Backing a ResourceBundle with Properties Files

This section steps through a sample program named PropertiesDemo.

1. Create the Default Properties File

A properties file is a simple text file. You can create and maintain a properties file with just about any text editor.

You should always create a default properties file. The name of this file begins with the base name of your ResourceBundle and ends with the .properties suffix. In the PropertiesDemo program the base name is LabelsBundle. Therefore the default properties file is called LabelsBundle.properties. This file contains the following lines:

# This is the default LabelsBundle.properties file
s1 = computer
s2 = disk
s3 = monitor
s4 = keyboard

Note that in the preceding file the comment lines begin with a pound sign (#). The other lines contain key-value pairs. The key is on the left side of the equal sign and the value is on the right. For instance, s2 is the key that corresponds to the value disk. The key is arbitrary. We could have called s2 something else, like msg5 or diskID. Once defined, however, the key should not change because it is referenced in the source code. The values may be changed. In fact, when your localizers create new properties files to accommodate additional languages, they will translate the values into various languages.

2. Create Additional Properties Files as Needed

To support an additional Locale, your localizers will create a new properties file that contains the translated values. No changes to your source code are required, because your program references the keys, not the values.

For example, to add support for the German language, your localizers would translate the values in LabelsBundle.properties and place them in a file named LabelsBundle_de.properties. Notice that the name of this file, like that of the default file, begins with the base name LabelsBundle and ends with the .properties suffix. However, since this file is intended for a specific Locale, the base name is followed by the language code (de). The contents of LabelsBundle_de.properties are as follows:

# This is the LabelsBundle_de.properties file
s1 = Computer
s2 = Platte
s3 = Monitor
s4 = Tastatur

The PropertiesDemo sample program ships with three properties files:

LabelsBundle.properties
LabelsBundle_de.properties
LabelsBundle_fr.properties

3. Specify the Locale

The PropertiesDemo program creates the Locale objects as follows:

Locale[] supportedLocales = {
    Locale.FRENCH,
    Locale.GERMAN,
    Locale.ENGLISH
};

These Locale objects should match the properties files created in the previous two steps. For example, the Locale.FRENCH object corresponds to the LabelsBundle_fr.properties file. The Locale.ENGLISH has no matching LabelsBundle_en.properties file, so the default file will be used.

4. Create the ResourceBundle

This step shows how the Locale, the properties files, and the ResourceBundle are related. To create the ResourceBundle, invoke the getBundlemethod, specifying the base name and Locale:

ResourceBundle labels =
    ResourceBundle.getBundle("LabelsBundle", currentLocale);

The getBundle method first looks for a class file that matches the base name and the Locale. If it can't find a class file, it then checks for properties files. In the PropertiesDemo program we're backing the ResourceBundle with properties files instead of class files. When the getBundle method locates the correct properties file, it returns a PropertyResourceBundle object containing the key-value pairs from the properties file.

5. Fetch the Localized Text

To retrieve the translated value from the ResourceBundle, invoke the getString method as follows:

String value = labels.getString(key);

The String returned by getString corresponds to the key specified. The String is in the proper language, provided that a properties file exists for the specified Locale.

6. Iterate through All the Keys

This step is optional. When debugging your program, you might want to fetch values for all of the keys in a ResourceBundle. The getKeys method returns an Enumeration of all the keys in a ResourceBundle. You can iterate through the Enumeration and fetch each value with the getString method. The following lines of code, which are from the PropertiesDemo program, show how this is done:

ResourceBundle labels =
    ResourceBundle.getBundle(
        "LabelsBundle", currentLocale);
Enumeration bundleKeys = labels.getKeys();

while (bundleKeys.hasMoreElements()) {
    String key = (String)bundleKeys.nextElement();
    String value = labels.getString(key);
    System.out.println(
        "key = " + key + ", " + "value = " + value);
}

7. Run the Demo Program

Running the PropertiesDemo program generates the following output. The first three lines show the values returned by getString for various Locale objects. The program displays the last four lines when iterating through the keys with the getKeys method.

Locale = fr, key = s2, value = Disque dur
Locale = de, key = s2, value = Platte
Locale = en, key = s2, value = disk

key = s4, value = Clavier
key = s3, value = Moniteur
key = s2, value = Disque dur
key = s1, value = Ordinateur