Applet Programing

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Applet Programming


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What is Applet

An applet is a special kind of Javaprogram which is used for internet

programming.

The applets reside in a server and theycan be downloaded from the server to the

client machine and executed on client

machine provided that the browser shouldbe enabled with Java technology.


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Types of Applet

Applets are basically of two types.1. Local Applet These are applets which are

created for standalone PC's where the same

PC is distinguished as server and client.

Hence, to execute a local applet there is no

need of internet connection.

2. Remote Applet The applets which are

developed for an environment where theserver and the client machine are physically

apart from each other are called remote

applets


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Needs of Applet

1. Applet is a window based application.2. Applets are basically used to create

dynamic pages.

3. The difference of an applet program froma normal program is that it does not

contain the main function. So it has no

entry point.4. The program starts executing from some

functions present in Applet class.


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Need for applet

5. an applet program cannot be executedindependently as done in case of normal

application programs.

6. They must be executed with the help ofeither applet viewer or with the help of

any web browser.


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Creating an applet program

We need to import two packages for writing anapplet program

Import java.lang.awt.*;

Import java.lang.applet.*;

The java.lang.awt.* package provides a set ofclasses and using those classes we can create awindow component and it also contains a graphicclass;

the java.lang.applet.* package is used as we areusing an applet class which is present in appletpackage.


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An applet class is a member of JavaApplication Programming Interface(API)

Package.

An applet class is extended from the PanelClass, which is further extended from the

classes Container, Component and

Object. Object class is the member class of the

java.lang package and is called in the

program automatically.


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The component, container and Panelclasses are the members of java.awt

package that provide visual components

such as label,button and text fields. Anapplet is the only member of the

java.applet package.


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30Java Class Hierarchies


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10

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30Abstract windows Toolkit(AWT)

The awt package is a collection of classesand methods that allow an end user to

design and manage graphical user

interface(GUI) based application. AWT is used to support applet windows

and it also helps in creating independent

windows such as frame window or panelwindow that run in GUI environment.


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30Abstract windows Toolkit(AWT)

AWT is javas Largest Package that canbe included in java program by giving the

java.awt.* statement.

It is a part of java foundation classes(JFC). AWT has 63 classes and 14 interfaces

used for creating user interfaces.


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It provides graphics and imaging tools thatenable you to develop GUI.

It provides layout manager that enables an

end user to develop window layouts. It supports event handling. for example, an

error occurred in between execution of a

program.


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13

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30Hierarchy of java.awt package


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30Component Class

Component class is at the top of the AWTinheritance class hierarchy. It is the super

class and all others in the hierarchy inherit

its characteristics directly or indirectly. Component class is the abstract class that

encapsulated all the properties of visual

components or object.


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void setSize(int width, int height): definesthe size of a component in terms of wisth and

size.

void setLocation(int x,int y): specifies thelocation of a components, in the window, at the

position specified by the x and y coordinates.

void setBounds(int x,int y,int width ,intheigth): sets the bounds of a component in

terms of position x and y and size in terms of

width and height.


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Void setForeground(Color b): sets theforeground color of the component.

Void setBackground(Color b): sets the

background color of the component.


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Container class is the subclass of component

class.

Methods that are used to manipulate

components on container objects are:

void add(object o): Nest a Component or an

object inside a container.

void remove(object o): removes a component

or an object from a container.

void removeALL(): removes all the

components or objects that are placed inside a

container.


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When we want to execute an applet programwith the help of a web browser, we need to

write an HTML file.

ExampleofApplet


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Applet Programming

importjava.lang.awt.*;importjava.lang.applet.*;publicclassmessageextendsApplet

{

Stringmsg;publicvoidinit()

{

msg=Welcometoappletprogramming;

}

publicvoidpaint(Graphicsg)

{ g.drawString(msg,100,400); }}


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R i th l t i W b b


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Running the applet in a Web browser


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APPLET LIFE CYCLE


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Applet Life Cycle


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Applet

Public keyword is used because we run anapplet program in a media different from

that in which we write it.

Paint() method used in the above examplecalls drawString() function which is a

function of Graphics class.

Applet class contains certain functions likeinit(), start(), stop(), paint(), destroy(), etc.

which are usually used in programs.


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Method init()

init() methodWhenever an applet isloaded into a web page this method is

automatically called. This method provides

an appropriate place for initialization of thevariables required and other set up

operations such as setting of background

color, defining the applets layout, parsing

parameters, etc.


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Method start()

start() method After the initialization iscompleted, start() method is called.

The difference between this method and the

init() method is that init() method is calledonly when the page is loaded, whereas

start() method is called every time the page

is loaded and every time the page is

restarted.

This method is mainly used while

implementing threads in Java.


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Method stop()

stop() method This method is calledwhenever the user navigates from one

page to another page.

It is always advisable to use stop()method if any type of animation or memory

consuming activities is being performed in

order to avoid wasting of systemresources.


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Method destroy()

destroy() method This method is calledimmediately before the applet exits.

The exiting operation is caused by either

the applet operation requesting for the exitor may be the web browser is shut down.

When it is called it directs the applet to

free up all the system resources beingused by it.


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Methods in applet

/* */

import java.awt.*;

import java.applet.*;public class balli1 extends Applet

{

String str="Hello ";


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30Methods in applet

public void init(){

str=str+"";

}

public void start()

{

str=str+" ";

}


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public void stop(){

str=str+"";

}

public void destroy()

{

str=str+"";

}

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public void paint(Graphics g){

str=str+"";

g.drawString(str,50,50);

}

}

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30Out Put of the Program


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1. http://aspalliance.com/1309_Introducing_Java_Applets.1

2. The Complete Reference by Patrick Naughton and Herbert Schildt

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http://aspalliance.com/1309_Introducing_Java_Applets.1http://aspalliance.com/1309_Introducing_Java_Applets.1


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The End

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Applet Programing

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