introduction to objects classes

8/13/2019 Introduction to Objects Classes

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Programming with Objects and Classes

OO Programming ConceptsDeclaring and Creating Objects

ConstructorsModifiers ( public , private )


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Object

represents an entity in the real world thatcan be distinctly identified.

it has a unique identity, state and behaviors. the state of an object is represented by datafields (also known as properties)

The behavior of an object is defined by as set ofmethods.

is an instance of a class


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Class

is a template or blueprint that defines whatan objects data and methods will be.

one can create many instances of a class.Creating an instance is referred to asinstantiation.

is a construct that defines objects of thesame type.


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OO Programming Concepts

data field 1

method n

data field n

method 1

An object

...

...

State

Behavior

Data Fieldradius = 5

MethodfindArea

A Circle object


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Class and Objects

Circle

radius

findArea

circle1: Circle

radius = 2

new Circle()

circlen: Circle

radius = 5

new Circle()

...

Graphicalnotation for classes

Graphicalnotation for ob ects


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Class Declaration

class Circle{

double radius = 1.0;

double findArea(){

return radius*radius*3.14159;}

}


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Declaring Objects

ClassName objectName;

Example:Circle myCircle;


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Creating Objects

objectName = new ClassName();

Example:myCircle = new Circle();


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Declaring/Creating Objects

in a Single StepClassName objectName = new

ClassName();

Example:Circle myCircle = new Circle();


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Differences between variables of primitive Data types and object types

1

c: Circle

radius = 5

Primitive type int i = 1 i

Object type Circle c c reference

Created usingnew Circle(5)


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Copying Variables of PrimitiveData Types and Object Types

1

c1: Circle

radius = 5

Primitive type assignmenti = j

Before:

i

2 j

2

After:

i

2 j

Object type assignmentc1 = c2

Before:

c1

c2

After:

c1

c2

c2: Circle

radius = 9


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Accessing Objects

Referencing the objects data: objectName.data

myCircle.radius

Referencing the objects method: objectName.method

myCircle.findArea()


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Example 1

Using Objects Objective: Demonstrate creating objects,accessing data, and using methods. Sample: TestCircle.java


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Constructors

is a special kind of methodare invoked when a new object is created.are designed to perform initializing actions

such as initializing the data fields ofobjects.are used to construct objects, has exactly the

same name as the defining class.can be overloaded (multiple constructorswith the same name but different

signatures)


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Constructors

Circle(double r){

radius = r;}

Circle(){

radius = 1.0;}

myCircle = new Circle(5.0);


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Example 2

Using Constructors Objective: Discuss the role of constructorsand use them to create objects.Sample: TestCircleWithConstructors.java


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TestCircle1.javaCircle1.java


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Passing Objects to Methods

Passing by reference

Passing by value

Example : Passing Objects as Arguments

TestPassingObject.java


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Example 4Using the private Modifier

and Accessor Methods In this example, private data are used for the

radius and the accessor methods getRadius andsetRadius are provided for the clients to retrieve

and modify the radius.

TestCircleWithPrivateModifier.java


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Exercise:Write a class named Rectangle to represent rectangles. The

data fields are height and width declared as private. Youneed to provide the assessor methods for the propertiesand a method findArea() for computing the area of therectangle.

Write a client program to test the class Rectangle. In theclient program, create two Rectangle objects.

Assign any widths and heights to the two objects. Display both object's properties and find their areas . Call yoursetter method that would update the width and height ofthe rectangle by 20%.


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Sample Output:

introduction to objects classes

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