11
Object-Oriented Design
Simple Program Design
Third Edition
A Step-by-Step Approach
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Objectives
• To introduce object-oriented design
• To define objects, classes, attributes,
methods and information hiding
• To list the steps required to create an
object-oriented design to a problem
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Introduction to Object-Oriented
Design
• Object-oriented design asks that we interact with the
problem in much the same way that we interact with
our world — we treat it as a set of separate objects
that perform actions and relate to each other
• An object-oriented program is a set of interacting
objects rather than a set of functions
• In object-oriented design you need to identify the data
in the objects as well as the processes or actions that
can be performed on that data
• Objects are said to encapsulate their data, and the
processes that act on those data
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Objects
• An object can be considered as a container for a
set of data, and the operations that need to be
performed on it
• An object has the following properties:
– It has a name that is unique for the lifetime of the object
– It has data in the form of a set of characteristics or attributes,
each of which has a value at any given point in time
– There is a set of operations or methods that can be performed
on the data
– It is an instance (example) of a class
• In a computer program an object’s attributes are
the properties or characteristics that describe it
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Methods
• Objects receive messages from other objects,
asking them to perform services or operations
• Each object has a number of methods or set of
operations, sometimes called procedures or
functions
• Methods can manipulate the values stored in an
object’s attributes, but can only act on the data
inside the object or on values passed to the
object
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Classes and Objects
• An object is created from a template or
pattern, called a class, which defines the
basic attributes and the methods available
to objects of that class
• The process of creating objects from
classes is called instantiation, and an
object is described as an instance, or
example of its class
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Constructors
• The process of instantiating an object from the
class template is performed by a special method
known as a constructor
• Every class should have a default constructor
that initializes the class attributes
• The constructor may:
– have no parameters
– have parameters that initialize the attributes with those
particular values
• Classes can be progressively created from other
classes by copying attributes and methods from
the base classes
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Constructors
• In a banking program, an Account class is
created to manage customer accounts
• The class Account includes attributes such as
accountName and balance, as well as methods
such as deposit and withdraw money (refer to the
diagram on page 177 of the textbook)
• Every ChequeAccount object that is created by
the program, contains all the properties and
methods of the Account class as well as the extra
features of the ChequeAccount class
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Information Hiding
• In object-oriented design, each object can be
regarded as a ‘black box’ whose internal
workings are hidden from all other objects
• This principle of information hiding simplifies
the use of objects, because the rest of the
system does not need to know how they are
structured or how they perform their operations
• The goal of information hiding is to make the
object as robust and independent as possible
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Public and Private Methods
• An object’s methods, or set of operations,
can be public or private
• The public methods of an object are those
producing services requested by other
objects
• Objects must be able to communicate and
interact with the rest of the program, and this
communication is achieved by the passing of
messages
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Steps in Creating an ObjectOriented Solution
• The steps in creating an OO solution are:
– Identify the objects and the methods to be
performed
– Determine the relationship between the objects
– Design the algorithms for the methods, using
structured design
– Develop the mainline algorithm
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Programming Example Using
Object-Oriented Design
• Example 11.2 Print student results
• Design a class to manage student results in a
subject. Each student is identified by a unique
student number. During the course of the
subject, each student completes three
assignments representing 40% of the final mark
but each scored out of 100, and an examination,
also scored out of 100 marks
• Use the four steps referred to in the textbook on
pages 184 to the top of 187 to do this
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Interface and GUI Objects
• An interface is a device in a program that connects
the user’s responses to the computer’s actions
• Many popular programming languages provide a
graphical user interface (GUI), which enables the
programmer to select the elements of the program’s
user interface from a pre-existing range of options
• These languages are call ‘visual’ languages, and
include Visual Basic, Visual C, and Visual J
• Java also shares these features
• The user interface options may include windows,
buttons, menus, boxes to hold text, drop-down lists
and many more
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Example 11.3 Library locater
interface
• Use this example as a guide, on pages 187
to the middle of page 189 of the textbook
• Also refer to the diagrams on pages 188
and 189 of the textbook
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Summary
• Object-oriented design is a fundamentally
different process to procedural design
• Instead of decomposing the problem into
functions, the problem is broken up into the
objects in the system, and the attributes and
methods for each object are identified
• Objects encapsulate their data and methods,
and can be regarded as ‘black boxes’ for the
purposes of large system design
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Summary
• Objects are instantiated from classes that are
templates defining the attributes and
methods for objects of the type, but
individual objects of the same type may store
different data values in their attributes
• Interface design for visual programming
languages uses object-oriented design
principles
• Interface objects have methods and attributes
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