An applet is a small Java program that runs in a web browser. It allows for interactive graphics and animation to be embedded in HTML pages. Applets differ from standalone Java applications in that they have no main method and run within the context of a web page. The lifecycle of an applet involves initialization, starting, stopping, and destruction as the user interacts with the web page. Parameters can be passed to applets using the <param> tag in HTML. The applet code uses methods like init(), paint(), start(), and stop() to control its execution and display output.

1. Applet
Programming
Chapter-5
Marks-20

2. Introduction
• There are two kinds of Java programs-
–Applications (stand-alone programs)
–Applets.
• An Applet is a small Internet-based program that
has the Graphical User Interface (GUI), written in
the Java programming language.

3. Introduction
• A Java applet is a small application written in
Java and delivered to users in the form
of bytecode.
• A applet is typically embedded in a Web page
and can be run from a browser.
• Applets can be transported over the Internet
from one computer to another.

4. How applet differ from application
• Applets are the small programs, applications are
larger programs.
• Applets don't have the main method while in an
application execution starts with the main
method.
• Applications run independently while Applets
cannot. They run from inside the web page.
• Applets cannot read from or write to the files in
the local computer.

5. Introduction
• An applet can perform
–arithmetic operation,
–display graphics,
–play sounds,
–accept user input,
–create animation and
–play interactive games.

6. Local and Remote Applets
• We can embed applets into web pages in
two ways-
1. Create our own applets ( Local )
2. Download an applet from remote computer
system (Remote Applet )

7. Local Applet
• An applet developed locally and stored in local
system is known as local applet.
• When a web page trying to find a local applet, it
does not need to use the Internet.
• It simply searches the directories in the local
system and locates and loads the specified
applet. local applet

8. Remote Applet
• A remote applet is that which is developed by
someone else and stored on the remote
computer connected to internet.
• Users can download the remote applet onto
there system and run it.
Local Computer Remote Computer
Internet

9. Steps involved in developing applets
1. Building an applet code (. java file )
2. Creating an executable applet ( .class file )
3. Designing a Web Page using HTML tags.
4. Preparing <APPLET> tag.
5. Incorporating <APPLET> tag into the Web
Page.
6. Creating HTML file.
7. Testing the Applet Code.

10. Building Applet Code
• It is essential that applet code uses services of
two classes
–Applet class and
–Graphics class.

11. Chain of classes inherited by Applet class
java.lang.Object
|
+----java.awt.Component
|
+----java.awt.Container
|
+----java.awt.Panel
|
+----java.applet.Applet

12. The Applet Class
• The java.applet package is the smallest package
in Java API.
• The Applet class is the only class in the package.
• The Applet class provides life and behaviour to
applet through its methods.
• The Applet class has many methods that are used
to display images, play audio files etc but it has
no main() method.

13. Methods of Applet Class
• init() :
– used for whatever initializations are needed for
applet.
– Applets can have a default constructor, but it is better
to perform all initializations in the init method
instead of the default constructor.
• start() :
– This method is automatically called after Java calls
the init method.

14. • stop() :
– This method is automatically called when the user
moves off the page where the applet sits.
• destroy():
– Java calls this method when the browser shuts
down.

15. paint() Method of Applet Class
• Paint method actually displays the result of applet
code on the screen.
• The output may be text, graphics, or sound.
• Syntax-
public void paint(Graphics g)
• The paint method requires graphics object as an
argument.
• Hence we require to import Graphics class from
java.awt package.

16. Applet Life Cycle
Born
Running Idle
Dead End
Stopped
Destroyed
Exit browser
Begin
(Load applet) Initialization
Display
paint()
start()
stop()
start()
destroy()
Applet state transition diagram

17. Born or Initialization State
• An applet begins its life when the web browser
loads its classes and calls its init() method.
• This is called exactly once in Applets lifecycle.
• init() method provides initialization code such as
initialization of variables, loading images or
fonts.
Eg.
public void init()
{
//initialization

18. Running State
• Once the initialization is complete, the web
browser will call the start() method in the applet.
• This method must called atleast once in the
Applets lifecycle.
• The start() method can also be called if the
Applet is in “Idle” state.
Eg. public void start()
{
//Code
}

19. Stopped State
• The web browser will call the Applets stop()
method, if the user moved to another web page
while the applet was executing.
• The stop() method is called atleast once in
Applets Lifecycle.
Eg. public void stop()
{
//Code
}

20. Dead State
• Finally, if the user decides to quit the web
browser, the web browser will free up system
resources by killing the applet before it closes.
• To do so, it will call the applets destroy()
method.
Eg.
public void destroy()
{
// Code
}

21. Display State
• Applet moves to the display state whenever it
has to perform the output operations on the
screen.
• This happens immediately after the applet
enters into the running state.
• The paint() method is called to accomplish this
task.
Eg.
public void paint(Graphics g)
{
//Display Statements

22. General format of Applet Code
import java.awt.*;
import java.applet.*;
……………………
public class applet_class_name extends Applet
{ ……………
public void paint(Graphics g)
{
………….//applet operation code
}
}

23. Example.
import java.awt.*;
import java.applet.*;
public class SimpleApplet extends Applet
{
public void paint(Graphics g)
{
g.drawString("My First Applet",40,40);
}
}

24. Designing a Web Page
• To execute an applet in a web browser, write a
short HTML text file that contains the
appropriate APPLET tag.
• For above example it is
<html>
<body>
<applet code="SimpleApplet.class" width=200
height=100>
</applet>
</body>
</html>

25. The Applet tag
<applet
[codebase=codebaseURL]
code=”Applet file”
[ALT=”alternative text]
[name=AppletInstanceName]
Width=pixels
height= pixels
[align= alignment]
>
[<param name=”Attributename” value =”Attribute value”]
[<param name=”Attributename” value =”Attribute value”]
</applet>

26. Passing Parameter to Applet
• We can supply user-defined parameters to an
applet using <param.....> tag.
• Each <param....> tag has a name ,and a value
attribute.
• For e.g. the color of the text can be changed to
red by an applet using a <param...> tag as
follows
<applet ……>
<param name=“color” value=“RED”>

27. Passing Parameters to Applet
• To handle parameters, we need to do two things.
1) Include <param.....> tags in HTML document.
2) Provide code in the applet to pass these parameters.
• We can define the init() method in the applet to
get parameters defined in the <param> tags.
•
• This is done using the getParameter() method,
which takes one string argument representing
the name of the parameter and returns a string
containing the value of that parameter.

28. Example :Parameter Passing
import java.awt.*;
import java.applet.*;
public class HelloJava extends Applet
{ String str;
public void init()
{ str=getParameter(“String”);
if(str==null)
str=“Java”;
str=“Hello ” +str;
}
Public void paint(Graphics g)
{ g.drawString(“str”,10,10)

29. <html>
<body>
<applet code=“HelloJava.class”
width=400
height=400 >
<param name= “String” value= “Applet!” >
</applet>
</body>
</html>

30. Displaying numerical values
import java.awt.*;
import java.applet.*;
public class Hellojava extends Applet{
public void paint(Graphics g)
{ int value1 =10;
int value2 = 20;
int sum =value1+value2;
String s= "Sum:"+String.valueOf(sum);
g.drawString(s,10,100);
}

31. Using Control Structure
import java.awt.*;
import java.applet.*;
public class Hellojava extends Applet {
public void paint(Graphics g) {
int y=50;
for(int i=1;i<=10;i++)
{ g.drawString(i+” ”,50,y);
y=y+10;
} } }
/*<applet code=“Hellojava.class” width=300 height=100>
</applet> */