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Applet as java applications tutorial - contributed by Pradip
Patil
Applet
Introduction
Applets are small java programs which are executed and displayed in java
browser. Applet is a predefined class available in “java.applet” package. We
can run Applets using a web browser or using appletviewer. It is mandatory to
import java.awt package at the time of writing an applet program as applet
requires support from windows and also applets are not executed by the runtime
interpreter.
Applets do not start with the main () method. Outputs of the applets are handled
by awt (abstract window toolkit) method instead of System.out.println ()
method.
Applets can be included in HTML file by using APPLET tag. If you want to include
applet tag in a program itself then you have to include it by using a comment
like
/*
<APPLET code=”classname” width=200 height=500>
</APPLET>
*/
Applet life cycle
When applets are loaded, they go through different phases or we can say
different states and based on those states, we can create applet state diagram
i.e. applet life cycle. The different Applet states are as follows.
These are the states of applet.In diagrammatic manner we can draw it as follow
Methods of the Applet class
Some of the predefined methods of the applet class are given below in the table.
| Method |
Description |
| Void init() |
To initialize applet. It is called by browser or applet viewer |
| Void start() |
start the execution of the applet
|
| Void destroyed() |
When applet is about to dispose, this method is called up. |
| Void play(URL url ,String name) |
Play the audio clip from given URL |
| Void resize(Dimension d) |
Request the applet to be resized |
| Void stop() |
It tells applet to stop execution |
You can run an applet program in 2 ways
1) Create a separate HTML page.
2) Embed HTML code into a java file.
1) In this type we have to run the html page and call the java code(the applet
program)
Sample.Html
<Html>
<Head>
<Body>
<Applet code=Demo.class width=200 height =200>
</applet>
</body>
</head>
</html>
Demo.java
import java.applet.*;
import java.awt.*;
public class Demo extends Applet
{
public void paint(Graphics g)
{
g.drawString(“Hello Applet”,100,100);
}
}
2)In this type applet code is embedded into a java code
Demo.java
import java.applet.Applet;
import java.awt.*;
/*
<Applet code=Demo.class width=200 height =200>
</applet>
*/
public class Demo extends Applet
{
public void paint(Graphics g)
{
g.drawString(“Hello Applet”,100,100);
}
}
An Applet Skeleton
In order to control the execution of the applets, following are the methods
defined by the applet
1)init()
2)start()
3)stop()
4)destroy()
One more method is defined by the awt component class which is
1)paint()
These 5 methods can be assembled into skeleton as follows
//applet skeleton
import java .awt.*;
import java.applet.*;
/*<applet code=Demo.class Height=200 weight=200>
</applet>
*/
public class Demo extends Applet
{
public void init()
{//initialization}
public void start()
{//start execution}
public void stop()
{//stop execution}
public void destroy()
{//shutdown activities}
public void paint(Graphics g)
{//redisplay the contain of window}
}
This is the skeleton format used by the applet code
The HTML APPLET Tag with all attributes.
The Applet tag is used to start the applet from html document as well as from
the appletviewer.
Various attributes available with the standard applet tag are as follows:
| Codebase |
It is an optional attribute. Specifies the base url of the applet code |
| Code
|
It is a mandatory attribute used to specify the name of the file containing
your applet compiled .class file |
| Alt |
Alt tag is an optional attribute used to specify short text message that is
displayed in the browser. |
| Name |
It is an optional attribute used to specify name for the applet instances |
| Width and height |
Gives the size of the applet display area in pixel |
| Align |
Optional attribute specifies alignment of the applet |
| Vspace and hspace |
Optional attribute; vspace specifies the space above and below in pixel hsapce
specifies the space on each side of the applet |
| Param name and values
|
Param tag allows us to specify argument in html page using getParammeter()
applet access attribute. |
Creating an Applet
To create an applet it is compulsory to import 2 packages which are
a) applet Package and b) awt package to draw the applet in the paint method.
import java.applet.Applet;
import java.awt.*;
/*
<Applet code=Demo.class width=200 height =200>
</applet>
*/
public class Demo extends Applet
{
public void paint(Graphics g)
{
g.drawString(“Hello Java Applet”,100,100);
}
}
To display the output using appletviewer we have to write
Appletviewer Demo.java and you would be able to see the output on appletviewer
If you want to see the output on a browser then following changes are to be
made.
1)create separate html file
2)embed applet tag in it
3)create separate java file
4)double click on the html file .
Sample.Html
<Html>
<Head>
<Body>
<Applet code=Demo.class width=200 height =200>
</applet>
</body>
</head>
</html>
Demo.java
import java.applet.*;
import java.awt.*;
public class Demo extends Applet
{
public void paint(Graphics g)
{
g.drawString(“Hello Applet”,100,100);
}
}
Open the html file through a browser and the applet program should run through
web browser.
Passing parameters to applet
For passing the parameter to the applet we have to use the param tag.
import java.applet.Applet;
import java.awt.*;
/*
<Applet code=Demo.class width=200 height =200>
<param name=str value=”Hello Java Applet”>
</applet>
*/
public class Demo extends Applet
{
public void paint(Graphics g)
{
g.drawString(getParameter(“str”),100,100);
}
}
Applet Vs Application
| Applet |
Application |
| Applet needs explicit installation on the local machine. |
Application must run on local machine |
| Applet start its execution with init() method |
Application starts its execution with main()method. |
| Applet must run with GUI |
Application can run with or without GUI |
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