Dealing with Compound Messages

A compound message may contain several kinds of variables: dates, times, strings, numbers, currencies, and percentages. To format a compound message in a locale-independent manner, you construct a pattern that you apply to a MessageFormat object, and store this pattern in a ResourceBundle.

By stepping through a sample program, this section demonstrates how to internationalize a compound message. The sample program makes use of the MessageFormat class. The full source code for this program is in the file called MessageFormatDemo.java. The German locale properties are in the file called MessageBundle_de_DE.properties.

1. Identify the Variables in the Message

Suppose that you want to internationalize the following message:

Notice that we've underlined the variable data and have identified what kind of objects will represent this data.

2. Isolate the Message Pattern in a ResourceBundle

Store the message in a ResourceBundle named MessageBundle, as follows:

ResourceBundle messages =
   ResourceBundle.getBundle("MessageBundle", currentLocale);

This ResourceBundle is backed by a properties file for each Locale. Since the ResourceBundle is called MessageBundle, the properties file for U.S. English is named MessageBundle_en_US.properties. The contents of this file is as follows:

template = At {2,time,short} on {2,date,long}, \
    we detected {1,number,integer} spaceships on \
    the planet {0}.
planet = Mars

The first line of the properties file contains the message pattern. If you compare this pattern with the message text shown in step 1, you'll see that an argument enclosed in braces replaces each variable in the message text. Each argument starts with a digit called the argument number, which matches the index of an element in an Object array that holds the argument values. Note that in the pattern the argument numbers are not in any particular order. You can place the arguments anywhere in the pattern. The only requirement is that the argument number have a matching element in the array of argument values.

The next step discusses the argument value array, but first let's look at each of the arguments in the pattern. The following table provides some details about the arguments:

For a full description of the argument syntax, see the API documentation for the MessageFormat class.

3. Set the Message Arguments

The following lines of code assign values to each argument in the pattern. The indexes of the elements in the messageArguments array match the argument numbers in the pattern. For example, the Integer element at index 1 corresponds to the {1,number,integer} argument in the pattern. Because it must be translated, the String object at element 0 will be fetched from the ResourceBundle with the getString method. Here is the code that defines the array of message arguments:

Object[] messageArguments = {
    messages.getString("planet"),
    new Integer(7),
    new Date()
};

4. Create the Formatter

Next, create a MessageFormat object. You set the Locale because the message contains Date and Number objects, which should be formatted in a locale-sensitive manner.

MessageFormat formatter = new MessageFormat("");
formatter.setLocale(currentLocale);

5. Format the Message Using the Pattern and the Arguments

This step shows how the pattern, message arguments, and formatter all work together. First, fetch the pattern String from the ResourceBundle with the getString method. The key to the pattern is template. Pass the pattern String to the formatter with the applyPattern method. Then format the message using the array of message arguments, by invoking the format method. The String returned by the format method is ready to be displayed. All of this is accomplished with just two lines of code:

formatter.applyPattern(messages.getString("template"));
String output = formatter.format(messageArguments);

6. Run the Demo Program

The demo program prints the translated messages for the English and German locales and properly formats the date and time variables. Note that the English and German verbs ("detected" and "entdeckt") are in different locations relative to the variables:

currentLocale = en_US
At 10:16 AM on July 31, 2009, we detected 7
spaceships on the planet Mars.
currentLocale = de_DE
Um 10:16 am 31. Juli 2009 haben wir 7 Raumschiffe
auf dem Planeten Mars entdeckt.