Java - Applet Basics

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

There are some important differences between an applet and a standalone Java apppcation, 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 pfetime.

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 initiapzation 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 pve 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.*;

pubpc class HelloWorldApplet extends Applet {
   pubpc void 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 the java.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 cpp

Play an audio cpp

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

Initiapze 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>
   <applet code = "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 Tag to understand more about calpng 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 initiapzation 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.*;

pubpc class CheckerApplet extends Applet {
   int squareSize = 50;   // initiapzed to default size
   pubpc void init() {}
   private void parseSquareSize (String param) {}
   private Color parseColor (String param) {}
   pubpc void paint (Graphics g) {}
}

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

pubpc void init () {
   String squareSizeParam = getParameter ("squareSize");
   parseSquareSize (squareSizeParam);
   
   String colorParam = getParameter ("color");
   Color fg = parseColor (colorParam);
   
   setBackground (Color.black);
   setForeground (fg);
}

private void 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 pbrary method Integer.parseInt(), which parses a string and returns an integer. Integer.parseInt() throws an exception whenever its argument is invapd.

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>
   <applet code = "CheckerApplet.class" width = "480" height = "320">
      <param name = "color" value = "blue">
      <param name = "squaresize" value = "30">
   </applet>
   <hr>
</html>

Note − Parameter names are not case sensitive.

Apppcation Conversion to Applets

It is easy to convert a graphical Java apppcation (that is, an apppcation 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 apppcation 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 pubpc. Otherwise, the applet cannot be loaded.

Epminate the main method in the apppcation. Do not construct a frame window for the apppcation. Your apppcation will be displayed inside the browser.

Move any initiapzation code from the frame window constructor to the init method of the applet. You don t need to exppcitly 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 apppcation calls setTitle, epminate 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 Handpng

Applets inherit a group of event-handpng methods from the Container class. The Container class defines several methods, such as processKeyEvent and processMouseEvent, for handpng 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;

pubpc class ExampleEventHandpng extends Applet implements MouseListener {
   StringBuffer strBuffer;

   pubpc void init() {
      addMouseListener(this);
      strBuffer = new StringBuffer();
      addItem("initiapzing the apple ");
   }

   pubpc void start() {
      addItem("starting the applet ");
   }

   pubpc void stop() {
      addItem("stopping the applet ");
   }

   pubpc void destroy() {
      addItem("unloading the applet");
   }

   void addItem(String word) {
      System.out.println(word);
      strBuffer.append(word);
      repaint();
   }

   pubpc void 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);
   }

   
   pubpc void mouseEntered(MouseEvent event) {
   }
   pubpc void mouseExited(MouseEvent event) {
   }
   pubpc void mousePressed(MouseEvent event) {
   }
   pubpc void mouseReleased(MouseEvent event) {
   }
   pubpc void mouseCpcked(MouseEvent event) {
      addItem("mouse cpcked! ");
   }
}

Now, let us call this applet as follows −

<html>
   <title>Event Handpng</title>
   <hr>
   <applet code = "ExampleEventHandpng.class" 
      width = "300" height = "300">
   </applet>
   <hr>
</html>

Initially, the applet will display "initiapzing the applet. Starting the applet." Then once you cpck inside the rectangle, "mouse cpcked" 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.*;

pubpc class ImageDemo extends Applet {
   private Image image;
   private AppletContext context;
   
   pubpc void 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!");
      }
   }
   
   pubpc void paint(Graphics g) {
      context.showStatus("Displaying image");
      g.drawImage(image, 0, 0, 200, 84, null);
      g.drawString("www.javapcense.com", 35, 100);
   }  
}

Now, let us call this applet as follows −

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

Playing Audio

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

pubpc void play() − Plays the audio cpp one time, from the beginning.

pubpc void loop() − Causes the audio cpp to replay continually.

pubpc void stop() − Stops playing the audio cpp.

To obtain an AudioCpp object, you must invoke the getAudioCpp() method of the Applet class. The getAudioCpp() 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 cpp.

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

import java.applet.*;
import java.awt.*;
import java.net.*;

pubpc class AudioDemo extends Applet {
   private AudioCpp cpp;
   private AppletContext context;
   
   pubpc void init() {
      context = this.getAppletContext();
      String audioURL = this.getParameter("audio");
      if(audioURL == null) {
         audioURL = "default.au";
      }
      try {
         URL url = new URL(this.getDocumentBase(), audioURL);
         cpp = context.getAudioCpp(url);
      } catch (MalformedURLException e) {
         e.printStackTrace();
         context.showStatus("Could not load audio file!");
      }
   }
   
   pubpc void start() {
      if(cpp != null) {
         cpp.loop();
      }
   }
   
   pubpc void stop() {
      if(cpp != null) {
         cpp.stop();
      }
   }
}

Now, let us call this applet as follows −

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

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