Chapter 6-1

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.
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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();
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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
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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
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14. Creating Objects
 Generally, we use the new operator to create
an object
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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”;
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16. Constructing String objects
 New String objects are created whenever the
String constructors are used:
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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
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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
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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
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"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:
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num1 38
num2 96
Before:
num2 = num1;
num1 38
num2 38
After:

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

23. Questions
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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
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25. Graphical Representation of a
Class
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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
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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
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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( )
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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
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32. Method Control Flow
 If the called method is in the same class, only
the method name is needed
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myMethod();
myMethodcompute

33. Method Control Flow
 The called method is often part of another
class or object
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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?
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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
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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.
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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
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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:
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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