java.util.ResourceBundle - Resource Bundle Abstract Class

JDK Tutorials - Herong's Tutorial Examples

∟Locales, Localization Methods and Resource Bundles

∟java.util.ResourceBundle - Resource Bundle Abstract Class

This section provides a tutorial example on how to create resource bundles (localization key names with their localized text messages) as subclasses of the java.util.ResourceBundle abstract class.

The ResourceBundle class, java.util.ResourceBundle, is an abstract class providing a foundation for subclasses to represent a specific map of resource keys and the associated localized resource objects.

Resource bundles are commonly used to localize the text messages in an application, where a key name is assigned to each individual text message. There are 3 ways to map key names to the localized text messages:

Implement a series of subclasses of ResourceBundle class
Implement a series of subclasses of ListResourceBundle class
Implement a series of property files, and let the system to automatically create objects of PropertyResourceBundle class

First let's define some localization key names and localized text messages for some error messages using subclasses of java.util.ResourceBundle class:

1. Mapping ErrorMessage key names to localized text messages for the default locale:

/* ErrorMessage.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.*;
public class ErrorMessage extends ResourceBundle {
   public Object handleGetObject(String key) {
      if (key.equals("NO_FILE")) return "File doesn't exist.";
      if (key.equals("FILE_EMPTY")) return "File is empty.";
      return null;
   }
   public Enumeration<String> getKeys() {
      Vector<String> keys = new Vector<String>();
      keys.addElement("NO_FILE");
      keys.addElement("FILE_EMPTY");
      return keys.elements();
   }
}

Note that:

Only the two required methods handleGetObject() and getKeys() are implemented. They are declared as abstract in the ResourceBundle class.

2. Mapping ErrorMessage key names to localized text messages for the fr locale:

/* ErrorMessage_fr.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.*;
public class ErrorMessage_fr extends ErrorMessage {
   public Object handleGetObject(String key) {
      if (key.equals("NO_FILE")) return "Document n'exist pas.";
      if (key.equals("FILE_EMPTY")) return "Rien.";
      return null;
   }
}

Note that:

I am extending ErrorMessage_fr from ErrorMessage, so I don't have to repeat the handleGetObject() method.
The resource bundle name is the class name of the default locale. The class names for other locales must have a suffix of language code.

3. Mapping ErrorMessage key names to localized text messages for the fr and CA locale:

/* ErrorMessage_fr_CA.java
 * Copyright (c) HerongYang.com. All Rights Reserved.
 */
import java.util.*;
public class ErrorMessage_fr_CA extends ErrorMessage {
   public Object handleGetObject(String key) {
      if (key.equals("NO_FILE")) return "Dossier n'exist pas.";
      return null;
   }
}

Note that:

The class names have the same hierarchical structure as the locales. Default locale is at the top level; language specific locales are at the middle level; language and country specific locales are at the bottom level.
For a given key and a given locale, the lookup mechanism searches in the class that matches the locale first. If failed, it will continue with the class at the parent level. In my example, for the key, "FILE_EMPTY", and the locale, "fr and CA", the lookup mechanism will search the ErrorMessage_fr_CA class first, then the ErrorMessage_fr class, then the ErrorMessage class. So the resulting object will be the String, "Rien." from the ErrorMessage_fr class.