[Chapter 13] 13.3 AWTError

13.3 AWTError

AWTError is a subclass of Error that is used when a serious run-time error has occurred within AWT. For example, an AWTError is thrown if the default Toolkit cannot be initialized or if you try to create a FileDialog within Netscape Navigator (since that program does not permit local file system access). When an AWTError is thrown and not caught, the virtual machine stops your program. You may throw this Error to indicate a serious run-time problem in any subclass of the AWT classes. Using AWTError is slightly preferable to creating your own Error because you don't have to provide another class file. Since it is part of Java, AWTError is guaranteed to exist on the run-time platform.

Methods are not required to declare that they throw AWTError. If you throw an error that is not caught, it will eventually propagate to the top level of the system.

AWTError Method

Constructor

public AWTError (String message): The sole constructor creates an AWTError with a detail message of message. This message can be retrieved using getMessage(), which it inherits from Error (and is required by the Throwable interface). If you do not want a detailed message, message may be null.

Throwing an AWTError

The code in Example 13.1 throws an AWTError if it is executed with this command:

java -Dawt.toolkit=foo throwme

The error occurs because the Java interpreter tries to use the toolkit foo, which does not exist (assuming that class foo does not exist in your CLASSPATH). Therefore, getDefaultToolkit() throws an AWTError, and the next statement never executes.

Example 13.1: The throwme class

import java.awt.Toolkit;
public class throwme {
    public static void main (String[] args) {
        System.out.println (Toolkit.getDefaultToolkit());
        System.out.println ("Never Gets Here");
    }
}