Localization in Java – A Practical Guide

Understanding Locale in Java

Locale in Java represents a specific geographical, political, or cultural region. It consists of language, country, and sometimes variant codes. Knowing how to correctly define a locale is essential for internationalizing your application.

Valid Locale Strings

A valid locale string follows the format language_country, where language is a lowercase ISO-639 code and country is an uppercase ISO-3166-1 code. Optionally, a variant can be added.


Locale locale = new Locale("en", "US");
System.out.println(locale); // Output: en_US
        

Using Locale.Builder

Instead of directly using the Locale constructor, Java provides Locale.Builder, which allows you to set properties like language and country more flexibly.


Locale locale = new Locale.Builder()
                          .setLanguage("fr")
                          .setRegion("FR")
                          .build();
System.out.println(locale); // Output: fr_FR
        

Resource Bundles in Java

Java uses resource bundles to provide localized content based on the current locale. Resource bundles are collections of key-value pairs, where the keys are typically string identifiers, and the values are the localized content for a particular locale.

Using Resource Bundles

The ResourceBundle class is used to load locale-specific data. When you call getBundle(), Java looks for the appropriate resource bundle file based on the current locale.


ResourceBundle bundle = ResourceBundle.getBundle("MessagesBundle", locale);
System.out.println(bundle.getString("greeting")); // Output: Bonjour (if locale is fr_FR)
        



Fallback for Missing Keys

If a resource key is missing, the default behavior is to return null. However, you can provide fallback values using the getProperty() method.


ResourceBundle bundle = ResourceBundle.getBundle("MessagesBundle", locale);
System.out.println(bundle.getString("greeting")); // Output: null (if key is missing)
System.out.println(bundle.getProperty("unknownKey", "Default Value")); // Output: Default Value
        

Understanding Resource Bundle Hierarchy

Java follows a hierarchy when searching for resource bundles. It first looks for a bundle with both language and country, then it searches with just the language, and finally, it defaults to the base bundle if no match is found.

For instance, if you request a fr_FR bundle, and it's not found, Java will attempt to load fr and then fall back to the default base bundle.

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