Working with Exceptions

Handling Hardcoded Exception Messages

Suppose you're calling a method that throws an exception, and you want to display the exception's message when an error occurs. If this method belongs to a class in a package written by someone else, you won't have any control over the message text unless you have access to the source code. This can be a problem if the message text has been hardcoded, because then it can't be translated into other languages. In the following code example, we encounter this problem when opening a file that does not exist:

static public void defaultMessage() {

   try {
      FileInputStream in = new FileInputStream("vapor.txt");
   }
   catch (FileNotFoundException e) {
      System.out.println("e.getMessage = " + e.getMessage());
      System.out.println("e.getLocalizedMessage = " + e.getLocalizedMessage());
      System.out.println("e.toString = " + e.toString());
   }
}

The catch clause of the defaultMessage method generates the following output:

e.getMessage = vapor.txt
e.getLocalizedMessage = vapor.txt
e.toString = java.io.FileNotFoundException: vapor.txt

The messages printed are inadequate, for several reasons. First, the message text is hardcoded in the FileNotFoundException class and cannot be localized. We tried printing the String returned by getLocalizedMessage, but because getLocalizedMessage hasn't been implemented, it merely returns the same message as the getMessage method. Besides, getMessage only returns the file name, which by itself is not a translatable message. Although we may find the file name useful during debugging, our end-users will need a more informative message.

In general, the messages provided by subclasses of the Exception class, such as FileNotFoundException, are meant for programmers, not for end-users. For example, a word processor should not display, "java.io.FileNotFoundException: vapor.txt," when the user tries to open a file that does not exist. This message, though technically correct, doesn't make sense to most people. Instead, when the word processor catches the FileNotFoundException, it should display a message like: "Cannot find the file named vapor.txt. Make sure the file exists."

In an application such as a word processor, the text returned by FileNotFoundException.getMessage is not appropriate for end-users. We should provide a message that is easier to understand, and that can be translated. We'll show you how to do this in the example that follows.

Since the message text needs to be translated, we'll isolate it in a ResourceBundle. We're going to store our messages in properties files, which will back up a ResourceBundle named ExceptionBundle. The message in the ExceptionBundle_en_US.properties file is as follows:

template = Cannot find the file named {0}.  Make sure the file exists.

In the following code example, we display a locale-sensitive message when the FileNotFoundException is caught. First, we create a ResourceBundle for the proper Locale. Next, we retrieve the translated message pattern from the ResourceBundle. Finally, we apply the format pattern to insert the file name into the message.

static public void customMessage(Locale currentLocale) {

   System.out.println("Locale: " + currentLocale.toString());
   String fileName = "vapor.txt";

   try {
      FileInputStream in = new FileInputStream(fileName);
   }
   catch (FileNotFoundException e) {
      ResourceBundle messages =
          ResourceBundle.getBundle("ExceptionBundle",currentLocale);
      Object[] messageArguments = {fileName};
      MessageFormat formatter = new MessageFormat("");
      formatter.setLocale(currentLocale);
      formatter.applyPattern(messages.getString("template"));
      String errorOut = formatter.format(messageArguments);
      System.out.println(errorOut);
   }
}

The output of the customMessage method follows. It's quite an improvement over, "java.io.FileNotFoundException: vapor.txt."

Locale: en_US
Cannot find the file named vapor.txt.  Make sure the file exists.

Locale: de_DE
Die Datei vapor.txt konnte nicht gefunden werden. 
Stellen Sie sicher, daß die Datei existiert.

Working with Exceptions