An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API at its disposal.

There are some important differences between an applet and a standalone Java application, including the following −

An applet is a Java class that extends the java.applet.Applet class.
A main() method is not invoked on an applet, and an applet class will not define main().
Applets are designed to be embedded within an HTML page.
When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user’s machine.
A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment.
The JVM on the user’s machine creates an instance of the applet class and invokes various methods during the applet’s lifetime.
Applets have strict security rules that are enforced by the Web browser. The security of an applet is often referred to as sandbox security, comparing the applet to a child playing in a sandbox with various rules that must be followed.
Other classes that the applet needs can be downloaded in a single Java Archive (JAR) file.

Life Cycle of an Applet

Four methods in the Applet class gives you the framework on which you build any serious applet −

init − This method is intended for whatever initialization is needed for your applet. It is called after the param tags inside the applet tag have been processed.
start − This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages.
stop − This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet.
destroy − This method is only called when the browser shuts down normally. Because applets are meant to live on an HTML page, you should not normally leave resources behind after a user leaves the page that contains the applet.
paint − Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. The paint() method is actually inherited from the java.awt.

A “Hello, World” Applet

Following is a simple applet named HelloWorldApplet.java −

import java.applet.*;import java.awt.*;publicclassHelloWorldAppletextendsApplet{publicvoid paint (Graphics g){
      g.drawString ("Hello World",25,50);}}

These import statements bring the classes into the scope of our applet class −

java.applet.Applet
java.awt.Graphics

Without those import statements, the Java compiler would not recognize the classes Applet and Graphics, which the applet class refers to.

The Applet Class

Every applet is an extension of thejava.applet.Applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context.

These include methods that do the following −

Get applet parameters
Get the network location of the HTML file that contains the applet
Get the network location of the applet class directory
Print a status message in the browser
Fetch an image
Fetch an audio clip
Play an audio clip
Resize the applet

Additionally, the Applet class provides an interface by which the viewer or browser obtains information about the applet and controls the applet’s execution. The viewer may −

Request information about the author, version, and copyright of the applet
Request a description of the parameters the applet recognizes
Initialize the applet
Destroy the applet
Start the applet’s execution
Stop the applet’s execution

The Applet class provides default implementations of each of these methods. Those implementations may be overridden as necessary.

The “Hello, World” applet is complete as it stands. The only method overridden is the paint method.

Invoking an Applet

An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser.

The <applet> tag is the basis for embedding an applet in an HTML file. Following is an example that invokes the “Hello, World” applet −

<html><title>The Hello, World Applet</title><hr><appletcode="HelloWorldApplet.class"width="320"height="120">
      If your browser was Java-enabled, a "Hello, World"
      message would appear here.
   </applet><hr></html>

Note − You can refer to HTML Applet Tagto understand more about calling applet from HTML.

The code attribute of the <applet> tag is required. It specifies the Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. The applet directive must be closed with an </applet> tag.

If an applet takes parameters, values may be passed for the parameters by adding <param> tags between <applet> and </applet>. The browser ignores text and other tags between the applet tags.

Non-Java-enabled browsers do not process <applet> and </applet>. Therefore, anything that appears between the tags, not related to the applet, is visible in non-Java-enabled browsers.

The viewer or browser looks for the compiled Java code at the location of the document. To specify otherwise, use the codebase attribute of the <applet> tag as shown −

<applet codebase = "https://amrood.com/applets" code = "HelloWorldApplet.class"
   width = "320" height = "120">

If an applet resides in a package other than the default, the holding package must be specified in the code attribute using the period character (.) to separate package/class components. For example −

<applet  = "mypackage.subpackage.TestApplet.class" 
   width = "320" height = "120">

Getting Applet Parameters

The following example demonstrates how to make an applet respond to setup parameters specified in the document. This applet displays a checkerboard pattern of black and a second color.

The second color and the size of each square may be specified as parameters to the applet within the document.

CheckerApplet gets its parameters in the init() method. It may also get its parameters in the paint() method. However, getting the values and saving the settings once at the start of the applet, instead of at every refresh, is convenient and efficient.

The applet viewer or browser calls the init() method of each applet it runs. The viewer calls init() once, immediately after loading the applet. (Applet.init() is implemented to do nothing.) Override the default implementation to insert custom initialization code.

The Applet.getParameter() method fetches a parameter given the parameter’s name (the value of a parameter is always a string). If the value is numeric or other non-character data, the string must be parsed.

The following is a skeleton of CheckerApplet.java −

import java.applet.*;import java.awt.*;publicclassCheckerAppletextendsApplet{int squareSize =50;// initialized to default sizepublicvoid init(){}privatevoid parseSquareSize (String param){}privateColor parseColor (String param){}publicvoid paint (Graphics g){}}

Here are CheckerApplet’s init() and private parseSquareSize() methods −

publicvoid init (){String squareSizeParam = getParameter ("squareSize");
   parseSquareSize (squareSizeParam);String colorParam = getParameter ("color");Color fg = parseColor (colorParam);
   setBackground (Color.black);
   setForeground (fg);}privatevoid parseSquareSize (String param){if(param ==null)return;try{
      squareSize =Integer.parseInt (param);}catch(Exception e){// Let default value remain}}

The applet calls parseSquareSize() to parse the squareSize parameter. parseSquareSize() calls the library method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invalid.

Therefore, parseSquareSize() catches exceptions, rather than allowing the applet to fail on bad input.

The applet calls parseColor() to parse the color parameter into a Color value. parseColor() does a series of string comparisons to match the parameter value to the name of a predefined color. You need to implement these methods to make this applet work.

Specifying Applet Parameters

The following is an example of an HTML file with a CheckerApplet embedded in it. The HTML file specifies both parameters to the applet by means of the <param> tag.

<html><title>Checkerboard Applet</title><hr><appletcode="CheckerApplet.class"width="480"height="320"><paramname="color"value="blue"><paramname="squaresize"value="30"></applet><hr></html>

Note − Parameter names are not case sensitive.

Application Conversion to Applets

It is easy to convert a graphical Java application (that is, an application that uses the AWT and that you can start with the Java program launcher) into an applet that you can embed in a web page.

Following are the specific steps for converting an application to an applet.

Make an HTML page with the appropriate tag to load the applet code.
Supply a subclass of the JApplet class. Make this class public. Otherwise, the applet cannot be loaded.
Eliminate the main method in the application. Do not construct a frame window for the application. Your application will be displayed inside the browser.
Move any initialization code from the frame window constructor to the init method of the applet. You don’t need to explicitly construct the applet object. The browser instantiates it for you and calls the init method.
Remove the call to setSize; for applets, sizing is done with the width and height parameters in the HTML file.
Remove the call to setDefaultCloseOperation. An applet cannot be closed; it terminates when the browser exits.
If the application calls setTitle, eliminate the call to the method. Applets cannot have title bars. (You can, of course, title the web page itself, using the HTML title tag.)
Don’t call setVisible(true). The applet is displayed automatically.

Event Handling

Applets inherit a group of event-handling methods from the Container class. The Container class defines several methods, such as processKeyEvent and processMouseEvent, for handling particular types of events, and then one catch-all method called processEvent.

In order to react to an event, an applet must override the appropriate event-specific method.

import java.awt.event.MouseListener;import java.awt.event.MouseEvent;import java.applet.Applet;import java.awt.Graphics;publicclassExampleEventHandlingextendsAppletimplementsMouseListener{StringBuffer strBuffer;publicvoid init(){
      addMouseListener(this);
      strBuffer =newStringBuffer();
      addItem("initializing the apple ");}publicvoid start(){
      addItem("starting the applet ");}publicvoid stop(){
      addItem("stopping the applet ");}publicvoid destroy(){
      addItem("unloading the applet");}void addItem(String word){System.out.println(word);
      strBuffer.append(word);
      repaint();}publicvoid paint(Graphics g){// Draw a Rectangle around the applet's display area.
      g.drawRect(0,0, 
      getWidth()-1,
      getHeight()-1);// display the string inside the rectangle.
      g.drawString(strBuffer.toString(),10,20);}publicvoid mouseEntered(MouseEventevent){}publicvoid mouseExited(MouseEventevent){}publicvoid mousePressed(MouseEventevent){}publicvoid mouseReleased(MouseEventevent){}publicvoid mouseClicked(MouseEventevent){
      addItem("mouse clicked! ");}}

Now, let us call this applet as follows −

<html><title>Event Handling</title><hr><appletcode="ExampleEventHandling.class"width="300"height="300"></applet><hr></html>

Initially, the applet will display “initializing the applet. Starting the applet.” Then once you click inside the rectangle, “mouse clicked” will be displayed as well.

Displaying Images

An applet can display images of the format GIF, JPEG, BMP, and others. To display an image within the applet, you use the drawImage() method found in the java.awt.Graphics class.

Following is an example illustrating all the steps to show images −

import java.applet.*;import java.awt.*;import java.net.*;publicclassImageDemoextendsApplet{privateImage image;privateAppletContext context;publicvoid init(){
      context =this.getAppletContext();String imageURL =this.getParameter("image");if(imageURL ==null){
         imageURL ="java.jpg";}try{
         URL url =new URL(this.getDocumentBase(), imageURL);
         image = context.getImage(url);}catch(MalformedURLException e){
         e.printStackTrace();// Display in browser status bar
         context.showStatus("Could not load image!");}}publicvoid paint(Graphics g){
      context.showStatus("Displaying image");
      g.drawImage(image,0,0,200,84,null);
      g.drawString("www.javalicense.com",35,100);}}

Now, let us call this applet as follows −

<html><title>The ImageDemo applet</title><hr><appletcode="ImageDemo.class"width="300"height="200"><paramname="image"value="java.jpg"></applet><hr></html>

Playing Audio

An applet can play an audio file represented by the AudioClip interface in the java.applet package. The AudioClip interface has three methods, including −

public void play() − Plays the audio clip one time, from the beginning.
public void loop() − Causes the audio clip to replay continually.
public void stop() − Stops playing the audio clip.

To obtain an AudioClip object, you must invoke the getAudioClip() method of the Applet class. The getAudioClip() method returns immediately, whether or not the URL resolves to an actual audio file. The audio file is not downloaded until an attempt is made to play the audio clip.

Following is an example illustrating all the steps to play an audio −

import java.applet.*;import java.awt.*;import java.net.*;publicclassAudioDemoextendsApplet{privateAudioClip clip;privateAppletContext context;publicvoid init(){
      context =this.getAppletContext();String audioURL =this.getParameter("audio");if(audioURL ==null){
         audioURL ="default.au";}try{
         URL url =new URL(this.getDocumentBase(), audioURL);
         clip = context.getAudioClip(url);}catch(MalformedURLException e){
         e.printStackTrace();
         context.showStatus("Could not load audio file!");}}publicvoid start(){if(clip !=null){
         clip.loop();}}publicvoid stop(){if(clip !=null){
         clip.stop();}}}

Now, let us call this applet as follows −

<html><title>The ImageDemo applet</title><hr><appletcode="ImageDemo.class"width="0"height="0"><paramname="audio"value="test.wav"></applet><hr></html>

You can use test.wav on your PC to test the above example.

Java – Documentation Comments

The Java language supports three types of comments −

Sr.No.	Comment & Description
1	*/ text /* The compiler ignores everything from /* to */.
2	//text The compiler ignores everything from // to the end of the line.
3	/ documentation /* This is a documentation comment and in general its called doc comment. The JDK javadoc tool usesdoc comments when preparing automatically generated documentation.

Sr.No.

Comment & Description

/* text */

The compiler ignores everything from /* to */.

//text

The compiler ignores everything from // to the end of the line.

/** documentation */

This is a documentation comment and in general its called doc comment. The JDK javadoc tool usesdoc comments when preparing automatically generated documentation.

This chapter is all about explaining Javadoc. We will see how we can make use of Javadoc to generate useful documentation for Java code.

What is Javadoc?

Javadoc is a tool which comes with JDK and it is used for generating Java code documentation in HTML format from Java source code, which requires documentation in a predefined format.

Following is a simple example where the lines inside /*….*/ are Java multi-line comments. Similarly, the line which preceeds // is Java single-line comment.

Example

/**
* The HelloWorld program implements an application that
* simply displays "Hello World!" to the standard output.
*
* @author  Zara Ali
* @version 1.0
* @since   2014-03-31 
*/publicclassHelloWorld{publicstaticvoid main(String[] args){/* Prints Hello, World! on standard output.
      System.out.println("Hello World!");
   }
}

You can include required HTML tags inside the description part. For instance, the following example makes use of <h1>….</h1> for heading and <p> has been used for creating paragraph break −

Example

/**
* <h1>Hello, World!</h1>
* The HelloWorld program implements an application that
* simply displays "Hello World!" to the standard output.
* <p>
* Giving proper comments in your program makes it more
* user friendly and it is assumed as a high quality code.
* 
*
* @author  Zara Ali
* @version 1.0
* @since   2014-03-31 
*/publicclassHelloWorld{publicstaticvoid main(String[] args){/* Prints Hello, World! on standard output.
      System.out.println("Hello World!");
   }
}

The javadoc Tags

The javadoc tool recognizes the following tags −

Tag	Description	Syntax
@author	Adds the author of a class.	@author name-text
{@code}	Displays text in code font without interpreting the text as HTML markup or nested javadoc tags.	{@code text}
{@docRoot}	Represents the relative path to the generated document’s root directory from any generated page.	{@docRoot}
@deprecated	Adds a comment indicating that this API should no longer be used.	@deprecated deprecatedtext
@exception	Adds a Throwssubheading to the generated documentation, with the classname and description text.	@exception class-name description
{@inheritDoc}	Inherits a comment from the nearestinheritable class or implementable interface.	Inherits a comment from the immediate surperclass.
{@link}	Inserts an in-line link with the visible text label that points to the documentation for the specified package, class, or member name of a referenced class.	{@link package.class#member label}
{@linkplain}	Identical to {@link}, except the link’s label is displayed in plain text than code font.	{@linkplain package.class#member label}
@param	Adds a parameter with the specified parameter-name followed by the specified description to the “Parameters” section.	@param parameter-name description
@return	Adds a “Returns” section with the description text.	@return description
@see	Adds a “See Also” heading with a link or text entry that points to reference.	@see reference
@serial	Used in the doc comment for a default serializable field.	@serial field-description \| include \| exclude
@serialData	Documents the data written by the writeObject( ) or writeExternal( ) methods.	@serialData data-description
@serialField	Documents an ObjectStreamField component.	@serialField field-name field-type field-description
@since	Adds a “Since” heading with the specified since-text to the generated documentation.	@since release
@throws	The @throws and @exception tags are synonyms.	@throws class-name description
{@value}	When {@value} is used in the doc comment of a static field, it displays the value of that constant.	{@value package.class#field}
@version	Adds a “Version” subheading with the specified version-text to the generated docs when the -version option is used.	@version version-text

Example

Following program uses few of the important tags available for documentation comments. You can make use of other tags based on your requirements.

The documentation about the AddNum class will be produced in HTML file AddNum.html but at the same time a master file with a name index.html will also be created.

importjava.io.*;/** * <h1>Add Two Numbers!</h1> * The AddNum program implements an application that * simply adds two given integer numbers and Prints * the output on the screen. * <p> * <b>Note:</b> Giving proper comments in your program makes it more * user friendly and it is assumed as a high quality code. * * @author Zara Ali * @version 1.0 * @since 2014-03-31 */publicclassAddNum{/** * This method is used to add two integers. This is * a the simplest form of a class method, just to * show the usage of various javadoc Tags. * @param numA This is the first paramter to addNum method * @param numB This is the second parameter to addNum method * @return int This returns sum of numA and numB. */publicintaddNum(intnumA,intnumB){returnnumA+numB;}/** * This is the main method which makes use of addNum method. * @param args Unused. * @return Nothing. * @exception IOException On input error. * @see IOException */publicstaticvoidmain(Stringargs[])throwsIOException{AddNumobj=newAddNum();intsum=obj.addNum(10,20);System.out.println("Sum of 10 and 20 is :"+sum);}}

Now, process the above AddNum.java file using javadoc utility as follows −

$ javadoc AddNum.java
Loading source file AddNum.java...
Constructing Javadoc information...
Standard Doclet version 1.7.0_51
Building tree for all the packages and classes...
Generating /AddNum.html...
AddNum.java:36: warning - @return tag cannot be used in method with void return type.
Generating /package-frame.html...
Generating /package-summary.html...
Generating /package-tree.html...
Generating /constant-values.html...
Building index for all the packages and classes...
Generating /overview-tree.html...
Generating /index-all.html...
Generating /deprecated-list.html...
Building index for all classes...
Generating /allclasses-frame.html...
Generating /allclasses-noframe.html...
Generating /index.html...
Generating /help-doc.html...
1 warning
$

QCOM

Java – Applet Basics

Life Cycle of an Applet

A “Hello, World” Applet

The Applet Class

Invoking an Applet

Getting Applet Parameters

Specifying Applet Parameters

Application Conversion to Applets

Event Handling

Displaying Images

Playing Audio

Java – Documentation Comments

What is Javadoc?

Example

Example

The javadoc Tags

Example

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Life Cycle of an Applet

A “Hello, World” Applet

The Applet Class

Invoking an Applet

Getting Applet Parameters

Specifying Applet Parameters

Application Conversion to Applets

Event Handling

Displaying Images

Playing Audio

Java – Documentation Comments

What is Javadoc?

Example

Example

The javadoc Tags

Example

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