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Trail: Internationalization
Lesson: Isolating Locale-Specific Data
Isolating Locale-Specific Data
About the ResourceBundle Class
Preparing to Use a ResourceBundle
Backing a ResourceBundle with Properties Files
Using a ListResourceBundle
Customizing Resource Bundle Loading
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About the ResourceBundle Class

### How a ResourceBundle is Related to a Locale

> Conceptually each `ResourceBundle` is a set of related subclasses that share the same base name. The list that follows shows a set of related subclasses. `ButtonLabel` is the base name. The characters following the base name indicate the language code, country code, and variant of a `Locale`. `ButtonLabel_en_GB`, for example, matches the `Locale` specified by the language code for English (`en`) and the country code for Great Britain (`GB`).

> >     ButtonLabel
> >     ButtonLabel_de
> >     ButtonLabel_en_GB
> >     ButtonLabel_fr_CA_UNIX

> To select the appropriate `ResourceBundle`, invoke the `ResourceBundle.getBundle` method. The following example selects the `ButtonLabel` `ResourceBundle` for the `Locale` that matches the French language, the country of Canada, and the UNIX platform.

> >     Locale currentLocale = new Locale("fr", "CA", "UNIX");
> >     ResourceBundle introLabels =
> >         ResourceBundle.getBundle("ButtonLabel", currentLocale);

> If a `ResourceBundle` class for the specified `Locale` does not exist, `getBundle` tries to find the closest match. For example, if `ButtonLabel_fr_CA_UNIX` is the desired class and the default `Locale` is `en_US`, `getBundle` will look for classes in the following order:

> >     ButtonLabel_fr_CA_UNIX
> >     ButtonLabel_fr_CA
> >     ButtonLabel_fr
> >     ButtonLabel_en_US
> >     ButtonLabel_en
> >     ButtonLabel

> Note that `getBundle` looks for classes based on the default `Locale` before it selects the base class (`ButtonLabel)`. If `getBundle` fails to find a match in the preceding list of classes, it throws a `MissingResourceException`. To avoid throwing this exception, you should always provide a base class with no suffixes.

### The ListResourceBundle and PropertyResourceBundle Subclasses

> The abstract class `ResourceBundle` has two subclasses: `PropertyResourceBundle` and `ListResourceBundle`.

> A `PropertyResourceBundle` is backed by a properties file. A properties file is a plain-text file that contains translatable text. Properties files are not part of the Java source code, and they can contain values for `String` objects only. If you need to store other types of objects, use a `ListResourceBundle` instead. The section Backing a ResourceBundle with Properties Files shows you how to use a `PropertyResourceBundle`.

> The `ListResourceBundle` class manages resources with a convenient list. Each `ListResourceBundle` is backed by a class file. You can store any locale-specific object in a `ListResourceBundle`. To add support for an additional `Locale`, you create another source file and compile it into a class file. The section Using a ListResource Bundle has a coding example you may find helpful.

> The `ResourceBundle` class is flexible. If you first put your locale-specific `String` objects in a `PropertyResourceBundle` and then later decided to use `ListResourceBundle` instead, there is no impact on your code. For example, the following call to `getBundle` will retrieve a `ResourceBundle` for the appropriate `Locale`, whether `ButtonLabel` is backed up by a class or by a properties file:

> >     ResourceBundle introLabels =
> >         ResourceBundle.getBundle("ButtonLabel", currentLocale);

### Key-Value Pairs

> `ResourceBundle` objects contain an array of key-value pairs. You specify the key, which must be a `String`, when you want to retrieve the value from the `ResourceBundle`. The value is the locale-specific object. The keys in the following example are the `OkKey` and `CancelKey` strings:

> >     class ButtonLabel_en extends ListResourceBundle {
> >         // English version
> >         public Object[][] getContents() {
> >     	return contents;
> >         }
> >         static final Object[][] contents = {
> >     	{"OkKey", "OK"},
> >     	{"CancelKey", "Cancel"},
> >         };
> >     }

> To retrieve the `OK` `String` from the `ResourceBundle`, you would specify the appropriate key when invoking `getString`:

> >     String okLabel = ButtonLabel.getString("OkKey");

> A properties file contains key-value pairs. The key is on the left side of the equal sign, and the value is on the right. Each pair is on a separate line. The values may represent `String` objects only. The following example shows the contents of a properties file named `ButtonLabel.properties`:

> >     OkKey = OK
> >     CancelKey = Cancel

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