creating object

7/27/2019 Creating Object

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Chapter 6 Objects and Classes

OO Programming ConceptsCreating Objects and Object Reference Variables Differences between primitive data type and object type

Automatic garbage collectionConstructorsModifiers ( public , private and static )

Instance and Class Variables and MethodsScope of VariablesUse the this KeywordCase Studies ( Mortgage class and Count class)


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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 1 : Circle

radius = 2

new Circle()

circle n : Circle

radius = 5

new Circle()

...

UML Graphical notation for classes

UML Graphical notationfor objects

Circle

radius: double

findArea(): double

UML Graphical notation for fields

UML Graphical notation for methods


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Class Declaration

class Circle {double radius = 1.0;

double findArea(){return radius * radius * 3.14159;

}}


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Declaring Object Reference Variables

ClassName objectReference;

Example:Circle myCircle;


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Creating Objects

objectReference = new ClassName();

Example:myCircle = new Circle();

The object reference is assigned to the objectreference variable.


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Declaring/Creating Objects

in a Single StepClassName objectReference = 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 = 1

Primitive type int i = 1 i

Object type Circle c c reference

Created usingnew Circle()


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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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Garbage Collection

As shown in the previous figure,after the assignment statement c1 =c2, c1 points to the same objectreferenced by c2. The objectpreviously referenced by c1 is nolonger useful. This object is knownas garbage. Garbage isautomatically collected by JVM.


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Garbage Collection, cont

TIP: If you know that an object is nolonger needed, you can explicitlyassign null to a reference variablefor the object. The Java VM willautomatically collect the space if theobject is not referenced by anyvariable.


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Accessing Objects

Referencing the objects data: objectReference.data

myCircle.radius

Invoking the objects method: objectReference.method

myCircle.findArea()


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Example 6.1 Using Objects

Objective: Demonstrate creating objects,

accessing data, and using methods.

TestCircle Run
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestCircle.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestCircle.htm


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Constructors

Circle(double r) {radius = r;

}

Circle() {radius = 1.0;

}

myCircle = new Circle(5.0);

Constructors are aspecial kind of methods that areinvoked to constructobjects.


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Constructors, cont.

A constructor with no parameters is referred toas a default constructor .

Constructors must have the same name

as the class itself. Constructors do not have a return type

not even void.

Constructors are invoked using the newoperator when an object is created.Constructors play the role of initializingobjects.


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Example 6.2 Using Classes from

the Java Library Objective: Demonstrate using classes from theJava library. U se the JFrame class in the

javax.swing package to create two frames;use the methods in the JFrame class to setthe title, size and location of the frames andto display the frames.

TestFrame Run
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestFrame.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestFrame.htm


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Example 6.3 Using Constructors

Objective: Demonstrate the role of constructors and use them to createobjects.

TestCircleWithConstructors Run
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestCircleWithConstructors.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestCircleWithConstructors.htm


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Visibility Modifiers andAccessor Methods

By default, the class, variable, or data can beaccessed by any class in the same package.

public

The class, data, or method is visible to any class in any package.

private The data or methods can be accessed only by the declaringclass.

The get and set methods are used to read and modify private properties.


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Example 6.4Using the private Modifier

and Accessor Methods

TestCircleWithAccessors Run

In this example, private data are used for theradius and the accessor methods getRadius andsetRadius are provided for the clients to retrieve

and modify the radius.
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestCircleWithAccessors.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestCircleWithAccessors.htm


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Passing Objects to Methods

Passing by value (the value is the referenceto the object)

Example 6.5 Passing Objects as Arguments

TestPassingObject Run
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestPassingObject.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestPassingObject.htm


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Passing Objects to Methods, cont.

mainmethod

Reference myCircle

5n 5

times

printAreasmethod

Reference

c

myCircle: Circle

radius = 1

Pass by value (here the value is 5)

Pass by value (here the value is thereference for the object)


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InstanceVariables, and Methods

Instance variables belong to a specific instance.

Instance methods are invoked by an instance of the class.


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Class Variables, Constants,and Methods, cont.

To declare class variables, constants, and methods,use the static modifier.


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Class Variables, Constants,and Methods, cont.

CircleWithStaticVariable

-radius-numOfObjects

+getRadius(): double+setRadius(radius: double): void+getNumOfObjects(): int+findArea(): double

1 radiuscircle1:Circle

-radius = 1-numOfObjects = 2

instantiate

instantiate

Memory

2

5 radius

numOfObjects

radius is an instancevariable, andnumOfObjects is aclass variable

UML Notation:+: public variables or methods-: private variables or methodsunderline: static variables or metods

circle2:Circle

-radius = 5-numOfObjects = 2


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Scope of Variables

The scope of instance and class variables is theentire class. They can be declared anywhere inside

a class.The scope of a local variable starts from its

declaration and continues to the end of the block

that contains the variable. A local variable must bedeclared before it can be used.


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The Keyword this

Use this to refer to the current object.

Use this to invoke other constructors of the

object.


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Array of Objects

Circle[] circleArray = new Circle[10];

An array of objects is actually anarray of reference variables . Soinvoking circleArray[1].findArea()involves two levels of referencing asshown in the next figure. circleArrayreferences to the entire array.circleArray[1] references to a Circleob ect.


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Array of Objects, cont.

reference Circle object 0circleArray[0]

circleArraycircleArray[1]

circleArray[9] Circle object 9

Circle object 1

Circle[] circleArray = new Circle[10];


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Class Abstraction

Class abstraction means to separate classimplementation from the use of the class. Thecreator of the class provides a description of theclass and let the user know how the class can beused. The user of the class does not need toknow how the class is implemented. The detailof implementation is encapsulated and hiddenfrom the user.


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Example 6.8 The Mortgage Class

Mortgage

-annualInterestRate: double-numOfYears: int

-loanAmount: double

+Mortgage()+Mortgage(annualInterestRate: double,

numOfYears: int, loanAmount: double)+getAnnualInterestRate(): double+getNumOfYears(): int

+getLoanAmount(): double+setAnnualInterestRate(annualInteresteRate: double): void+setNumOfYears(numOfYears: int): void+setLoanAmount(loanAmount: double): void+monthlyPayment(): double+totalPayment(): double

TestMortgageClass

Run

Mortgage
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestMortgageClass.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/Mortgage.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/Mortgage.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestMortgageClass.htm


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Example 6.9 The Count Class

Run TestV oteCandidate
http://localhost/var/www/apps/conversion/tmp/scratch_8/TestCountClass.htmhttp://localhost/var/www/apps/conversion/tmp/scratch_8/TestCountClass.htm


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Java API and Core Java classes

java.lang Contains core Java classes, such as numericclasses, strings, and objects. This package is

implicitly imported to every Java program. java.awt Contains classes for graphics.

java.applet Contains classes for supporting applets.


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java.io Contains classes for input and outputstreams and files.

java.util Contains many utilities, such as date.

java.net Contains classes for supporting

network communications.

Java API and Core Java classes,cont.


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java.awt.image Contains classes for managing bitmap images.

java.awt.peer Platform-specific GUI implementation.

Others:

java.sql java.rmi

Java API and Core Java classes,cont .

creating object

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