Java AWT Reference

13

AWT Exceptions and Errors

In this chapter:

• AWTException
• IllegalComponentStateException
• AWTError

This chapter describes AWTException, IllegalComponentStateException, and AWTError. AWTException is a subclass of Exception. It is not used by any of the public classes in java.awt; you may, however, find it convenient to throw AWTException within your own code. IllegalComponentStateException is another Exception subclass, which is new to Java 1.1. This exception is used when you try to do something with a Component that is not yet appropriate. AWTError is a subclass of Error that is thrown when a serious problem occurs in AWT—for example, the environment is unable to get the platform's Toolkit.

13.1 AWTException

AWTException is a generic exception that can be thrown when an exceptional condition has occurred within AWT. None of the AWT classes throw this. If you subclass any of the AWT classes, you can throw an AWTException to indicate a problem. Using AWTException is slightly preferable to creating your own Exception subclass because you do not have to generate another class file. Since it is a part of Java, AWTException is guaranteed to exist on the run-time platform.

If you throw an instance of AWTException, like any other Exception, it must be caught in a catch clause or declared in the throws clause of the method.

13.1.1 AWTException Method

Constructor

public AWTException (String message)

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

13.1.2 Throwing an AWTException

An AWTException is used the same way as any other Throwable object. Here's an example:

if (someProblem) {
    throw new AWTException ("Problem Encountered While Initializing");
}

13.2 IllegalComponentStateException

IllegalComponentStateException is a subclass of IllegalStateException; both are new to Java 1.1. This exception is used when you try to do something with a Component that is not yet appropriate. With the standard AWT components, this can happen only in three instances:

• If you call setCaretPosition() to set the cursor position of a text component before the component's peer exists.
• If you call getLocale() to get the locale of a component that does not have one and is not in a container that has one.
• If you call getLocationOnScreen() for a component that is not showing.

In these cases, the operation isn't fundamentally illegal; you are just trying to perform it before the component is ready. When you create your own components, you should consider using this exception for similar cases.

Since IllegalComponentStateException is a subclass of RunTimeException, you do not have to enclose method calls that might throw this exception within try/catch blocks. However, catching this exception isn't a bad idea, since it should be fairly easy to correct the problem and retry the operation.

13.2.1 IllegalComponentStateException Method

Constructor

public IllegalComponentStateException ()

The first constructor creates an IllegalComponentStateException instance with no detail message.

public IllegalComponentStateException (String message)

This constructor creates an IllegalComponentStateException with a detail message of message. This message can be retrieved using getMessage(), which it inherits from Exception (and is required by the Throwable interface).

13.2.2 IllegalComponentStateException Example

The following code throws an IllegalComponentStateException. The Exception occurs because the TextField peer does not exist when setCaretPosition() is called. setCaretPosition() throws an IllegalComponentStateException, and the next statement never executes.

import java.awt.TextField;
public class illegal {
    public static void main (String[] args) {
        new TextField().setCaretPosition (24);
        System.out.println ("Never gets here");
    }
}

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.

13.3.1 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.

13.3.2 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");
    }
}