171-33B - Java Programming-Building Applets.ppt

By
Dr M. Senthilkumar
Assistant Professor
Department of Computer Science
Government Arts and Science College, Avinashi - 641654
Building Applets

2
Writing Applets
 When to use?
 How applets works?
 What sort of features an applet has?
 Where to start when we first create our own applets?

3
Writing Applets
 When we need something dynamic to be included in
the display of a Web Page
 When we need flash outputs
 Place useful program in Internet for others to use it
 Example:
 Share prices of companies in a list
 Bar Chart of Tables
 Playing Sounds, Animations, Special Effects

4
Writing Applets
 Install Java properly
 Ensure appletviewer or Java enabled Browser
 Building an applet code (.java file)
 Creating an executable applet (.class file)
 Designing a Web page using HTML tags
 Preparing <APPLET> tag
 Incorporating <APPLET> tag into the web page
 Creating HTML file
 Testing the applet code

5
Building an applet code (.java file)
 Applet and Graphics class
 Applet class is available within java.applet package
 Applet class provides life and behavior of applet
 Applet class contains init ( ), start ( ) and paint ( ) methods
 Applet methods are called automatically by Java instead
of main( )

6
 paint( ) displays result of the applet code
 paint( ) requires an Graphics object as an argument
 paint( ) outputs text, image, sound, animations using
methods of Graphics Object
 paint( ) is defined as public void paint(Graphics g)
 We must import java.awt package that contains Graphics

7
import java.awt.*;
import java.applet.*;
………….
………….
public class appletclassname extends Applet
{
………...
…………
public void paint(Graphics g)
{
…………
…………
}
………
}

8
Our First Applet
import java.awt.*;
public class HelloJava extends Applet
{
{
g.drawString(“Hello Java”, 10, 100);
}
}
100
Hello Java
10
(0,0)
D:jdk1.8.0_111jdk1.8.0_111bin>javac HelloJava.java
D:jdk1.8.0_111jdk1.8.0_111bin>appletviewer.exe HelloJava.html

9
Chain of Classes Inherited by Applet Class
java.lang.Object
java.awt.Component
java.awt.Container
java.awt.Panel
java.applet.Applet

10
Life Cycle of an Applet
stop ( )
Begin
(Load Applet)
start ( )
Born
Running Idle
start ( )
Idle
destroy ( )
paint ( )
End
Destroyed
Display
Stopped

11
Life Cycle of an Applet - Initialization State
 Initialization occurs only once in the applet’s life cycle
 Create Objects needed by the applet
 Setup initial values
 Load images or fonts
 Setup colors
public void init( )
{
……………
…………… (Action)
……………
}

12
Life Cycle of an Applet - Running State
 Applet enters running state using start( )
 start( ) can be called from Idle state
(Switching between web pages)
 start( ) Can be called more than once
 start( ) can be overridden to create a thread to
control applet
 Occurs automatically after initialization of applet
public void start( )
{
……………
……………
}

13
Life Cycle of an Applet – Idle / Stopped State
 Applet becomes idle when it is
stopped using stop( )
 Occurs automatically when leaving
a web page
(Switching between web pages)
 If we use a thread to control applet,
then we must use stop( ) to
terminate the thread by Overriding
the stop( )
public void start( )
{
……………
……………
}

14
Life Cycle of an Applet – Dead State
 Applet becomes dead when it is
removed from memory using destroy( )
 Occurs automatically when exiting from
browser
 Occurs only once in Life cycle
 Resources like threads are released by
Overriding destroy( )
public void destroy( )
{
……………
……………
}

15
Life Cycle of an Applet – Display State
 Occurs automatically after Running state using paint( )
 We must Override paint( ) to display Contents
{
……………
…………… (Display statements)
……………
}

16
Creating an Executable Applet
 Compile the applet and generate byte code
 javac HelloJava.java
import java.awt.*;
public class HelloJava extends Applet
{
{
g.drawString(“Hello Java”, 10, 100);
}
}

17
Designing a Web Page – HelloJava.html
<HTML>
<! Title Bar>
<HEAD>
<TITLE> Welcome to Java Applets </TITLE>
</HEAD>
<BODY>
<CENTER>
<H1> Welcome to the world of Applets </H1>
</CENTER>
<BR>
<CENTER>
<APPLET
CODE = HelloJava.class
WIDTH = 400
HEIGHT = 200>
</APPLET>
</CENTER>
</BODY>
</HTML>

18
References
 Programming with Java – A Primer - E. Balagurusamy, 3rd
Edition, TMH

171-33B - Java Programming-Building Applets.ppt

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