Page 10 - DCAP308_OBJECT_ORIENTED_ANALYSIS_AND_DESIGN

Object Oriented Analysis and Design

Notes Each object “knows how to perform its operation.” This burden is on the class and not the
caller of the operation.
OOP selects the method to implement an operation based on the name and class of the
operation. The user of an operation need not be aware of implementation details.
Inheritance: Inheritance is a mechanism for sharing similarities among classes while
preserving their differences. Classes are typically defined broadly (higher level of
abstraction) and then refined into subclasses (lower level of abstraction).
Each subclass “inherits” all of the properties of the superclass and adds its own unique
properties. You can use an existing class to derive a new class. Derived classes inherit the
data and methods of the superclass. However, they can overwrite existing methods, and
also add new ones. Properties (both data and behavior) of the superclass need not be
repeated in the subclass.

The ability to abstract common properties of several classes into one common superclass
and inherit the properties from the superclass can greatly reduce repetition within designs
and programs (at the cost of coupling).

Example: Class ‘employee’ and class ‘customer’ are derived from the class ‘person’
New classes can be added without changing code given that methods are provided for each
applicable operation on the new class. Inheritance is used to model an “is a” relationship. A
subclass is always an instance of the superclass.

!
Caution Be careful, the reverse is not true.

Task Explain the concept of “Identity” with example.

1.1.1 Uses of Object Orientation

Below are some of the advantages of object-oriented programming:
Complex software systems become easier to understand, since object-oriented structuring
provides a closer representation of reality than other programming techniques.

In a well-designed object-oriented system, it should be possible to implement changes at
class level, without having to make alterations at other points in the system. This reduces
the overall amount of maintenance required.
Through polymorphism and inheritance, object-oriented programming allows you to
reuse individual components.
In an object-oriented system, the amount of work involved in revising and maintaining
the system is reduced, since many problems can be detected and corrected in the design
phase.
Achieving these goals requires:

Object-oriented programming languages: Object-oriented programming techniques do
not necessarily depend on object-oriented programming languages. However, the efficiency

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