Remarks#

Java comes with a powerful and flexible mechanism for localizing your applications, but it’s also easy to misuse and wind up with a program that disregards or mangles the user’s locale, and therefore how they expect your program to behave.

Your users will expect to see data localized to the formats they’re used to, and attempting to support this manually is a fools errand. Here is just a small example of the different ways users expect to see content you might assume is “always” displayed a certain way:

General Resources

• Wikipedia: Internationalization and Localization

Java Resources

• Java Tutorial: Internationalization

• Oracle: Internationalization: Understanding Locale in the Java Platform

• JavaDoc: Locale

Automatically formatted Dates using “locale”

SimpleDateFormatter is great in a pinch, but like the name suggests it doesn’t scale well.

If you hard-code "MM/dd/yyyy" all over your application your international users won’t be happy.

Let Java do the work for you

Use the static methods in DateFormat to retrieve the right formatting for your user. For a desktop application (where you’ll rely on the default locale), simply call:

String localizedDate = DateFormat.getDateInstance(style).format(date);

Where style is one of the formatting constants (FULL, LONG, MEDIUM, SHORT, etc.) specified in DateFormat.

For a server-side application where the user specifies their locale as part of the request, you should pass it explicitly to getDateInstance() instead:

String localizedDate =
    DateFormat.getDateInstance(style, request.getLocale()).format(date);

String Comparison

Compare two Strings ignoring case:

"School".equalsIgnoreCase("school"); // true

Don’t use

text1.toLowerCase().equals(text2.toLowerCase());

Languages have different rules for converting upper and lower case. A ‘I’ would be converted to ‘i’ in English. But in Turkish a ‘I’ becomes a ‘ı’. If you have to use toLowerCase() use the overload which expects a Locale: String.toLowerCase(Locale).

Comparing two Strings ignoring minor differences:

Collator collator = Collator.getInstance(Locale.GERMAN);
collator.setStrength(Collator.PRIMARY);
collator.equals("Gärten", "gaerten"); // returns true

Sort Strings respecting natural language order, ignoring case (use collation key to:

String[] texts = new String[] {"Birne", "äther", "Apfel"};
Collator collator = Collator.getInstance(Locale.GERMAN);
collator.setStrength(Collator.SECONDARY); // ignore case
Arrays.sort(texts, collator::compare); // will return {"Apfel", "äther", "Birne"}

Locale

The java.util.Locale class is used to represent a “geographical, political or cultural” region to localize a given text, number, date or operation to. A Locale object may thus contain a country, region, language, and also a variant of a language, for instance a dialect spoken in a certain region of a country, or spoken in a different country than the country from which the language originates.

The Locale instance is handed to components that need to localize their actions, whether it is converting the input, output, or just need it for internal operations. The Locale class cannot do any internationalization or localization by itself

Language

The language must be an ISO 639 2 or 3 character language code, or a registered language subtag of up to 8 characters. In case a language has both a 2 and 3 character language code, use the 2 character code. A full list of language codes can be found in the IANA Language Subtag Registry.

Language codes are case insensitive, but the Locale class always use lowercase versions of the language codes

Creating a Locale

Creating a java.util.Locale instance can be done in four different ways:

Locale constants
Locale constructors
Locale.Builder class
Locale.forLanguageTag factory method 

Java ResourceBundle

You create a ResourceBundle instance like this:

Locale locale = new Locale("en", "US");
ResourceBundle labels = ResourceBundle.getBundle("i18n.properties");
System.out.println(labels.getString("message"));

Consider I have a property file i18n.properties:

message=This is locale

Output:

This is locale

Setting Locale

If you want to reproduce the state using other languages, you can use setDefault() method. Its usage:

 setDefault(Locale.JAPANESE); //Set Japanese