1. Object-Oriented Programming
Lecture 05
Introduction to
Object-Oriented Programming
Jaeki Song

2. Outlines
Characteristics of OOP
Object
Class
Inheritance
Polymorphism

3. Principles of Good Programming
Modularity
Reusability
Extensibility
Ease of maintenance
Preservation of data integrity
Efficient debugging

4. Characteristics of OOP
Everything is an object
A program is a bunch of objects telling each other what to do by sending messages
Each object has its own memory made up of other objects
Every object has a type
All objects of a particular type can receive the same message

5. Object A natural “thing”
Any object can be identified, classified, and defined
E.g., dog, desk, television set, bicycle
Objects are key to understanding object-oriented technology
Each object has a set of attributes that describe it
Desk can be described in terms of its weight, color, and the material it is made of

6. Object Has a unique identity
Object knows (state) and can do (behavior)
The state consists of a set of fields with current values
The behavior is defined by a set of methods
Change
gear
Methods
(behavior)
Variables
(state)
3rd gear
brake

7. Object: Message
Objects interact and communicate with each other by sending message to each other
changeGears (lowGear)
Your car
You

8. Object: Message Important benefits
An object’s behavior is expressed through its methods
Message passing supports all possible interaction between objects
Object don’t need to be in the same process

9. Object Combination of data and code (or methods)
The process of packaging an objects’ data together with its code is termed “data encapsulation”
E.g.
Car (Object) has data variables (describing the state of car including car’s engine type, its current speed, its make, and its model).
A car also has methods (brake, accelate, changeGear) operating on its data

10. Object Encapsulation provides the ability to hide data “data hiding”
Objects are composed of internal (private) sections and external (public) sections
Private is a combination of internal data and methods
External section is often referred to as its “interface”
Presents everything that the external users of the object need to know or are allowed to know

11. Class
Each individual object in a class is called an instance of that class
Change
gear
Change
gear
currentSpeed = 50
currentSpeed = 75
brake
brake
currentGear = 4
currentGear=3
MyCar
YourCar

12. Class
Can be created by defining all the data and associated methods for a given set of objects
Provides the benefit of reusability
Reuse the code of a given class many times to create new objects
Each object gets its own data but shares a single set of methods with other objects in its class

13. Class Hierarchical structure Supper class vs. subclass
Every class is derived from a superclass
classKeyword and classBody
classKeyword-identifier, nameOfClass
classBody – dataVariables (instance variables) and methods

14. Access Control
The goal of class creator (those who create class) is to build a class that exposes only what’s necessary to the client programmer (the class consumers who use the class in their application) and keeps everything else hidden
Class creator can change the hidden portion without worrying about the impact to anyone else
Class creator can change the internal working of the class without worrying about how it will affect the client programmer
Java use three boundaries
public, private, and protected

15. Reusability Use an object of the class directly
Place an object of the class inside a new class
“creating a member object”  composition or aggregation
car
engine
“A car has an engine”

16. Inheritance
A new class can be defined which inherits all the attributes that have been defined for an existing class
Inheritance provides the benefit of code reusability

17. Inheritance
Take the existing class, clone it, and then make additions and modifications to the clone
However, if the original class (bass or super or parent class) is changed, the modified “clone” (inherited or sub or child class) reflects those changes

18. Inheritance
A super class contains all of the characteristics and behaviors that are shared among the classes derived from it
Create a supper class to represent the core of your idea and derive other classes to express the different ways that this core can be realized

19. Inheritance Shape draw() erase() move() Circle Square Triangle draw()
FlipVertical()
FlipHorizontal()
extension

20. Polymorphism Greek words
“Multiple” and “Shape”
Perform the same operation on different types of classes as long as they share a common trait
Define a generic command which is implemented by a number of related classes
Helps avoid complexity and redundancy

21. Polymorphism Interchangeable objects
Allows an instruction to be given to an object using a generalized, rather than a specifically detailed
Two forms
Method overloading
Different types of information in the message being sent to an object
Method overriding
Dynamic binding