Applet and graphics programming

Applet and Graphics
Programming

Applet Programming
• Applets are small Java programs that are primarily used
in Internet computing.
• They can be transported over the Internet from one
computer to another and run using the Applet Viewer or
any Web browser that supports Java.
• An applet, like any application program, can do many
things for us. It can perform arithmetic operations,
display graphics, play sounds, accept user input, create
animation, and play interactive games.

Local and Remote Applets
• An applet developed locally and stored in a local system
is known as a local applet.
• A remote applet is that which is developed by someone
else and stored on a remote computer connected to the
Internet.
• In order to locate and load a remote applet, we must
know the applet’s address on the Web. This address is
known as Uniform Resource Locator(URL).

How Applets differ from
Applications
• Applets do not use the main() methods for initiating the
execution of the code.
• Unlike stand-alone applications, applets cannot be run
independently. They are run from inside a Web page
using a special feature known as HTML tag.
• Applets cannot read from or write to the files in the local
computer.
• Applets cannot communicate with other servers on the
network.
• Applets cannot run any program the local computer.
• Applets are restricted form using libraries form other
languages such as C or C++.

To Write Applets
• 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

Chain of classes inherited by Applet
Class
java..lang.Object
java.applet.Applet
java.awt.Panel
java.awt.Container
java.awt.Component

Applet Life Cycle
• The applet states include:
– Born or initialization state
– Running state
– Idle state
– Dead or destroyed state

Born
Running Idle
Dead
Begin
(Load Applet)
Initalization
Start( )
stop( )
start( )
Stopped
destroy ( )
EndDestroyed
Exit of Broswer
paint ( )
Display
An applet’s state transition diagram

Applet Life Cycle
• Initialization State
– Applet enters the initialization state when it is first loaded. This
is achieved by calling the init( ) method of Applet Class.
– The applet is born.
– The initialization occurs only once in the applets’life cycle.
– To provide any of the behaviours mentioned above, we must
override the init ( ) method:
public void init( )
{
//statements
}

• Running State
– Applet enters the running state when the system calls the start
( ) method of Applet Class.
– The occurs automatically after the applet is initialized.
– Starting can also occur if the applet is already in “stopped”
(Idle ) state.
– Unlike init( ) method, the start ( ) method may be called more
than once.
public void start()
{
// statements
}
Applet Life Cycle

• Idle or Stopped State
– An applet becomes idle when it is stopped from running.
Stopping occurs automatically when we leave the page
containing the currently running applet.
public void stop( )
{
//statements
}
Applet Life Cycle

• Dead State
– An applet is said to be dead when it is removed from memory.
This occurs automatically by invoking the destroy ( ) method
when we quit the browser.
– Like initialization, destroying stage occurs only once in the
applet’s life cycle.
public void destroy( )
{
//statements
}
Applet Life Cycle

• Display state
– Applet moves to the display state whenever it has to perform some
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.
– Almost every applet will have a paint( ) method.
– Paint( ) method is inherited form the Component class , a super class
of Applet.
public void paint (Graphics g)
{
//statements
}
Applet Life Cycle

import java.awt.*;
import java.applet.*;
public class HelloJava extends Applet
{
public void paint(Graphics g)
{
g.drawString("Hello Java",10,100);
}
}
<html>
<body>
<applet code = "HelloJava.class"
height=“100” width =“200”>
</applet>
</body></html>

Applet Tag
• The <Applet. .>tag supplies the name of the applet to be
loaded and tells the browser how much space the applet
requires.
• <APPLET> tag specifies three things:
– Same of the applet
– Width of the applet (in pixels)
– Height of the applet (in pixels)

Passing Parameters to Applets
• We can supply user-defined parameters to an applet using
<PARAM…> tags.
• Each <PARAM…>tag has a name attribute such as color and a value
attribute such as red.
• Inside the applet code, the applet can refer to that parameter by name
to find its value.
• To set up and handle parameters, we need to do two things:
– Include appropriate <PARAM…> tags in the HTML document
– Provide Code in the applet to parse these parameters.
• Parameters are passed to an applet when it is loaded. We can define
the init() method in the applet to get hold of the 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 the parameter.

import java.awt.*;
import java.applet.*;
public class JavaParam extends Applet
{
String s;
public void init()
{
s =
getParameter("uname");
if(s==null)
s = “ABC”;
s = "Hello" + s;
}
public void paint(Graphics g)
{
g.drawString(s,10,100);
}
}
<html>
<body>
<APPLE CODE =
JavaParam.class
width=400 height = 200>
<PARAM NAME = "uname"
VALUE="ruchita">
</APPLE>
</body>
</html>

Applet and graphics programming

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