Applets

General Information

Introduction

Two Java libraries and provide the mechanisms for building graphical user interfaces (GUI) and applets. GUI's allow programs to communicate with users using bitmap displays and gestures with a mouse (as opposed to a textual, command-line approach). Applets are GUI applications that are part of HTML documents and are delivered over the internet using WWW browsers.

Applet Overview

• An on-line version of Euclid's Elements contains the figures as java applets. Here is a captured screen.
• Binary Search Tree
• Animation: NOAA Temp and Wind animation
• real-time example? Shuttle data
• e-commerce example?
• Multimedia: JavaTuner
• Word games at Meriam-Webster

Java Class: Applet

• API Documentation for class
• API Documentation for class

Applet running in browswer

Applet running in appletviewer

Tip: Put an applet tag in the comments of the java source file. Then the source file can be used by appletviewer to test and debug the applet without writing a separate html document.

How does it Work?

Client-side execution

The "applet" tag in HTML

<applet code="ConicApplet" width=400 height=400></applet>

Attributes

ALIGN tag controls "flow"

Applet Tag Details

<applet CODE="applet.class" WIDTH=200 HEIGTH=200>
text for non-JAVA Browsers 
</applet>

<applet CODE="applet.class" WIDTH=200 HEIGTH=200> 
<PARAM NAME="DBHOME" VALUE="dbdirectory">
<PARAM NAME="LANG" VALUE="US-ENGLISH">
text for non-JAVA Browsers 
< /APPLET>

<applet CODEBASE="http://www.somplace.com/java/classes"
           CODE="applet.class" WIDTH=200 HEIGTH=200> 

'<' 'applet'
    ['ARCHIVE '=' archiveList]
    ['CODEBASE' '=' codebaseURL]
    'CODE' '=' appletFile
    ['ALT' '=' alternateText]
    ['NAME' '=' appletInstanceName]
    'WIDTH' '=' pixels 'HEIGHT' '=' pixels
    ['ALIGN' '=' "LEFT"|"RIGHT"|"TOP"|"ABSMIDDLE"|"ABSBOTTOM"|
                 "TEXTTOP"|"MIDDLE"|"BASELINE"|"BOTTOM"]
    ['VSPACE' '=' pixels] ['HSPACE' '=' pixels]
'>' 
['<' 'PARAM' 'NAME' '=' appletAttribute1 'VALUE' '=' value '>']
['<' 'PARAM' 'NAME' '=' appletAttribute2 'VALUE' '=' value '>']
. . .
[alternateHTML]
'</applet>'

ARCHIVE = archiveList

This optional attribute describes one or more archives containing classes and other resources that will be "preloaded". The classes are loaded using an instance of an AppletClassLoader with the given CODEBASE. The archives in archiveList are separated by ",". NB: in Java Development Kit multiple applet tags with the same CODEBASE share the same instance of a ClassLoader. This is used by some client code to implement inter-applet communication.

CODEBASE = codebaseURL

This optional attribute specifies the base URL of the applet -- the directory that contains the applet's code. If this attribute is not specified, then the document's URL is used.

CODE = appletFile

This required attribute gives the name of the file that contains the applet's compiled Applet subclass. This file is relative to the base URL of the applet. It cannot be absolute.

ALT = alternateText

This optional attribute specifies any text that should be displayed if the browser understands the APPLET tag but can't run Java applets.

NAME = appletInstanceName

This optional attribute specifies a name for the applet instance, which makes it possible for applets on the same page to find (and communicate with) each other.

WIDTH = pixels & HEIGHT = pixels

These required attributes give the initial width and height (in pixels) of the applet display area, not counting any windows or dialogs that the applet brings up.

ALIGN = alignment

This required attribute specifies the alignment of the applet. The possible values of this attribute are the same as those for the IMG tag: left, right, top, texttop, middle, absmiddle, baseline, bottom, absbottom.

VSPACE = pixels & HSPACE = pixels

These option attributes specify the number of pixels above and below the applet (VSPACE) and on each side of the applet (HSPACE). They're treated the same way as the IMG tag's VSPACE and HSPACE attributes.

Pie Chart Example

<applet codebase="http://www.cs.fit.edu/~ryan/java/applets/piechart" code="PieChart.class" width=250 height=200>
 <param name=showlabel   value="yes">
 <param name=showpercent value="yes">
 <param name=bgcolor value="white">
 <param name=columns value="2">
 <param name=Plabel1 value="main">
 <param name=Pvalue1 value="2779">
 <param name=Pcolor1 value="red">
 <param name=Plabel2 value="off campus">
 <param name=Pvalue2 value="1399">
 <param name=Pcolor2 value="blue">
</applet>

Netscape

Communicator > Tools > Java Console

IE

1. From the Menu Bar, select Tools.
2. Select Internet Options.
3. Select the Advanced tab.
4. >Make sure you have a check mark next to Java Console Enabled (requires restart), JIT Compiler Enabled, and Java Logging Enabled.

Java Plug-In

• Sun's Plug-in Page
• Documentation

Applet lifecycle and methods

• -- called first and called only once
• -- called everytime applet is displayed on screen
• -- called when browser no longer is displaying applet
• -- called when browser decides that the applet is no longer needed at all

Usual GUI operation in inherited by from and :

• -- called when applet display is damaged
• -- called by

  Default value in :

        public void update(Graphics g) {
  	 g.setColor(getBackground());
  	 g.fillRect(0, 0, width, height);
  	 g.setColor(getForeground());
  	 paint(g);
        }
        

  Don't change background in paint or you will get a flash of color.
• -- force the applet to repaint the whole display as soon as possible

Other methods in class Applet provide the interface to the outside world.

• getAudioClip(URL url)
• getCodeBase()
• getDocumentBase()
• getImage(URL url)
• getLocale() -- for internationalization
• -- named parameters in HTML file (already discussed)
• showStatus()

public class Applet extends Panel {
    public boolean isActive()
    public URL getDocumentBase()
    public URL getCodeBase()
    public String getParameter(String name)
    public AppletContext getAppletContext()
    public void showStatus(String msg) {
        getAppletContext().showStatus(msg);
    }
    public Image getImage(URL url) {
        return getAppletContext().getImage(url);
    }
    public AudioClip getAudioClip(URL url) {
        return getAppletContext().getAudioClip(url);
    }
    public void play(URL url)
    public void init() {}
    public void start() {}
    public void stop() {}
    public void destroy() {}
}

These methods are illustrated in the next few sections.

Methods

Audio

• audio

Jumping to another URL

• switch -- Source code: Switch.java
• Search.java -- showDocument, showStatus

  Use AltaVista to search a particular WWW site (www.cs.fit.edu/~ryan) for:

  _self	current frame
  _parent	parent frame
  _top	top frame
  _blank	new top-level frame
  other	new top-level frame named other

• click

Communicating Applets

• using static
• List.java

  Works with Internet Explorer, but with Netscape it causes a security exception.

Applet Security

• Can't access, read or write files
• Can't create, delete or rename files or directory
• Can't list directory contents
• Can't make a network connection to any other computer except the one form which the applet itself was loaded
• A warning on top-level window to prevent spoofing
• Can't access most of the system properities
• Can't run Reuntime.exec() methods
• Can't execute System.exit()
• Can't manipulate any thread that is not part of the same ThreadGroup as its own.
• Can't create its own class loader
• Can't create its own SecurityManager

Netscape priveledges