of communications

between remote objects message bus CORBA industry standard

IIOP Internet Interoperability ORB

protocol DCOM Microsoft

Distributed Component Object Model

RMI Sunsoft Remote Method Invocation JAVA

Messaging Services

Location transparency location service with ORB

Remoteness transparencyhides the fact that an object is located on another machine

PROXY objects

• local representative of a remote object

• the proxy presents the object’s interface to other objects on the local machine

• it forwards requests for services to the real object

• proxies allow objects to be moved without affecting any of the systems since they can be replaced by a new proxy

• Proxies can talk to other proxies

Mobile Objects

The ideal solution for distributed object systems is to allow objects to move themselves freely from machine to machine within a virtual object space.

JAVA offers the ability to define objects that can move themselves over a network.

Mobile objects support load balancing

Mobile objects permit flexible architectures.

Remote Messaging

Location Transparency

Remoteness Transparency

Mobile Objects

The Need for Rules

Objects contain methods for executing procedures and they have variables to hold the inputs and outputs of those procedures

Objects lack a good mechanism for capturing rules that determine when and how these procedures should be carried out.

Policies can be hard-coded into objects but that makes them rigid and hard to modify and not adaptive.

Apart from procedures and data, objects need rules to make them adaptive and intelligent.

Rules address the “when” and “whether” questions.

Design Business Rules

Business rules should remain visible and modifiable to managers instead of hiding them in the code of a method.

The business rules should be encapsulated within the objects and not be stored in a separate rule base.

This facilitates the integration of knowledge from different human experts (rules from marketing and from production)

Rules can also be designed as separate objects invoked from other object methods.

Rule Objects

Simplest form of a Rule class

1. Condition: variable or method that evaluates to true or false

2. Action: method defined within the class that contains the rule

Composite rules can be usefulthey can be made dynamic because they can

manage lists of other rulesit is possible to build rule classes that perform

automatic code generation (facilitated by dynamic languages like Smalltalk or JAVA)

Objects as Agents

If objects are mobile and use rules to guide their actions, they can take on a great deal of responsibility within a business system.

An object becomes an active agent, representing and guiding the actions of its real-world object.

Agent objects can :monitor the information systemswatch for changesscan the internet for useful informationcarry out routine business transactions….

Agent technology can be seen as an alternative for object technology, but it is better seen as an extension