1. java applications and applets

8/14/2019 1. Java Applications and Applets

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2008, University of Colombo School of Computing 2

Applications and Applets

Applications and Applets both are

programs written in Java

Depending on the way these programs are

run/executed they vary from each other


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What is an Application

type of Java Program


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An ApplicationStartStarta suitablea suitable

We are going to use theNotepadutility as the Editor(ie. To type the Java source

program)


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An ApplicationIntroduce a classIntroduce a class

class FirstApp{

}


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An ApplicationStartStarta suitablea suitable

So here we have opened

an untitled notepad

document


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An ApplicationIntroduce the main methodIntroduce the main method

class FirstApp{

public static void main(String args[ ]) {

}

}


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An ApplicationLets declare two variablesLets declare two variables

class FirstApp{


int myNum1,myNum2;

}

}


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An ApplicationLets declare two variablesLets declare two variables

class FirstApp{


int myNum1,myNum2;

}

}myNum1

myNum2


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An ApplicationLets initialize two variablesLets initialize two variables

class FirstApp{


int myNum1,myNum2;

myNum1=3; myNum2=6;

}}


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An ApplicationLets initialize two variablesLets initialize two variables

class FirstApp{


int myNum1,myNum2;

myNum1=3; myNum2=6;

}

}

myNum1

myNum2


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An ApplicationAdd two values and display the total on the screenAdd two values and display the total on the screen

class FirstApp{

public static void main(String args[]) {

int myNum1,myNum2;myNum1=3;myNum2=6;

System.out.println(Total+(myNum1+myNum2));

}

}


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An ApplicationAdd two values and display them on the screenAdd two values and display them on the screen

class Application{



System.out.println(Result+(myNum1+myNum2));

}

}

Total 9


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An Application File name must agree with the class name

File extension is .java

So our file name is FirstApp.java

Java is case sensitive FirstApp and firstapp are two different words

in Java

When saving the fileWhen saving the file


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An ApplicationSave the fileSave the file

class FirstApp{



System.out.println(Total+(myNum1+myNum2));

}

}


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An Application javac FirstApp.javaWhen compilingWhen compiling

JavaCompiler

o

utp

ut

FirstApp.class


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An Application java FirstAppWhen executingWhen executing

JavaInterpreter

outp

ut


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What is an Applet

Tiny Java program, dynamicallydownloaded across the network

An intelligent program not just an

animation or media file

It can react to user input and dynamically

change


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Applets They are on the Internet server

Transported over the Internet

Automatically installed

Run as part of a web document After arriving in the client machine it has

limited access to resources of the client

machine high security


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Your first applet

Lets sayLets say HelloHellousing an appletusing an applet

Open a suitable

editor to work with

Here we use notepad

utility


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Applets It begins with two import statements

Use import statement to bring certain classes, orentire packages into visibility

The first statement imports the Abstract Window

Toolkit(AWT) package Applets interact with the user through the AWT,

not through the console-based I/O classes

The AWT contains support for a window basedgraphical interface


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Applets

The second import statement importsthe applet package

It contains the class Applet

Every applet that you create must be asubclassof Applet What is a subclass?

We call it inheritance, getting features of asuper class to a sub class


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import java.awt.*;

import java.applet.*;

Lets say Hellousing an applet

Your first appletYour first applet


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Extend(Inherit)

the existing features from

the applet class to our own

Lets sayLets say HelloHellousing an appletusing an appletYour first appletYour first applet


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Applets.. program declares the class MyApplet

This class must be declared as publicbecause it will be accessed by code that isoutside the program


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import java.awt.*;

public class MyApplet extends Applet {

}

Our className



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Make use of the paint method in theApplet class

There is one parameter

which is an instance of the graphic

class in AWT



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2008, University of Colombo School of Computing29

Applets Inside MyApplet, paint() method is

declared paint() is called each time the applet must

redisplay its output

paint method has one parameter of typeGraphics

This parameter contains the graphicscontent


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2008, University of Colombo School of Computing30


import java.awt.*;


public void paint(Graphics g) {

}

}



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2008, University of Colombo School of Computing

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Your first applet

Draw the string

on the graphical user interface



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Applets Inside the paint() method is a call to

drawString(), which is a member methodof the Graphics class

This method outputs a string beginning atthe specified x,y location (coordinates)

It has the following general form

Void drawString(String message, int x , int y)


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In a Java windows upper left

corner is location 0,0

0,0

Java window


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Your first applet

import java.applet.*;import java.awt.*;


public void paint(Graphics g) {g.drawString(Hello, 20, 20);

}

}



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Your first applet

import java.applet.*;import java.awt.*;public class MyApplet extends Applet {

public void paint(Graphics g) {

g.drawString(Hello, 20, 20);

}

}



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Your first applet

Save the file giving the class name

as the file name with .java fileextension



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Your first applet

Compile the source file



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Your first applet


Java compiler


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Your first applet


Can you do this?


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Applets..

Notice that the applet does not have a

main() method unlike java applicationprograms

Applet cannot be run from thecommand prompt like we do anapplication type of java program


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Running an applet There are two ways one can run an applet

Executing the applet within a Java compatibleweb browser such as Internet Explorer orNetscape Navigator

Using an appletviewer


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Running an appletHTML( Hyper Text Markup Language)

Basics To execute an applet in a web browser or

appletviewer write a HTML text file usingnotepad


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My first applet

Structure of HTML fileStructure of HTML file


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Running an applet

Within the body tag include the applet tag

Then save the file giving .html or .htm fileextension with any name (Myapp.html)


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Running an applet

Executing the applet within a Javacompatible web browser such as InternetExplorer

R i l t


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Running an applet

Using a java compatible browserUsing a java compatible browser


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Running an applet Using appletviewer

To execute the MyAppletin the applet viewerone has to use an html file

Type the following command in the command

promptE.g.:

C:\java>appletviewer myapp.html

R i l t


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Running an applet

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