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Applets
For a tutorial, see
http://java.sun.com/docs/books/tutorial/applet/overview/index.html
Applets
Special Java programs (without a “main”) callable
from HTML and executed in a graphic context.
They can be executed by:
a Java enabled Web Browser;
ad-hoc programs (e.g. Sun AppletViewer).
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Applets
Every applet is implemented by creating a
subclass of the Applet class.
The hierarchy determines much of what an
applet can do and how.
Applet Lifecycle
An applet can react to major events in the following
ways:
It can initialize itself.
init()
It can start running.
start()
It can draw some graphics. paint()
It can respond to user-generated events (Mouse,
keyboard, menus…). handleEvent()
It can stop running.
stop()
It can perform a final cleanup, in preparation for
being unloaded.
destroy()
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Applet Lifecycle
init()
Multithreading!
start()
handleEvent()
stop()
destroy()
Whenever it’s needed,
at lower priority
paint()
Actually, more threads are active
behind the scenes.
handleEvent()
This code is part of the AWT (1.0 Event Model)
public boolean handleEvent(Event evt) {
switch (evt.id) {
case Event.MOUSE_ENTER: return mouseEnter(evt, evt.x, evt.y);
case Event.MOUSE_EXIT:
return mouseExit(evt, evt.x, evt.y);
case Event.MOUSE_MOVE: return mouseMove(evt, evt.x, evt.y);
case Event.MOUSE_DOWN: return mouseDown(evt, evt.x, evt.y);
case Event.MOUSE_DRAG: return mouseDrag(evt, evt.x, evt.y);
case Event.MOUSE_UP:
return mouseUp(evt, evt.x, evt.y);
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handleEvent()
case
case
case
case
Event.KEY_PRESS:
Event.KEY_ACTION:
return keyDown(evt, evt.key);
Event.KEY_RELEASE:
Event.KEY_ACTION_RELEASE: return keyUp(evt, evt.key);
case Event.ACTION_EVENT:
case Event.GOT_FOCUS:
return action(evt, evt.arg);
return gotFocus(evt, evt.arg);
case Event.LOST_FOCUS:
return lostFocus(evt, evt.arg);
}
return false;
}
Applets-Event handling
To react to an event, an applet must override either the
appropriate event-specific method or the
handleEvent method.
For example, adding the following code to the Simple
applet makes it respond to mouse clicks.
import java.awt.Event;
...
public boolean mouseDown(Event event, int x, int y) { addItem("click!... ");
return true;
}
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HTML 3.2: Java Applet Support
<applet code=“Name.class”height=“150” width=“300”>
<param name=“time” value=“100”>
<param name=“color” value=“red”>
This Browser does not support applets
</Applet>
ted
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in
4.0
L
HT M
Objects (HTML 4.0)
The new tag OBJECT is defined. It passes to the browser
Info needed to load non-native datatypes (applets, plugins,
Active-X controls ecc.)
The PARAM tag allows to pass parameters to the applet
Or to the plugin.
<OBJECT HEIGHT=… WIDTH=… CLASSID=…>
<PARAM NAME=… VALUE=…>
</OBJECT>
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Applet example
package demoApplet;
import java.awt.*;
import java.applet.*;
public class SimpleApplet extends Applet {
String var0;
StringBuffer buffer;
//Construct the applet
public SimpleApplet() {
}
//Initialize the applet
public void init() {
buffer = new StringBuffer();
addItem("initializing... ");
}
Applet example
//Start the applet
public void start() {
addItem("starting... ");
addItem("Parameter is: "+getParameter("param0"));
}
//Stop the applet
public void stop() {
addItem("stopping... ");
}
//Destroy the applet
public void destroy() {
addItem("destroying... ");
}
void addItem(String newWord) {
System.out.println(newWord);
buffer.append(newWord);
repaint();
}
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Applet example
public void paint(Graphics g) {
int red = (int)(Math.random() * 255);
int green = (int)(Math.random() * 255);
int blue = (int)(Math.random() * 255);
g.setColor(new Color(red, green, blue));
System.out.println(red+green+blue);
//Draw a Rectangle around the applet's display area.
g.fillRect(0, 0, size().width - 1, size().height - 1);
g.setColor(new Color(0, 0, 0));
//Draw the current string inside the rectangle.
g.drawString(buffer.toString(), 5, 15);
}
}
HTML page (tag Applet)
<head>
<title>
HTML Test Page
</title>
</head>
<body>
demoApplet.SimpleApplet will appear below in a Java enabled browser.<br>
<applet
codebase = "."
code = "demoApplet.SimpleApplet.class"
name
= "TestApplet"
width = "400"
height = "300"
hspace = "0"
vspace = "0"
align = "middle"
>
Your browser cannot show applets
<param name = "param0" value = "hello">
</applet>
</body>
</html>
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HTML page (tag Object)
<head>
<title>
HTML Test Page
</title>
</head>
<body>
demoApplet.SimpleApplet will appear below in a Java enabled browser.<br>
<object classid="java:demoApplet.SimpleApplet.class">
<param name = "param0" value = "hello" valuetype="data">
Your browser cannot show applets
<param name = "param0" value = "hello">
</object>
</body>
</html>
JApplet
An extended version of java.applet.Applet that adds support for
the JFC/Swing component architecture. You can find taskoriented documentation about using JApplet in The Java Tutorial,
in the section How to Make Applets.
The JApplet class is slightly incompatible with java.applet.Applet.
JApplet contains a JRootPane as it's only child. The contentPane
should be the parent of any children of the JApplet. This is
different than java.applet.Applet, e.g. to add a child to an an
java.applet.Applet you'd write:
applet.add(child);
However using JApplet you need to add the child to the JApplet's
contentPane instead:
applet.getContentPane().add(child);
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Java-JavaScript interaction: JSObject
JSObject allows Java to manipulate objects that are defined in JavaScript.
Values passed from Java to JavaScript are converted as follows:
JSObject is converted to the original JavaScript object.
Any other Java object is converted to a JavaScript wrapper, which can be
used to access methods and fields of the Java object.
Converting this wrapper to a string will call the toString method on the
original object, converting to a number will call the floatValue method if
possible and fail otherwise.
Converting to a boolean will try to call the booleanValue method in the
same way.
Java arrays are wrapped with a JavaScript object that understands
array.length and array[index].
A Java boolean is converted to a JavaScript boolean.
Java byte, char, short, int, long, float, and double are converted to
JavaScript numbers.
Note If you call a Java method from JavaScript, this conversion happens
automatically--you can pass in "int" argument and it works.
Java-JavaScript interaction: JSObject
Values passed from JavaScript to Java are converted as follows:
Objects that are wrappers around Java objects are unwrapped.
Other objects are wrapped with a JSObject.
Strings, numbers, and booleans are converted to String, Float, and
Boolean objects respectively.
Examples
(String) window.getMember("name")
(JSObject) window.getMember("document")
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Java-JavaScript interaction: JSObject
The netscape.javascript.JSObject class has the following methods:
Method
Description
Call
Calls a JavaScript method
Eval
Evaluates a JavaScript expression
getMember
Retrieves a named member of a JavaScript object
getSlot
Retrieves an indexed member of a JavaScript object
removeMember Removes a named member of a JavaScript object
setMember
Sets a named member of a JavaScript object
setSlot
Sets an indexed member of a JavaScript object
toString
Converts a JSObject to a string
The netscape.javascript.JSObject class has the following static methods:
getWindow
Gets a JSObject for the window containing the given applet
Java-JavaScript interaction: applet side
package javascript;
import netscape.javascript.*;
import java.applet.*;
import java.awt.*;
public class MyApplet extends Applet {
private JSObject mainWindow;
private JSObject pageDoc;
private JSObject location;
private String s;
public String comment="instanceVarContent";
public void init() {
System.out.println("initing");
mainWindow = JSObject.getWindow(this);
pageDoc = (JSObject) mainWindow.getMember("document");
location = (JSObject) mainWindow.getMember("location");
s = (String) location.getMember("href"); // document.location.href
}
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Java-JavaScript interaction: applet side
public int r=0;
public int g=255;
public int b=0;
public void start(){
s=(String)mainWindow.call("f",null);
String[] stringArgs = new String[1];
stringArgs[0] = "5";
s=(String)mainWindow.call("g", stringArgs);
System.out.println (" Calling g returned "+s);
}
public void paint(Graphics gra) {
if (s==null) s="NULL";
gra.setColor(new Color(r,g,b));
Dimension d=this.getSize();
gra.fillRect(0,0,d.width,d.height);
gra.setColor(new Color(0,0,0));
gra.drawString("VERSION 1",80,80);
gra.drawString(s,30,30);
}
Java-JavaScript interaction: applet side
void changeColor(String s) {
int x=Integer.parseInt(s);
r=x;
this.repaint();
}
public String square(String sx) {
int x=Integer.parseInt(sx);
return new Integer(x*x).toString();
}
}
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Java-JavaScript interaction: JavaScript side
<HTML>
<head>
<script>
function g(x){return x+x}
function f(){return "Called f()";}
</script>
</head>
<body>
<script type="text/JavaScript">
<!-- //hide script from old browsers
document.write("<h2>JavaScript is enabled.</h2>")
// end hiding contents from old browsers -->
</script>
<noscript><h2>JavaScript is not enabled, or your browser has
restricted this file from showing active
content.</h2></noscript>
Java-JavaScript interaction: JavaScript side
<script>
<!-- //hide script from old browsers
var jEnabled = navigator.javaEnabled();
if (jEnabled){
document.write("<h2>JAVA is enabled.</h2>")
}else{
document.write("<h2>JAVA is <i>NOT</i> enabled.</h2>")
}
// end hiding contents from old browsers -->
</script>
<APPLET code="javascript.MyApplet.class" name="app"
codebase="classes/" align="baseline"
width="200" height="200"
MAYSCRIPT>
<PARAM NAME="param" VALUE="1">
If your browser is blocking the content, please click on the
bar above.
</APPLET>
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Java-JavaScript interaction: JavaScript side
<script language="Javascript">
document.write(f());
</script>
<script language="Javascript">
document.write(app.comment);
app.r=255;
document.write(app.square("3");
</script>
<form>
<input name="ChangeColorButton" value="Change color"
type="button"
onclick="app.r=(app.r+100)%256;app.repaint()";/>
</form>
<form>
<input title="writeButton" value="write on console"
type="button" onclick='java.lang.System.out.println("a java
message");';/>
</form>
</body>
</html>
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