Overview of object concepts
Object concepts are not limited to computer
programming. Fundamentally, object concepts provide a framework for modelling in
terms of the objects which make up a system. Other, more traditional, modelling techniques
have concentrated on either the data managed by a system (e.g. data flow modelling)
or on the operations the system carries out (e.g. functional decomposition).
Object-oriented analysis and design allows both data and operations to be modelled
(encapsulated) together, in a way that can be used to design and implement computer
programs that preserve and exploit such an encapsulated view of systems and their
component objects.
The fundamental concept in all object-oriented
technologies is the object. At a high level of abstraction an object is a software
component that has:
- behaviour: an object can do work
- state: an object can remember aspects of its work
- individuality: an object can be distinguished from other objects without
having to look at any part of its state – i.e. each object can be referred to via
an individual identifier
An object is an instance of a class. There may be
many objects all based on the same class.
To imagine an instance of a class, consider the
Notepad application that is provided as an accessory in Windows operating systems.
(Whether or not Notepad is object-oriented does not matter, it behaves in an
object-oriented manner.) You can imagine starting Notepad a number of times (see later
activity) Each time you start an instance of Notepad is like creating an object of the
class Notepad. Each instance is different from another although we as humans can probably
only tell the difference by what is written on the pad or by the name we give to a note
pad document.
An important part of analysing what classes might be
involved in an object-oriented system also involves analysing what responsibilities
objects of a class might have and how they might collaborate with objects of other
classes. This should become apparent as you go through the module.
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