oop chapter 3 by jlncrnl

8/11/2019 OOP chapter 3 by JLNCRNL

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Chapter 3Using Classes andObjects


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2004 Pearson Addison-Wesley. All rights reserved 3-2

Using Classes and Objects

We can create more interesting programs usingpredefined classes and related objects

Chapter 3 focuses on:

object creation and object referencesthe String,Character and StringBuilder classand its methodsthe Java standard class librarythe Random and Math classes

formatting outputenumerated typeswrapper classesgraphical components and containerslabels and images


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OutlineCreating Objects

String, Character and StringBuilder

Packages

Formatting Output

Wrapper Classes

Components and Containers

Images


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Creating Objects

A variable holds either a primitive type or areference to an object

A class name can be used as a type to declare anob ject reference v ar iable

String title;

No object is created with this declaration

An object reference variable holds the address ofan object

The object itself must be created separately


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Creating Objects

Generally, we use the new operator to create anobject

title = new String ("Java Software Solutions");

This calls the String o stru tor which isa special method that sets up the object

Creating an object is calledinstant ia t ion

An object is an ins tance of a particular class


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Invoking Methods

We've seen that once an object has beeninstantiated, we can use the do t operato r to invokeits methods

count = title. length ()

A method may return a value , which can be usedin an assignment or expression

A method invocation can be thought of as askingan object to perform a service


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References

Note that a primitive variable contains the valueitself, but an object variable contains the addressof 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

"Steve Jobs"name1

num1 38


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Assignment Revisited

The act of assignment takes a copy of a value andstores it in a variable

For primitive types:

num1 38num2 96

Before:

num2 = num1;

num1 38

num2 38After:


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Reference Assignment

For object references, assignment copies theaddress:

name2 = name1;

name1

name2

Before:"Steve Jobs"

"Steve Wozniak"

name1

name2After:

"Steve Jobs"


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Aliases

Two or more references that refer to the sameobject are called aliases of each other

That creates an interesting situation: one objectcan be accessed using multiple reference

variables


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Garbage Collection

When an object no longer has any valid referencesto it, it can no longer be accessed by the program

The object is useless, and therefore is calledgarbage

Java performs autom atic garbage co l lect ion periodically, returning an object's memory to thesystem for future use


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String,Character and StringBuilder

Packages

Formatting Output

Wrapper Classes


Images


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Character is a class whose instances can hold asingle character value.

Strings is a class for working fixed-string data-that is unchanging data composed of multiple

characters. StringBuilder and StringBuffer are Classes for

storing and manipulating changeable datacomposed of multiple characters.


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The Character Class

Commonly used methods of the Character Class

isUpperCase() toUpperCase() isLowerCase()

toLowerCase() isDigit() isLetter()

isLetterOrDigit() isWhiteSpace()

The Chracter class is defined in java.lang


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The Character Classimport java.util.*;

public class TestCharacter { public static void main(String[]args){

char aChar;String aString;Scanner keyboard=new Scanner

(System.in);System.out.println("Enter a character:");aString=keyboard.nextLine();aChar=aString.charAt(0);System.out.println("The character is"+

aChar);

if(Character.isUpperCase(aChar))System.out.println(aChar +"is Upper

case");else

System.out.println(aChar+"is not Upper

case");if(Character.isLowerCase(aChar))

System.out.println(aChar +"is Lowercase");

elseSystem.out.println(aChar+"is not lower

case");

if (Character.isLetterOrDigit(aChar))System.out.println(aChar+"is a letter or a

digit");

elseSystem.out.println(aChar+"is neither a

letter nor a digit");

if (Character.isWhitespace(aChar))System.out.println(aChar+"is

whitespace");else

System.out.println(aChar+"is not awhitespace"); } }

Output:Enter a character: AThe character is A

A is Upper caseA is not lower caseAfter toLOwerCase(), aChar is aAfter UpperCase(), aChar is AA is a letter or a digitA is not a whitespace


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The String Class Java Strings is a class and each created String is a

class object

As an object a String variable name is not a simpledata type- it is a reference, a variable that holds a

memory address.title = "Java Software Solutions


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The String ClassString aGreeting=Hello;

Address 10876, nameaGreeting

aGreeting holds theaddress where Hello is stored.

address 26040

address 32564

String aGreeting=Hello; aGreeting= Bonjour

Address 10876, nameaGreeting

aGreeting holds theaddress where Hello is stored.

address 26040

address 32564

26040

Hello

XXYYZZ23

26040

Hello

XXYYZZ23


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String Methods Once a String object has been created, neither its

value nor its length can be changed

Thus we say that an object of the String class isimmutab le

However, several methods of the String classreturn new String objects that are modifiedversions of the original


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The String Class

Commonly used methods of the String Class

Length() toUpperCase() indexOf()

toLowerCase() charAt() endsWith()

startsWith() replace()

-The Chracter class is defined in java.lang


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String Indexes It is occasionally helpful to refer to a particular

character within a string

This can be done by specifying the character'snumeric index

The indexes begin at zero in each string

In the string "Hello" , the character 'H' is at index0 and the 'o' is at index 4


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StringBuilder Used to improve performance when a strings

contents must change.

Sufficient memory space is allocated toaccommodate the number of Unicode characters

in the strings. Contains a memory block called buffer .


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2004 Pearson Addison-Wesley. All rights reserved

StringBuilderimport javax.swing.*;public class StringBuilderDemo {

public static void main(String[]args){

StringBuilder nameString=new StringBuilder("Barbara");int nameStringCapacity=nameString.capacity();System.out.println("Capacity of name string is"+nameStringCapacity);

StringBuilder addressString=null;addressString=new StringBuilder("6311 Hickory nut grove road");

int addStringCapacity=addressString.capacity();System.out.println("Capacity of address String is:" + addStringCapacity);

nameString.setLength(20);System.out.println( "The name is:"+ nameString+"end");addressString.setLength(20);System.out.println("The address is:"+ addressString);}}

Output:Capacity of name string is 23Capacity of address String is:43The name is: Barbara endThe address is:6311 Hickory nut gro


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Packages

Formatting Output

Wrapper Classes


Images


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Class Libraries A class l ibrary is a collection of classes that we

can use when developing programs The Java s tand ard class l ibrary is part of any Java

development environment


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Packages The classes of the Java standard class library are

organized into packages

Some of the packages in the standard class library:

Package

java.langjava.appletjava.awt

javax.swingjava.netjava.utiljavax.xml.parsers

Purpose

General supportCreating applets for the webGraphics and graphical user interfaces

Additional graphics capabilitiesNetwork communicationUtilitiesXML document processing


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The Random Class The Random class is part of the java.util

package

It provides methods that generate pseudorandomnumbers

A Random object performs complicatedcalculations based on a seed value to produce astream of seemingly random values


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The Random Classimport java.util.Random;public class RandomNumbers{public static void main (String[] args) {

Random generator = new Random();int num1;float num2;num1 = generator.nextInt();

System.out.println ("A random integer: " + num1);num1 = generator.nextInt(10);System.out.println ("From 0 to 9: " + num1);num1 = generator.nextInt(10) + 1;System.out.println ("From 1 to 10: " + num1);num1 = generator.nextInt(15) +20;System.out.println ("From 20 to 34: " + num1);num1 = generator.nextInt(20) -10 ;System.out.println ("From -10 to 9: " + num1);num2 = generator.nextFloat();System.out.println ("A random float (between 0-1): " + num2);num2 = generator.nextFloat() * 6; // 0.0 to 5.999999num1 = (int)num2 + 1;

System.out.println ("From 1 to 6: " + num1);}


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The Math Classpublic class TrigonometricDemo {

public static void main(String[] args) {double degrees = 45.0;double radians =

Math.toRadians(degrees);

System.out.format("The value of pi " + "is%.4f%n",

Math.PI);

System.out.format("The sine of %.1f " +"degrees is %.4f%n",degrees, Math.sin(radians));

System.out.format("The cosine of %.1f " +"degrees is %.4f%n",

degrees, Math.cos(radians));

System.out.format("The tangent of %.1f " +"degrees is %.4f%n",

degrees, Math.tan(radians));

System.out.format("The arcsine of %.4f " +"is %.4f degrees %n",

Math.sin(radians),

Math.toDegrees(Math.asin(Math.sin(radians))));

System.out.format("The arccosine of %.4f "+ "is %.4f degrees %n",

Math.cos(radians),

Math.toDegrees(Math.acos(Math.cos(radians))));

System.out.format("The arctangent of %.4f" + "is %.4f degrees %n",

Math.tan(radians),Math.toDegrees(Math.atan(Math.tan(radians))));

}}

Output:The value of pi is 3.1416The sine of 45.0 degrees is 0.7071The cosine of 45.0 degrees is 0.7071

The tangent of 45.0 degrees is 1.0000The arcsine of 0.7071 is 45.0000 degreesThe arccosine of 0.7071 is 45.0000 degreesThe arctangent of 1.0000 is 45.0000 degrees


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The Date Class SimpleDateFormat to control the date/time display

format.

Use java.util.Calendar to extract year, month, day,hour, minute, and second, or manipulating these field(e.g., 7 days later, 3 weeks earlier).

Use java.text.DateFormat to formata Date (form Date to text) and parse a date string(from text to Date). SimpleDateForamt is a subclass ofDateFormat.

j il d j Si l


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java.util.Date and java.text.SimpleDateFormatimport java.text.SimpleDateFormat;import java.util.Date;

public class DateTest { public static void main(String[] args) {

Date now = new Date();System.out.println("toString(): " + now);

SimpleDateFormat dateFormatter = new SimpleDateFormat("E, y-M-d 'at'h:m:s a z");

System.out.println("Format 1: " + dateFormatter.format(now));

dateFormatter = new SimpleDateFormat("E yyyy.MM.dd 'at' hh:mm:ss a zzz");System.out.println("Format 2: " + dateFormatter.format(now));

dateFormatter = new SimpleDateFormat("EEEE, MMMM d, yyyy");System.out.println("Format 3: " + dateFormatter.format(now));}}

Output:toString(): Sat Sep 25 21:27:01 SGT 2010Format 1: Sat, 10-9-25 at 9:27:1 PM SGTFormat 2: Sat 2010.09.25 at 09:27:01 PM SGTFormat 3: Saturday, September 25, 2010

j il C l d


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java.util.Calendarimport java.util.Calendar;public class Cal {

public static void main(String[] args) {Calendar cal = Calendar.getInstance();

int year = cal.get(Calendar.YEAR);int month = cal.get(Calendar.MONTH); int day =

cal.get(Calendar.DAY_OF_MONTH);int hour = cal.get(Calendar.HOUR_OF_DAY);int minute = cal.get(Calendar.MINUTE);int second = cal.get(Calendar.SECOND);

System.out.printf("Now is %4d/%02d/%02d%02d:%02d:%02d\n", year, month+1, day, hour, minute, second);

}}Output:Now is 2014/07/21 09:32:43


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Packages

Formatting Output

Wrapper Classes


Images


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Formatting Output The Java standard class library contains classes

that provide formatting capabilities

The NumberFormat class allows you to formatvalues as currency or percentages

The DecimalFormat class allows you to formatvalues based on a pattern

Both are part of the java.text package


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F tti O t t


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Formatting OutputThe NumberFormat class

import java.util.*;import java.text.NumberFormat;

public class NumFormat{public static void main (String[] args)

{final double TAX_RATE = 0.06; // 6% sales tax

int quantity;double subtotal, tax, totalCost, unitPrice;

Scanner scan = new Scanner (System.in);

NumberFormat fmt1 =NumberFormat.getCurrencyInstance(Locale.US);

NumberFormat fmt2 =

NumberFormat.getPercentInstance();

System.out.print ("Enter the quantity: ");quantity = scan.nextInt();

System.out.print ("Enter the unit price: ");unitPrice = scan.nextDouble();

tax = subtotal * TAX_RATE;totalCost = subtotal + tax;

// Print output with appropriate formattingSystem.out.println ("Subtotal: " +

fmt1.format(subtotal));System.out.println ("Tax: " + fmt1.format(tax) + "

at "+ fmt2.format(TAX_RATE));

System.out.println ("Total: " +fmt1.format(totalCost));

}}

Output:Enter the quantity: 5Enter the unit price: 20Subtotal: $100.00Tax: $6.00 at 6%Total: $106.00


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Formatting Output The DecimalFormat class can be used to format a

floating point value in various ways

For example, you can specify that the numbershould be truncated to three decimal places

The constructor of the DecimalFormat classtakes a string that represents a pattern for theformatted number


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Using the printf() method System.out.printf() method is used to format

numeric values.

Two types of arguments:

A format string (string of characters)

A list of arguments

A format specifier is a placeholder for numericvalue.


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Packages

Formatting Output

Wrapper Classes


Images


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Wrapper Classes The java.lang package contains wrapper

classes that correspond to each primitive type:

Primitive Type Wrapper Class

byte Byte

short Shortint Integer

long Long

float Float

double Double

char Character

boolean Boolean

void Void


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Wrapper Classes The following declaration creates an Integer

object which represents the integer 40 as an object

Integer age = new Integer(40);

An object of a wrapper class can be used in anysituation where a primitive value will not suffice

For example, some objects serve as containers ofother objects

Primitive values could not be stored in suchcontainers, but wrapper objects could be


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Wrapper Classes Wrapper classes also contain static methods that

help manage the associated type

For example, the Integer class contains amethod to convert an integer stored in a String to

an int value:num = Integer.parseInt( str ) ;

The wrapper classes often contain useful

constants as well The Integer class contains MIN_VALUE and

MAX_VALUE which hold the smallest and largestint values


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The String Class

Packages

Formatting Output

Enumerated Types

Wrapper Classes


Images


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GUI Components A GUI co m p o n en t is an object that represents a

screen element such as:

button text field label

scroll bar menu check box

Allows user to interact with program

GUI-related classes are defined primarily in thejava.awt and the javax.swing packages

The Abst rac t Window ing Too lk i t (AWT) was theoriginal Java GUI package


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GUI Components The Swing package provides additional and more

versatile components

Both packages are needed to create a Java GUI-based program


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GUI Containers A GUI conta iner is a component that is used to

hold and organize other components

A f rame is a container that is used to display aGUI-based Java application

A frame is displayed as a separate window with atitle bar

The frame can be repositioned and resized on the

screen as needed


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GUI Containers A panel is a container that cannot be displayed on

its own but is used to organize other components

A panel must be added to another container to bedisplayed

Frames from JFrame class

Panels from JPanel class

Frame has multiple panels

The visible elements of the interface are displayedin the content pane


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GUI Containers A GUI container can be classified as either

heavyweight or lightweight

A heavyweigh t con ta iner is one that is managed bythe underlying operating system

A l igh tweigh t con ta iner is managed by the Javaprogram itself

Occasionally this distinction is important

A frame is a heavyweight container and a panel isa lightweight container


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Labels A label is a GUI component that displays a line of

text

Labels are usually used to display information oridentify other components in the interface

Let's look at a program that organizes two labelsin a panel and displays that panel in a frame

See Authority.java

This program is not interactive, but the frame canbe repositioned and resized

Labels
http://localhost/var/www/apps/conversion/tmp/examples/chap03/Authority.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap03/Authority.java


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Labelsimport java.awt.*;import javax.swing.*;

public class Authority{ public static void main (String[] args) {

JFrame frame = new JFrame ("Authority");

frame.setDefaultCloseOperation (JFrame.EXIT_ON_CLOSE);

JPanel primary = new JPanel(); primary.setBackground (Color.yellow);primary.setPreferredSize (new Dimension(250, 75));

JLabel label1 = new JLabel (Technological University of the Philippines,"); JLabel label2 = new JLabel (Manila");

primary.add (label1); primary.add (label2);

frame.getContentPane().add(primary);frame.pack();frame.setVisible(true);

}}


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Nested Panels Containers that contain other components make

up the con tainm ent h ierarchy of an interface

This hierarchy can be as intricate as needed tocreate the visual effect desired

The following example nests two panels inside athird panel note the effect this has as the frameis resized

See NestedPanels.java Every container is managed by an object called:

A Layout Manager
http://localhost/var/www/apps/conversion/tmp/examples/chap03/NestedPanels.javahttp://localhost/var/www/apps/conversion/tmp/examples/chap03/NestedPanels.java


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Images Images are often used in a programs with a

graphical interface

Java can manage images in both JPEG and GIFformats

As we've seen, a JLabel object can be used todisplay a line of text

It can also be used to display an image

That is, a label can be composed of text, andimage, or both at the same time


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Summary Chapter 3 focused on:

object creation and object referencesthe String , Character and StringBuilder class and itsmethodsthe Java standard class library

the Random and Math classesformatting outputwrapper classesgraphical components and containers

labels and images

oop chapter 3 by jlncrnl

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