This document provides an introduction to object-oriented programming (OOP) concepts. It discusses problem solving using both structured programming and OOP approaches. The key concepts of OOP covered include objects, classes, methods, encapsulation, and inheritance. It also provides examples of defining a Rectangle class with attributes like length and width, and methods to calculate the area and perimeter. The document demonstrates how to create Rectangle objects, call methods on them to get the area, and use them in an application class.

1. Introduction to
Object-Oriented
Programming

2. Problem Solving
 The key to designing a solution is breaking it
down into manageable pieces
 When writing software, we design separate
pieces that are responsible for certain parts of
the solution

3.  Two popular programming design
methods:
 Structured Programming (SP)
 Object-Oriented Programming (OOP)
Problem Solving

4. SP
 Procedural based approach
 Main problem will be divided into sub-
problems
 Analyze and refine each sub-problem
 All sub-problem solutions are implemented as
procedures and combined to solves the main
problem

5. Object-Oriented
Programming
 Object based approach
 objects as foundation for problem solving
 Identify objects from the main problem
 An OOP program is a collection of objects
that interacts each other

6. Object-Oriented
Programming
 More natural way to solve problem
 Objects can be used to represent real-world
entities
 For instance, an object might represent a
particular employee in a company
 Each employee object handles the
processing and data management related to
that employee

7. Objects
 Object-oriented programs use objects, which
represent real world objects.
 A real world object is a thing, both tangible and
intangible.
 An object has:
 state (it has various properties, which might change)
 behaviour (it can do things and can have things done
to it)
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8. Objects
 OOProgramming software objects
 Software objects have state
 Instance Variable/Data/Field
 Software objects have behaviour
 Method
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Object’s
members

9. Objects interactions
 OOP also involves interactions between
objects through calling (invoking) of
methods.
9
Method CallsThe OilSensor object calls the warning() method
of the Controller, which then invokes the OilLight
TurnOn() method.

10. Method can Return Values
 Return value: A result that the method has
computed and returns it to the caller
 Can returns 0 or 1 value
Scanner s = new Scanner(System.in);
int num = s.nextInt();
10
Continued…

11. Classes and Objects
 To create an object , we MUST provide a
definition/description for it
 A class is a description/blue print of a kind of
object
 It does not by itself create any objects
How to create an object ?
 An object is called an instance of a class
Object instantiation – process of creating an object
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12. Example of Java class:
The String Class
12

13. Creating Objects
 A variable holds either a primitive type or a
reference to an object
 A class name can be used as a type to declare an
object reference variable
String name;
 No object is created with this declaration
 An object reference variable holds the address of
an object
 The object itself must be created separately
13

14. Creating Objects
 Generally, we use the new operator to create
an object
14
name = new String (“Ali bin Ahmad");
This calls the String constructor, which is
a special method that sets up the object
• Creating an object is called instantiation
• An object is an instance of a particular class

15. Constructing String
objects
Strings stringRef = new String(stringLiteral);
eg.
String name = new String(“Muhammad Haziq”);
Since strings are used frequently, Java provides a
shorthand notation for creating a string:
String name = "Muhammad Haziq”;
15

16. Constructing String objects
 New String objects are created whenever the
String constructors are used:
16
String name4 = new String(); // Creates an object
String name5 = new String("Socrates");
String name6 = name4;

17. Invoking Methods
 We've seen that once an object has been instantiated, we
can use the dot operator to invoke its methods
name.length()
 A method may return a value, which can be used in an
assignment or expression
count = name.length();
S.o.p(“Num. of char in “ + name+ “=“ + count);
 A method invocation can be thought of as asking an object
to perform a service
17

18. Object without object reference
cannot be accessed
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String n1 = new String(“Ali“);
new String(“Abu“);
sv1: String
: String
value = “Ali”
value = “Abu”
n1
n1- object reference variable

19. Object References
 Primitive type variables ≠ object variables
19

20. References
 Note that a primitive variable contains the value
itself, but an object variable contains the address
of the object
 An object reference can be thought of as a pointer
to the location of the object
 Rather than dealing with arbitrary addresses, we
often depict a reference graphically
20
"Steve Jobs"name1
num1 38

21. Assignment Revisited
 The act of assignment takes a copy of a value
and stores it in a variable
 For primitive types:
21
num1 38
num2 96
Before:
num2 = num1;
num1 38
num2 38
After:

22. Object Reference
Assignment
 For object references, assignment copies the
address:
22
name2 = name1;
name1
name2
Before:
"Steve Jobs"
"Steve Austin"
name1
name2
After:
"Steve Jobs"

23. Questions
23
String stud1 = new String(“Ani”);
int studID = 65000;
What does variable stud1 contains?
What does variable studID contains?
Is this allowed? stud1 = studID;
String stud1;
stud1 = new String(“Ani”);
stud1 = new String(“Obi”);
How many objects were created by the program?
How many reference variables does the program contain?

24. Writing Classes
 The programs we’ve written in previous
examples have used classes defined in the Java
standard class library
 Now we will begin to design programs that rely
on classes that we write ourselves
 True object-oriented programming is based on
defining classes that represent objects with
well-defined characteristics and functionality
24

25. Graphical Representation of a
Class
25
The notation we used here is based on the industry standard notation
called UML, which stands for Unified Modeling Language.
A UML Class Diagram
Class Name
Variables
Method
Type of data
Access
Type of
return
value
A class can
contain data
declarations and
method
declarations

26. Object Instantiation
26
class
object

27. Object Design Questions
 What role will the object perform?
 What data or information will it need?
 Look for nouns.
 Which actions will it take?
 Look for verbs.
 What interface will it present to other objects?
 These are public methods.
 What information will it hide from other objects?
 These are private.
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28. Design Specification for a
Rectangle
 Class Name: Rectangle
 Role: To represent a geometric rectangle
 States (Information or instance variables)
- Length: A variable to store rectangle’s length
(private)
- Width: A variable to store rectangle's width (private)
 Behaviors (public methods)
- Rectangle(): A constructor method to set a rectangle’s
length and width
- calculateArea(): A method to calculate a rectangle’s
area
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29. UML Design Specification
29
UML Class Diagram
Class Name
What data does it need?
What behaviors
will it perform?
Public
methods
Hidden
information
Instance variables -- memory locations
used for storing the information needed.
Methods -- blocks of code used to
perform a specific task.

30. Method can has input (parameter) &
output (return value)
 Parameter : value given to method so that it can
do its task
 Can has 0 or more parameter
 Return value: A result that the method has
computed and returns it to the caller
 Can returns 0 or 1 value
 Eg. - pow(2,3)
- calculateArea()
- getBalance( )
- move( )
30
Continued…

31. Method Declarations
 A method declaration specifies the code that will
be executed when the method is invoked (called)
 When a method is invoked, the flow of control
jumps to the method and executes its code
 When complete, the flow returns to the place
where the method was called and continues
 The invocation may or may not return a value,
depending on how the method is defined
31

32. Method Control Flow
 If the called method is in the same class, only
the method name is needed
32
myMethod();
myMethodcompute

33. Method Control Flow
 The called method is often part of another
class or object
33
doIt helpMe
helpMe();obj.doIt();
main

34. Method Design
 What specific task will the method perform?
 What input data will it need to perform its
task?
 What result will the method produce?
 How input data are processed into result?
 What algorithm will the method use?
34

35. Method calculateArea()
Algorithm
 Method Name: calculateArea()
 Task: To calculate the area of a rectangle
 Data Needed (variables)
 length: A variable to store the rectangle's length
 width: A variable to store the rectangle's width
 area: A variable to store result of calculation
 Processing: area = length x width
 Result to be returned: area
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36. Coding into Java
36
public class Rectangle // Class header
{
private double length; // Instance variables
private double width;
public Rectangle(double l, double w) // Constructor method
{
length = l;
width = w;
}
public double calculateArea() // calculate area method
{
double area;
area = length * width;
return area;
} // calculateArea()
} // Rectangle class

37. Method calculatePerimeter()
Algorithm
 Write an algorithm to calculate the perimeter
of a rectangle.
 Write the method in Java.
37

38. calculatePerimeter()
Algorithm
 Method Name: calculatePerimeter()
 Task: To calculate the perimeter of a
rectangle
 Data Needed (variables)
 length
 width
 perimeter
 Processing: perimeter = 2 x(length + width)
 Result to be returned: perimeter
38

39. calculatePerimeter() in Java
code
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public double calculatePerimeter()
{
double perimeter;
perimeter = 2 * (length + width);
return perimeter;
} // calculatePerimeter()

40. Creating Rectangle Instances
 Create, or instantiate, two instances of the
Rectangle class:
40
The objects (instances)
store actual values.
Rectangle rectangle1 = new Rectangle(30,10);
Rectangle rectangle2 = new Rectangle(25, 20);

41. Using Rectangle Instances
 We use a method call to ask each object to tell
us its area:
41
rectangle1 area 300
rectangle2 area 500Printed output:
System.out.println("rectangle1 area " + rectangle1.calculateArea());
System.out.println("rectangle2 area " + rectangle2.calculateArea());
References to
objects
Method calls

42. Syntax : Object Construction
 new ClassName(parameters);
 Example:
 new Rectangle(30, 20);
 new Car("BMW 540ti", 2004);
 Purpose:
 To construct a new object, initialize it with the
construction parameters, and return a
reference to the constructed object.
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43. The RectangleUser Class
Definition
public class RectangleUser
{
public static void main(String argv[])
{
Rectangle rectangle1 = new Rectangle(30,10);
Rectangle rectangle2 = new Rectangle(25,20);
System.out.println("rectangle1 area " +
rectangle1.calculateArea());
System.out.println("rectangle2 area " +
rectangle2.calculateArea());
} // main()
} // RectangleUser
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An application must
have a main() method
Object
Use
Object
Creation
Class
Definition