The Java platform does not require you to use the same Locale throughout your program. If you wish, you can assign a different Locale to every locale-sensitive object in your program. This flexibility allows you to develop multilingual applications, which can display information in multiple languages.
However, most applications are not multi-lingual and their locale-sensitive objects rely on the default Locale. Set by the Java Virtual Machine when it starts up, the default Locale corresponds to the locale of the host platform. To determine the default Locale of your Java Virtual Machine, invoke the Locale.getDefault method.
It is possible to independently set the default locale for two types of uses: the format setting is used for formatting resources, and the display setting is used in menus and dialogs. Introduced in the Java SE 7 release, the
Locale.getDefault(Locale.Category) method takes a
Locale.Category parameter. Passing the FORMAT enum to the getDefault(Locale.Category) method returns the default locale for formatting resources. Similarly, passing the DISPLAY enum returns the default locale used by the UI. The corresponding
setDefault(Locale.Category, Locale) method allows setting the locale for the desired category. The no-argument getDefault method returns the DISPLAY default value.
On the Windows platform, these default values are initialized according to the "Standards and Formats" and "Display Language" settings in the Windows control panel.
You should not set the default Locale programmatically because it is shared by all locale-sensitive classes.
Distributed computing raises some interesting issues. For example, suppose you are designing an application server that will receive requests from clients in various countries. If the Locale for each client is different, what should be the Locale of the server? Perhaps the server is multithreaded, with each thread set to the Locale of the client it services. Or perhaps all data passed between the server and the clients should be locale-independent.
Which design approach should you take? If possible, the data passed between the server and the clients should be locale-independent. This simplifies the design of the server by making the clients responsible for displaying the data in a locale-sensitive manner. However, this approach won't work if the server must store the data in a locale-specific form. For example, the server might store Spanish, English, and French versions of the same data in different database columns. In this case, the server might want to query the client for its Locale, since the Locale may have changed since the last request.