Object Oriented Design Concept

©Ian Sommerville 1995 Software Engineering, 5th edition. Chapter 14 Slide 1
Object-oriented Design
Designing systems using self-
contained objects and object
classes

Objectives
To explain how a software design may be
represented as a set of interacting objects
To illustrate, with a simple example, the object-
oriented design process
To introduce various models which describe an
object-oriented design
To explain how objects may be represented as
concurrent processes

Topics covered
Objects, object classes and inheritance
Object identification
An object-oriented design example
Concurrent objects

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 and may execute
sequentially or in parallel

OOD structure
state 3
O3
state 4
O4
state 1
O1
state 6
O6
state 5
O5
state 2
O2

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 C++

Object-oriented design methods
Some methods which were originally based on
functions (such as the Yourdon method) have
been adapted to object-oriented design. Other
methods such as the Booch method have been
developed specifically for OOD
HOOD is an object-oriented design method
developed to support Ada programming.
JSD has an object-oriented flavour but does not
conceal entity state information.

OO Design method commonality
The identification of objects, their attributes and
services
The organisation of objects into an aggregation
hierarchy
The construction of dynamic object-use
descriptions which show how services are used
The specification of object interfaces

Objects, classes and inheritance
Objects are entities in a software system which
represent instances of real-world and system
entities
Object classes are templates for objects. They
may be used to create objects
Object classes may inherit attributes and services
from other object classes

Objects
An object is an entity which has a state and a defined set of
operations which operate on that state. The state is represented as a
set of object attributes. The operations associated with the object
provide services to other objects (clients) which request these
services when some computation is required. Objects are created
according to some object class definition. An object class definition
serves as a template for objects. It includes declarations of all the
attributes and services which should be associated with an object of
that class.

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 calls
• Name = procedure name.
• Information = parameter list.

A mail message object class
Replace with portrait slide

Inheritance
Objects are members of classes which define
attribute types and operations
Classes may be arranged in a class hierarchy
where one class is derived from an existing
class (super-class)
A sub-class inherits the attributes and
operations from its super class and may add
new methods or attributes of its own

A class or type hierarchy

Multiple inheritance

Advantages of inheritance
It is an abstraction mechanism which may be
used to classify entities
It is a reuse mechanism at both the design and the
programming level
The inheritance graph is a source of
organisational knowledge about domains and
systems

Inheritance and OOD
There are differing views as to whether
inheritance is fundamental to OOD.
• View 1. Identifying the inheritance hierarchy or network is a
fundamental part of object-oriented design. Obviously this can
only be implemented using an OOPL.
• View 2. Inheritance is a useful implementation concept which
allows reuse of attribute and operation definitions. Identifying
an inheritance hierarchy at the design stage places unnecessary
restrictions on the implementation.

Object identification
Identifying objects 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. Used in the
ObjectOry method

Objects and operations
Nouns in the description give pointers to objects
in the system
Verbs give pointers to operations associated with
objects
Approach assumes that the designer has a
common sense knowledge of the application
domain as not all objects and services are likely
to be mentioned in the description

An office information system
The Office Information Retrieval System (OIRS) is
an automatic file clerk which can file documents
under some name in one or more indexes, retrieve
documents, display and maintain document
indexes, archive documents and destroy
documents. The system is activated by a request
from the user and always returns a message to the
user indicating the success or failure of the request.

Preliminary object identification
File
Retrieve
A rchive
Destroy
Docume nt
Name
Display
Delete entry
A dd entry
I ndex
Name
Get command
Put message
U se r R e trieva l
sy ste m
User command

Object interface design
Concerned with specifying the detail of the object
interfaces. This means defining attribute types
and the signatures and semantics of object
operations
Representation information should be avoided
Precise specification is essential so a
programming language description should be
used

Concurrent objects
The nature of objects as self-contained entities
make them suitable for concurrent
implementation
The message-passing model of object
communication can be implemented directly if
objects are running on separate processors in a
distributed system

Active and passive objects
Passive objects.
• The object is implemented as a parallel process (server)
with entry points corresponding to object operations. If no
calls are made to it, the object suspends itself
Active objects
• Objects are implemented as parallel processes and the
internal object state may be changed by the object itself and
not simply by external calls

OOD is design with information hiding.
Representations may be changed without
extensive system modifications
An object has a private state with associated
constructor and access operations. Objects
provide services (operations) to other objects.
Object identification is a difficult process.
Identifying nouns and verbs in a natural
language description can be a useful starting
point for object identification.
Key points

Key points
Object interfaces must be precisely defined. A
programming language such as Ada or C++ may
be used for this
Useful documentation of an OOD include object
hierarchy charts and object interaction
diagrams
Objects may be implemented as either
sequential or concurrent entities. Concurrent
objects may be active or passive

Object Oriented Design Concept

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