Introduction

• An applet  is a Java program embedded within a Web page and run by a Web browser.
• An applet has a graphical user interface (GUI ).
• Java applet programming is event-driven programming.
• Applets are much like java applications with the following exceptions:
• You use the Applet class instead of the JFrame class to build your application.
• Like JFrame,  Applet already has a content pane with a default FlowLayout, you may set its layout manager
• There is no "main" method. Instead, a method called init() is called first.
• init() is where you should add the code to build and initialize the content pane.
• You don't have to setup the size of the applet in the java code, instead, this is done in the HTML code.
• Currently many browsers don't support version 2 of Java and/or they don't have the swing classes in their code libraries. Therefore its best to stick with Java version 1 conventions. Before you use a class check the help file for the class and see when it originated. If it says "Since 1.0" then you are ok. To be safe don't use any Swing classes (JApplet, JButton, JTextField, i.e. all the J... classes) instead use the AWT classes (Applet, Button, TextField, Label, etc). There is a way to make it work (by adding in a Java plug-in to the browser, running JAVA as a plugin is the future). Not all browsers support all functions. For example, Netscape version 4.76 does not support the getImage routine (used in chapter 8).
• After learning the Swing classes (JLabel, JButton, etc.) you will be a little disappointed with the AWT versions of the same items. For example, the buttons don't support icons and tool tip text.

Figure 4-2 Java's AWT Objects often used in Applets

Figure 4-7 Sample Hierarchy for SimpleApplet Example

Example : SimpleApplet

• This applet mere displays a button that changes each time you press it.
• Code for this applet:

import java.applet.*;
import java.awt.*;
import java.awt.event.*;

public class SimpleApplet extends Applet implements ActionListener
{
    private Button toggle;       // From java.awt.Button
 

   /*
    * The init() method instantiates the toggle Button, assigns
    * it an ActionListener, and adds it to the applet. This causes
    * the button to appear on the applet's window.
    */
    public void init()
    {
        toggle = new Button ("The machine is off");
        toggle.addActionListener(this);
        add(toggle);
    } // init()

    /*
     * The actionPerformed() method is called whenever the toggle
     * button is clicked. It reverses the button's label.
     */
    public void actionPerformed(ActionEvent e)
    {
        String str = toggle.getLabel();              // Get the toggle Button's label
        if (str.equals("The machine is on"))         // and change it
            toggle.setLabel("The machine is off");
        else                                         // or
            toggle.setLabel("The machine is on");    // change it back
    } // actionPerformed()
} // SimpleApplet

Due to security Applets can't:

• read or write files on the user's system
• communicate with an internet site other than the one they originated on
• run or load any programs in the user's system
• You can read files from the server where the applet resides and communicate with that server

Other programming notes:

• you can add any of the following methods to control interaction if needed:
• start() - this is called whenever the applet begins (after the init) and whenever the user leaves the applet page, then returns to it in the browser
• stop()  - called whenever a user leaves the applet page
• destroy() - allows the applet to perform cleanup before it is released from memory
• paint() is called whenever the applet needs to repaint itself, this occurs if the browser is minimized then restored, during scrolling and any other time the applet becomes visible. NOTE: you only have to provide a paint method if you are doing graphics with a Canvas or Graphics2D object. Buttons, lists, etc repaint themselves.
• If you rebuild your applet you will probably have to exit the browser then restart it to get the new version loaded. Pressing the reload button on the browser may not due the trick.
• Java Console
• Java Console is a window built into most browsers that display error messages from applets and allow a very limited amount of debugging of applets.
• You can use the java console like you did the dos window for displaying debugging messages.
• In Netscape the java console is found under menu item Communicator-Tools-Java Console.
• Internet Explorer  hides the java console, to access it you first must turn it on. To do this
• Start IE
• Select Tools-Internet Options
• Click on the Advanced tab
• Scroll down until you see the checkbox labled "java console enabled", turn that on
• exit IE
• restart IE (just IE not your computer)
• You  now may access the Java Console, Select View-Java Console from the menu.
• To display a message in the console use the System.out.println method, since there is no DOS window, the messages will go to the JAVA Console

The Applet HTML Tag

• Creating the HTML to show the applet.
• You should use the Applet (the newer Object tag can be used as well)

<applet
  CODE     = "Calculator.class"
  ARCHIVE  = "Calculator.jar"
  WIDTH    = 200
  HEIGHT   = 200
  HSPACE   = 0
  VSPACE   = 0
  ALIGN    = Middle>  </applet>
• The CODE parameter identifies the name of the Applet class
• The Archive parameter identifies the name of a JAR file that holds all the class files, gifs, and jpgs associated with the applet. This parameter is omitted if you do not create a jar file. Instead, be sure to have all the class files in the same directory as the one specified in the code parameter.
• The Width and Height specifiy how big a space on the web page to allocate for the applet. this is in pixels
• the HSPACE and VSPACE specify the amount of space around the applet to separate from text on the web page
• the Align positions the applet window on the web page. This works much the same way as aligning text and images on a web page.

JAR Files

• Creating JAR files
• If you applet requires more than one class file, or has jpg, wav, gifs, or other files associated with it, then you should create a JAR file.
• A JAR file takes all the files needed for an applet and zips them together into one file. This makes managing the files on the web site easier and downloading by clients faster.
• To create a JAR file, use the JAR utility (included with the JAVA SDK). For the Calculator Applet:
• jar cf  Calculater.jar *.class
or
• jar cf Calculator.jar Calculator.class KeyPad.class
• The c in the cf tells jar to create a new file. The f says that you will specify the name of that file.
• JAR files can also be used for applications to make them  run like .EXE files.

Calculator Applet

Click here to see the code

WebMenu Example

• This applet shows a couple of new things: how to put messages on the browser status line, and how an applet can interact with the browser.
• To display a message in the status line of the browser use the showStatus method of the Applet.
showStatus("the message");
• Loading Web pages.    For an applet to tell the browser to go to a different page you must first create a URL object then use the showDocument method of the AppletContext.
try{
       URL link = new URL(links[index]);
       getAppletContext().showDocument(link); // link is a string that has the URL
      }
         catch(MalformedURLException ex) {
        showStatus("Bad URL:"+links[index]);
     }
• optionally the showDocument method can take a second parameter that specifies where to show the document (similar to the target parameter on an HTML a href tag). e.g.
       getAppletContext().showDocument(link,"_blank");  // show the document in a new browser window.
 
• "_self"     Show in the window and frame that contain the applet.
• "_parent"   Show in the applet's parent frame. If the applet's frame has no parent frame, acts the same as  "_self".
• "_top"   Show in the top-level frame of the applet's window. If the applet's frame is the top-level frame,   acts the same as "_self".
• "_blank"   Show in a new, unnamed top-level window.
•  name     Show in the frame or window named name. If a target named name does not already exist, a  new top-level window with the specified name is created, and the document is shown there.

Example: An Applet that Sketches

• This program provides basic sketching features for a web page. It consists of two classes:
• Sample12 - is the applet that provides the structure using a border layout. on the west is a tool bar built from a panel with a number of color buttons. on the south is a status line showing the current color, and in the center is a Sketch object.
• Sketcher - is a class that provides a surface (Panel) to draw on and  handles input from the mouse. When the mouse is moved across the surface with the button down then a line appears in the current color. Sketcher inherits/extends Panel and implements MouseListener (for button clicks) and MouseMotionListener (for mouse move events). Chapter 10 talks more about JAVA graphics functions.
• New things:
• MouseListener - this was implemented in the Sketcher class. The constructor for Sketcher add an action listener for the MouseListener. The MouseListener events let Sketcher know when a button was pressed, when it was released, and when the mouse entered and exited the Sketcher's boundaries.
• MouseMotionListener this was implemented in the Sketcher class. The constructor for Sketcher add an action listener for the MouseMotionListener. The MouseMotionListener events let Sketcher know when the mouse moved (with or without a button down).
• MouseEvent - this object is generated and sent to the listeners.  It contains methods to determine the x,y location of the mouse at the time of the event and the state of the mouse buttons.
• Graphics class - Sketcher creates a graphics class whenever it needs to draw something on the screen. Graphics class provides many functions to draw shapes, lines, and text.  In this case I used the setColor and drawLine functions.
• Other notes:
• What's missing from this graphics application ??
• Answer - a paint function.  Sketch something on the applet, then minimize and restore your browser. The picture will be gone.  This is because there is no automatic way for JAVA to know what to restore to the drawing area. You have to provide a paint method (in this case added to the Sketcher class), and enough intelligence to recreate what was drawn.  To implement painting the sketcher should maintain a vector containing objects that represent line segments. Each line segment would have an x,y points for the start and end points and the color. Each time a mouse event draws another line segment it should create a line segment object and add it to the vector. Then when paint is called, the vector can be traversed and all the line segments redrawn.

Example: CyberPet Applet

• Class Name: CyberPetApplet
• Task: User interface for the CyberPet simulation.
• Information (instance variables)
• pet: A reference to a CyberPet
• nameLabel: A Label for the pet's name.
• stateField: A TextField for the pet's state.
• eatButton: A Button for the eat command.
• sleepButton: A Button for the sleep command.
• Manipulations Needed (public methods)
• init(): Creates the GUI components and initialize.
• actionPerformed(): Handles clicks on the Buttons

click here for the CyberPet code

Future of Applets

• Due to the ongoing battles between Sun and Microsoft support for JAVA applets from browsers is in question. Rumors have it that Microsoft XP will ship without a Java Virtual Machine (you can still install one from Sun, that's your best bet in any case).
• The best way around this is to run your applets as plugins. That way you do not have to worry about what version of what browser will be used, and you can use the full power of JAVA 2 and Swing's JApplet class.
• Java Plug-in software enables enterprise customers to direct applets or JavaBeansTM on their intranet web pages to run using Sun's Java 2 Runtime Environment, Standard Edition (JRE) instead of the web browser's default virtual machine. Sun's JRE provides a Java CompatibleTM environment

for today's widely adopted web browsers. That means consistency and reliability when running applets.
• In order to use the plugin the end user must have the plugin installed and the web page containing the applet must have HTML code that directs the plugin to run the applet.
• Sun provides an HTML converter that will change your APPLET tags to code that can check for and launch the Plugin.
• Click here for information on how to do this.

Summary

Keywords:

abstract class, abstract method, applet,Application Programming Interface (API), event-driven, Graphical User Interface (GUI), inheritance, inheritance hierarchy, interface, polymorphism

Suggested Exercises : 1, 2, 4, 5, 6, 12, 17, 18