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JavaScript-to-Java communication

because it is a public method. The method simply assigns the specified string to the

message

property,

then calls

repaint()

, which clears the display of the applet and calls the

paint()

method again.

Applets only require a default constructor (which throws a

HeadlessException

if the operating system

doesn’t have a graphical interface). If the applet’s purpose is solely to provide Java functionality to

JavaScript, you can just add public methods and set the applet to a width and height of 0 in the HTML.

If, however, the applet is to display something, you most likely need to defined a

paint()

method.

After the applet is defined and saved in a file with a

.java

extension, compile the file using the

javac

utility from the command line:

javac ExampleApplet.java

This command creates a file with a

.class

extension (the previous example creates

ExampleApplet.class

). The

.class

file must be placed in a Web server directory to allow

HTML pages to access it.

JavaScript-to-Java communication

Now that you have an applet with a public method, you can include it in a Web page and access it via

JavaScript. The following example uses

ExampleApplet

from the previous section and shows how

JavaScript can be used to change the message displayed in the applet:

<html>

<head>

<title>Applet Example</title>

function changeAppletMessage() {

var oApplet = document.getElementById(“ExampleApplet”);

var oTextbox = document.getElementById(“txtMessage”);

oApplet.setMessage(oTextbox.value);

}

</script>

</head>

<body>

<p>Enter the message you want to see in the applet.</p>

</p>

width=”100” height=”100” border=”1” id=”ExampleApplet”>

<applet code=”ExampleApplet.class” width=”100” height=”100”

name=”ExampleApplet”></applet>

</comment>

</object>

</body>

</html>

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