1. Java Applications And Applets

1. Java Applications and Applets

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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 Application… Start a suitable

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An Application… Start a suitable

We are going to use the Notepad utility as the Editor (ie. To type the Java source program) © 2008, University of Colombo School of Computing

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An Application… Introduce a class class FirstApp{

} © 2008, University of Colombo School of Computing

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An Application… Start a suitable

So here we have opened an untitled notepad document

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An Application… Introduce the main method class FirstApp{ public static void main(String args[ ]) {

} } © 2008, University of Colombo School of Computing

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An Application… Lets declare two variables class FirstApp{ public static void main(String args[ ]) { int myNum1,myNum2; } }

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An Application… Lets declare two variables class FirstApp{ public static void main(String args[ ]) { int myNum1,myNum2; } }

myNum2 myNum1

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An Application… Lets initialize two variables class FirstApp{ public static void main(String args[ ]) { int myNum1,myNum2; myNum1=3; myNum2=6; } } © 2008, University of Colombo School of Computing

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An Application… Lets initialize two variables class FirstApp{ public static void main(String args[ ]) { int myNum1,myNum2; myNum1=3; myNum2=6; } }

myNum2 myNum1 © 2008, University of Colombo School of Computing

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An Application… Add 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)); } } © 2008, University of Colombo School of Computing

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An Application… Add two values and display them on the screen class Application{ public static void main(String args[])

{

int myNum1,myNum2; myNum1=3;myNum2=6; Total 9 System.out.println(“Result”+(myNum1+myNum2)); } } © 2008, University of Colombo School of Computing

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An Application… When saving the file

• • • •

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

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An Application… Save the file class FirstApp{ public static void main(String args[])

{

int myNum1,myNum2; myNum1=3;myNum2=6; System.out.println(“Total”+(myNum1+myNum2)); } } © 2008, University of Colombo School of Computing

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An Application… When compiling

• javac FirstApp.java t u tp u o

Java Compiler

FirstApp.class

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An Application… When executing

• java FirstApp

t u tp u o

Java Interpreter © 2008, University of Colombo School of Computing

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What is an Applet • Tiny Java program, dynamically downloaded 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 © 2008, University of Colombo School of Computing

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Your first applet Lets say Hello using an applet Open a suitable editor to work with Here we use notepad utility © 2008, University of Colombo School of Computing

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Applets • It begins with two import statements – Use import statement to bring certain classes, or entire 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 based graphical interface © 2008, University of Colombo School of Computing

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Applets • The second import statement imports the applet package • It contains the class Applet • Every applet that you create must be a subclass of Applet – What is a subclass…? We call it inheritance, getting features of a super class to a sub class © 2008, University of Colombo School of Computing

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Your first applet…

Lets say Hello using an applet

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

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Your first applet… Lets say Hello using an applet

Extend(Inherit) the existing features from the applet class to our own

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Applets….. • program declares the class MyApplet • This class must be declared as public because it will be accessed by code that is outside the program

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Your first applet… Lets say Hello using an applet import java.applet.*; import java.awt.*;

public class MyApplet extends Applet {

}

Our class Name

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Your first applet… Lets say Hello using an applet Make use of the paint method in the Applet class There is one parameter which is an instance of the graphic class in AWT

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Applets • Inside MyApplet, paint() method is declared • paint() is called each time the applet must redisplay its output • paint method has one parameter of type Graphics • This parameter contains the graphics content © 2008, University of Colombo School of Computing

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Your first applet… Lets say Hello using an applet import java.applet.*; import java.awt.*; public class MyApplet extends Applet { public void paint(Graphics g) {

} }

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Your first applet… Lets say Hello using an 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 method of the Graphics class • This method outputs a string beginning at the 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… Lets say Hello using an 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… Lets say Hello using an applet import java.applet.*; import java.awt.*; public class MyApplet extends Applet { public void paint(Graphics g) { g.drawString(“Hello”, 20, 20); } } © 2008, University of Colombo School of Computing

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Your first applet… Lets say Hello using an applet

Save the file giving the class name as the file name with .java file extension

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Your first applet… Lets say Hello using an applet

Compile the source file

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Your first applet… Lets say Hello using an applet Java compiler

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Your first applet… Lets say Hello using an applet

Can you do this…? © 2008, University of Colombo School of Computing

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Applets.. • Notice that the applet does not have a main() method unlike java application programs • Applet cannot be run from the command prompt like we do an application 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 compatible web browser such as Internet Explorer or Netscape Navigator – Using an appletviewer

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Running an applet HTML( Hyper Text Markup Language) Basics • To execute an applet in a web browser or appletviewer write a HTML text file using notepad

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Structure of HTML file © 2008, University of Colombo School of Computing

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Running an applet • Within the body tag include the applet tag • Then save the file giving .html or .htm file extension with any name (Myapp.html) © 2008, University of Colombo School of Computing

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Running an applet • Executing the applet within a Java compatible web browser such as Internet Explorer

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Running an applet Using a java compatible browser

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Running an applet • Using appletviewer – To execute the MyApplet in the applet viewer one has to use an html file – Type the following command in the command prompt E.g.:

C:\java>appletviewer myapp.html

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Running an applet

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