Applets in Java

Introduction to Applets

 

Applets are small applications that are 

• accessed on an Internet server,
•  transported over the Internet, 
• Automatically installed, 
• and run as part of a web document

An applet is a Java program that runs in a Web browser. They can be executed by browsers on multiple platforms like Windows, Linux, and Mac. They can be executed by browsers on various platforms, including Windows, Linux, and macOS.

Java can be used to create two types of programs: applications and applets. 

Applet Vs Application Programs

Applet	Application
It is initialised through init().	Starts its execution through the main() method
It is a small program that uses another application (browser) program for its execution.	An application is a program executed on a computer independently.
Cannot run independently, requires API's	Can run alone but requires JRE.
Prior installation is not needed. Browsers with an applet plug-in are enough.	Requires prior explicit installation on the local computer.
The files cannot be read or written on the local computer through the applet.	Applications are capable of performing those operations on the files on the local computer.
Requires security for the system as the applets are untrusted.	No security concerns exist.
Created by extending the Applet class	User-defined class with main()
Uses a GUI interface to interact with users	Uses I/O stream classes

Sample Applet Program

import java. awt.*;

import java.applet.*;

public class MyApplet extends Applet 

{

     public void paint(Graphics g)

     {

             g.drawString("Hello World!", 20, 20);

     }

}

/*<applet code="MyApplet" height="300" width="500">

</applet>*/

There are two ways to run an applet:

•  Executing the applet within a Java-compatible Web browser, such as Netscape Navigator.
• Using an applet viewer, such as the standard JDK tool, appletviewer. 

An appletviewer executes the applet in a window. This is generally the fastest and easiest way to test an applet.

To execute an applet in a Web browser, it is essential to write a short HTML text file that contains the appropriate APPLET tag. 

<applet code="SimpleApplet" width=200 height=60>

</applet>

This tag can also be embedded within a Java file inside a comment line.

 Applet begins with two import statements. 

The first imports the Abstract Window Toolkit (AWT) classes. 

The second import statement imports the applet package, which contains the class Applet

Every AWT-based applet must be a subclass (either directly or indirectly) of Applet.

The class SimpleApplet must be declared as public, because it will be accessed by code that is outside the program.

Life Cycle of an Applet

An applet's life cycle includes the following methods

• init( )
• start( )
• paint( )
• stop( )
• destroy( )

When an applet begins, the AWT calls the following methods, in this sequence:

• init( )
• start( )
• paint( )

When an applet is terminated, the following sequence of method calls takes place:

• stop( )
• destroy( )

init() and start() methods

The init( ) method is the first method to be called.

This is where the variables are initialised. 

init( ) is called once—the first time an applet is loaded

The start( ) method is called after init( ). 

start( ) is called each time an applet's HTML document is displayed on screen. 

So, if a user leaves a web page and comes back, the applet resumes execution at start( ).

paint() method

The paint( ) method is called each time the applet's output must be redrawn.  

paint( ) is also called when the applet begins execution.

The paint( ) method has one parameter of type Graphics. 

This parameter will contain the graphics context, which describes the graphics environment in which the applet is running. 

This context is used whenever output to the applet is required.

stop() and destroy()

The stop( ) method is called when a web browser leaves the HTML document containing the applet.

 Applet uses stop( ) to suspend threads that don't need to run when the applet is not visible. 

The destroy( ) method is called when the environment determines that the applet needs to be removed from memory. 

The stop( ) method is always called before destroy( ).

repaint() and update()

The paint () method is called automatically by the browser whenever the applet window needs to be redrawn.

The user can call the repaint() method whenever the applet window needs to be redrawn.

The repaint () method will call the update () method of the Component class, which clears the window with the background colour of the applet and then calls the paint () method.

Types of Applets

Applets are of two types

• Simple applets
• JApplets

Simple applets can be created by extending Applet class

These applets use the Abstract Window Toolkit (AWT) to provide the graphical user interface.

This style of applet has been available since Java was first created. 

JApplets can be created by extending JApplet class of javax.swing package

Simple display methods in Applets

void drawString(String message, int x, int y);  //To display text on the applet window

void setBackground(Color newColor)            //To set the background colour of the applet window     

void setForeground(Color newColor)   //To set colour to the text and other components on the applet

setBackground(Color.green);

setForeground(Color.red);

Status Window

An applet can also output a message to the status window of the browser or applet viewer on which it is running.

To do so, call showStatus( ) with the string to be displayed.

The HTML APPLET tag

< APPLET

[CODEBASE = codebaseURL]

CODE = appletFile

[ALT = alternateText]

[NAME = appletInstanceName]

WIDTH = pixels HEIGHT = pixels

[ALIGN = alignment]

[VSPACE = pixels] [HSPACE = pixels]

>

[< PARAM NAME = AttributeName VALUE = AttributeValue>]

[< PARAM NAME = AttributeName2 VALUE = AttributeValue>]

Document Base – Filename from where the Java file containing the applet tag is loaded

Code Base – directory where the applet class file resides

Passing parameters to Applets

Parameters specify extra information that can be passed to an applet from the HTML page. 

Parameters are specified using the HTML’s param tag.

The < APPLET> tag in HTML is used to pass parameters to an applet. 

Param tag

The <param> tag is a subtag of the <applet> tag. 

The <param> tag contains two 

attributes: name and value 

which are used to specify the name of the parameter and its value, respectively.

For example, the param tags for passing name and age parameters 

<param name=”name” value=”Ramesh” />

<param name=”age” value=”25″ /> 

getParameter() method

The getParameter() method of the Applet class can be used to retrieve the parameters passed from the HTML page. 

The syntax of the getParameter() method is as follows:

 

                      String getParameter(String param-name)

Program using the param tag

import java. awt.*;

import java.applet.*;

public class MyApplet extends Applet 

{

String n;

String a;

public void init()

{

n = getParameter("name");

a = getParameter("age");

}

    public void paint(Graphics g)

    {

   g.drawString("Name is: " + n, 20, 20);

   g.drawString("Age is: " + a, 20, 40);

   }

}

/*

<applet code="MyApplet" height="300"  width="500">

<param name="name" value="Ramesh" />

<param name="age" value="25" />

</applet>

*/

Graphics Package

Graphics belongs to a package named java. awt

import java.awt.Graphics;

To use Graphics,  the above line must be placed at the very top of the applet program, before the public class header.

The window coordinate system

Each (x, y) position is a pixel  ("picture element").

Position (0, 0) is at the window's top-left corner.

x increases rightward and y increases downward.

The rectangle from (0, 0) to (200, 100) looks like this:

   

Graphics Methods

 

Method name	Description
g.drawLine(x1, y1, x2, y2);	line between points (x1, y1), (x2, y2)
g.drawOval(x, y, width, height);	Outline the largest oval that fits in a box of size width * height, with top-left at (x, y)
g.drawRect(x, y, width, height);	outline of a rectangle of size width * height with top-left at (x, y)
g.drawString(text, x, y);	text with bottom-left at (x, y)
g.fillOval(x, y, width, height);	fill the largest oval that fits in a box of size width * height with top-left at (x, y)
g.fillRect(x, y, width, height);	fill rectangle of size width * height with top-left at (x, y)
g.setColor(Color);	Set Graphics to paint any following shapes in the given colour