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COMP 144 Programming Language Concepts
Felix Hernandez-Campos
Lecture 22:
Object-Oriented Programming
Spring 2002
March 11
The University of North Carolina at Chapel Hill

2
Fundamental Concepts in OOP
• Encapsulation
– Data Abstraction
– Information hiding
– The notion of class and object
• Inheritance
– Code reusability
– Is-a vs. has-a relationships
• Polymorphism
– Dynamic method binding

3
Encapsulation
• Data abstraction allow programmers to hide data
representation details behind a (comparatively)
simple set of operations (an interface)
• What the benefits of data abstraction?
– Reduces conceptual load
» Programmers need to knows less about the rest of the program
– Provides fault containment
» Bugs are located in independent components
– Provides a significant degree of independence of program
components
» Separate the roles of different programmer
Software
Engineering
Goals

4
Encapsulation
Classes, Objects and Methods
• The unit of encapsulation in an O-O PL is a class
– An abstract data type
» The set of values is the set of objects (or instances)
• Objects can have a
– Set of instance attributes (has-a relationship)
– Set of instance methods
• Classes can have a
– Set of class attributes
– Set of class methods
Method calls are
known as messages
• The entire set of methods of an object is known as
the message protocol or the message interface of the
object

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Inheritance
• Encapsulation improves code reusability
– Abstract Data Types
– Modules
– Classes
• However, it is generally the case that the code a
programmer wants to reuse is close but not exactly
what the programmer needs
• Inheritance provides a mechanism to extend or
refine units of encapsulation
– By adding or overriding methods
– By adding attributes

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Inheritance
Notation
Java.awt.Dialog
Java.awt.FileDialog
Base Class
(or Parent Class
or Superclass)
Derived Class
(or Child Class
or Subclass)
Is-a relationship

7
Polymorphism
• The is-a relationship supports the development of
generic operations that can be applied to objects of a
class and all its subclasses
– This feature is known as polymorphism
– E.g. paint() method
• The binding of messages to method definition is
instance-dependent, and it is known as dynamic
binding
– It has to be resolved at run-time
– Dynamic binding requires the virtual keyword in C++
– Static binding requires the final keyword in Java

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Encapsulation
Modules and Classes
• The basic unit of OO, the class, is a unit of scope
– This idea originated in module-based languages in the
mid-70s
» E.g. Clu, Modula, Euclid
• Rules of scope enforce data hiding
– Names have to be exported in order to be accessible by
other modules
– What kind of data hiding mechanisms we have in Java?
» http://java.sun.com/docs/books/tutorial/java/javaOO/accesscontrol
.html
– And in Python?
» http://www.python.org/doc/current/tut/node11.html#SECTION00
11600000000000000000

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Classes and Encapsulation
Two Views
• Module-as-type
– A module is an abstract data type
– Standardized constructor and destructor syntax
– Object-oriented design is applied everywhere
– E.g. Java, Smalltalk, Eiffel, C++, Python
• Module-as-manager
– A module exports an abstract data type
– Create and destroy operations
– Object-oriented design is optional (OO as an extension)
– E.g. Ada 95, Modula-3, Oberon, CLOS, Perl

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Ada 95

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Ada 95

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Ada 95

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Reading Assignment
• Scott
– Read Ch. 10 intro
– Read Sect. 10.1
» Study the list and queue examples
– Read Sect. 10.2
» Go through the documents linked in slide 8

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