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11.6 Formatted Messages

We've seen that in order to internationalize programs, you must place all user-visible messages into resource bundles. This is straightforward when the text to be localized consists of simple labels like those on buttons and menu items. It is trickier, however, with messages that consist partially of static text and partially of dynamic values. For example, a compiler might have to display a message like "Error at line 5 of file 'hello.java'," in which the line number and filename are dynamic and locale-independent, while the rest of the message is static and needs to be localized.

The MessageFormat class of the java.text package helps tremendously with these types of messages. To use it, you store only the static parts of a message in the ResourceBundle and include special characters that indicate where the dynamic parts of the message are to be placed. For example, one resource bundle might contain the message: "Error at line {0} of file {1}." And another resource bundle might contain a "translation" that looks like this: "Erreur: {1}: {0}."

To use such a localized message, you create a MessageFormat object from the static part of the message, and then call its format() method, passing in an array of the values to be substituted. In this case, the array would contain an Integer object that specifies the line number and a String object that specifies the filename. The MessageFormat class knows about other Format classes defined in java.text. It creates and uses NumberFormat objects to format numbers and DateFormat objects to format dates and times. In addition, you can design messages that create ChoiceFormat objects to convert from numbers to strings--this is useful when working with enumerated types such as numbers that correspond to month names, or when you need to use the singular or plural form of a word based on the value of some number.

Example 11.6 demonstrates this kind of MessageFormat usage. It is a convenience class with a single static method for the localized display of exception and error messages. When invoked, the code attempts to load a ResourceBundle with the basename "Errors." If found, it looks up a message resource using the class name of the exception object that was passed. If such a resource is found, it is used to display the error message. An array of five values is passed to the format() method. The localized error message can include any or all of these arguments.

The LocalizedError.display() method defined in this example was used in Example 11.1 at the beginning of this chapter. Example 11.7 shows the default Errors.properties resource bundle used in conjunction with that example. Error message display for the program is nicely internationalized. Porting the program's error message to a new locale is simply a matter of translating (localizing) the Errors.properties file.

Example 11.6: Internationalizing Error Message Display with MessageFormat

import java.text.*;
import java.io.*;
import java.util.*;
/**
 * A convenience class that can display a localized exception message
 * depending on the class of the exception.  It uses a MessageFormat,
 * and passes five arguments that the localized message may include:
 *   {0}: the message included in the exception or error.
 *   {1}: the full class name of the exception or error.
 *   {2}: a guess at what file the exception was caused by.
 *   {3}: a line number in that file.
 *   {4}: the current date and time.
 * Messages are looked up in a ResourceBundle with the basename
 * "Errors," using the full class name of the exception object as
 * the resource name.  If no resource is found for a given exception
 * class, the superclasses are checked.
 */
public class LocalizedError {
  public static void display(Throwable error) {
    ResourceBundle bundle;
    // Try to get the resource bundle.
    // If none, print the error in a non-localized way.
    try { bundle = ResourceBundle.getBundle("Errors"); }
    catch (MissingResourceException e) {
      error.printStackTrace(System.err);
      return;
    }
    // Look up a localized message resource in that bundle, using the
    // classname of the error (or its superclasses) as the resource name.
    // If no resource was found, display the error without localization.
    String message = null;
    Class c = error.getClass();
    while((message == null) && (c != Object.class)) {
      try { message = bundle.getString(c.getName()); }
      catch (MissingResourceException e) { c = c.getSuperclass(); }
    }
    if (message == null) { error.printStackTrace(System.err);  return; }
    // Try to figure out the filename and line number of the
    // exception.  Output the error's stack trace into a string, and
    // use the heuristic that the first line number that appears in
    // the stack trace is after the first or  second colon.  We assume that
    // this stack frame is the first one the programmer has any control
    // over, and so report it as the location of the exception.
    String filename = "";
    int linenum = 0;
    try {
      StringWriter sw = new StringWriter();    // Output stream into a string.
      PrintWriter out = new PrintWriter(sw);   // PrintWriter wrapper.
      error.printStackTrace(out);              // Print stacktrace.
      String trace = sw.toString();            // Get it as a string.
      int pos = trace.indexOf(':');            // Look for first colon.
      if (error.getMessage() != null)          // If the error has a message
        pos = trace.indexOf(':', pos+1);       // look for second colon.
      int pos2 = trace.indexOf(')', pos);      // Look for end of line number.
      linenum = Integer.parseInt(trace.substring(pos+1,pos2));  // Get linenum.
      pos2 = trace.lastIndexOf('(', pos);      // Back to start of filename.
      filename = trace.substring(pos2+1, pos); // Get filename.
    }
    catch (Exception e) { ; }                  // Ignore exceptions.
    // Set up an array of arguments to use with the message.
    String errmsg = error.getMessage();
    Object[] args = {
      ((errmsg!= null)?errmsg:""), error.getClass().getName(),
      filename, new Integer(linenum), new Date()
    };
    // Finally, display the localized error message, using
    // MessageFormat.format() to substitute the arguments into the message.
    System.out.println(MessageFormat.format(message, args));
  }
}

Example 11.7 shows the resource bundle properties file used to localize the set of possible error messages that could be thrown by the ConvertEncoding class shown at the beginning of this chapter. With a resource bundle like this, ConvertEncoding produces error messages like the following:

Error: Specified encoding not supported
        Error occurred at line 46 of file "ConvertEncoding.java"
        at 7:55:28 PM on 08-Apr-97

Example 11.7: A ResourceBundle Properties File Containing Localized Error Messages

#
# This is the file Errors.properties.
# One property for each class of exceptions that our program might
# report.  Note the use of backslashes to continue long lines onto the
# next.  Also note the use of \n and \t for newlines and tabs.
#
java.io.FileNotFoundException: \
Error: File "{0}" not found\n\t\
Error occurred at line {3} of file "{2}"\n\tat {4}
java.io.UnsupportedEncodingException: \
Error: Specified encoding not supported\n\t\
Error occurred at line {3} of file "{2}"\n\tat {4,time} on {4,date}
java.io.CharConversionException:\
Error: Character conversion failure.  Input data is not in specified format.
# A generic resource.  Display a message for any error or exception that
# is not handled by a more specific resource.
java.lang.Throwable:\
Error: {1}: {0}\n\t\
Error occurred at line {3} of file "{2}"\n\t{4,time,long} {4,date,long}


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