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Handling Java exceptions

This example presents the user with a text box where a new message can be entered. When the user

clicks the Set Message button, it calls the

changeAppletMessage()

function, which gets a reference to

the applet and retrieves the text from the text box. Then, the function calls the applet’s

setMessage()

method, passing in the entered text. The message displayed in the applet changes from

“Hello

World!”

to whatever the user enters (sometimes this happens quickly, sometimes slowly).

Type conversion

Although JavaScript-to-Java communication is a powerful tool for developers, it is not without its issues.

In the previous example, a string was passed from JavaScript to a Java method without issue. That’s

because the JavaScript

String

object maps directly to the Java

String

object. The same is true for any

primitive value in JavaScript because they all have equivalent primitive types in Java. You can run into

trouble when trying to pass objects into a Java method because no equivalent class exists. For this rea-

son, it’s always best that any method you create for use with JavaScript accept only primitive values.

Handling Java exceptions

Exceptions are much more common in Java than in JavaScript, so you must be aware when they could

possibly occur. When accessing an applet method that could cause an error, you can prepare by wrap-

ping the call in

try..catch

statement. Yes, the JavaScript

try..catch

statement catches exceptions

thrown by an applet.

Suppose you change the

ExampleApplet

to throw an error if

setMessage()

is passed a zero-length

string, such as:

import java.applet.Applet;

import java.awt.Graphics;

import java.awt.HeadlessException;

public class ExampleApplet2 extends Applet {

private String message = “Hello World!”;

public ExampleApplet2() throws HeadlessException {

super();

}

public void paint(Graphics g) {

g.drawString(message, 20, 20);

}

public void setMessage(String message) throws Exception {

if (message.length()== 0) {

throw new Exception(“Message must have at least one character.”);

}

this.message = message;

repaint();

}

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Interacting with Plugins

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