java.util
Class ResourceBundle

java.lang.Object
  java.util.ResourceBundle

Direct Known Subclasses:

ListResourceBundle, PropertyResourceBundle

public abstract class ResourceBundle
extends Object

Resource bundles contain locale-specific objects. When your program needs a locale-specific resource, a String for example, your program can load it from the resource bundle that is appropriate for the current user's locale. In this way, you can write program code that is largely independent of the user's locale isolating most, if not all, of the locale-specific information in resource bundles.

This allows you to write programs that can:

• be easily localized, or translated, into different languages
• handle multiple locales at once
• be easily modified later to support even more locales

Resource bundles belong to families whose members share a common base name, but whose names also have additional components that identify their locales. For example, the base name of a family of resource bundles might be "MyResources". The family should have a default resource bundle which simply has the same name as its family - "MyResources" - and will be used as the bundle of last resort if a specific locale is not supported. The family can then provide as many locale-specific members as needed, for example a German one named "MyResources_de".

Each resource bundle in a family contains the same items, but the items have been translated for the locale represented by that resource bundle. For example, both "MyResources" and "MyResources_de" may have a String that's used on a button for canceling operations. In "MyResources" the String may contain "Cancel" and in "MyResources_de" it may contain "Abbrechen".

If there are different resources for different countries, you can make specializations: for example, "MyResources_de_CH" contains objects for the German language (de) in Switzerland (CH). If you want to only modify some of the resources in the specialization, you can do so.

When your program needs a locale-specific object, it loads the ResourceBundle class using the getBundle method:

 ResourceBundle myResources =
      ResourceBundle.getBundle("MyResources", currentLocale);
 

Resource bundles contain key/value pairs. The keys uniquely identify a locale-specific object in the bundle. Here's an example of a ListResourceBundle that contains two key/value pairs:

 public class MyResources extends ListResourceBundle {
     protected Object[][] getContents() {
         return new Object[][] {
             // LOCALIZE THE SECOND STRING OF EACH ARRAY (e.g., "OK")
             {"OkKey", "OK"},
             {"CancelKey", "Cancel"},
             // END OF MATERIAL TO LOCALIZE
        };
     }
 }
 

Keys are always Strings. In this example, the keys are "OkKey" and "CancelKey". In the above example, the values are also Strings--"OK" and "Cancel"--but they don't have to be. The values can be any type of object.

You retrieve an object from resource bundle using the appropriate getter method. Because "OkKey" and "CancelKey" are both strings, you would use getString to retrieve them:

 button1 = new Button(myResources.getString("OkKey"));
 button2 = new Button(myResources.getString("CancelKey"));
 

The getter methods all require the key as an argument and return the object if found. If the object is not found, the getter method throws a MissingResourceException.

Besides getString, ResourceBundle also provides a method for getting string arrays, getStringArray, as well as a generic getObject method for any other type of object. When using getObject, you'll have to cast the result to the appropriate type. For example:

 int[] myIntegers = (int[]) myResources.getObject("intList");
 

The Java Platform provides two subclasses of ResourceBundle, ListResourceBundle and PropertyResourceBundle, that provide a fairly simple way to create resources. As you saw briefly in a previous example, ListResourceBundle manages its resource as a list of key/value pairs. PropertyResourceBundle uses a properties file to manage its resources.

If ListResourceBundle or PropertyResourceBundle do not suit your needs, you can write your own ResourceBundle subclass. Your subclasses must override two methods: handleGetObject and getKeys().

ResourceBundle.Control

The ResourceBundle.Control class provides information necessary to perform the bundle loading process by the getBundle factory methods that take a ResourceBundle.Control instance. You can implement your own subclass in order to enable non-standard resource bundle formats, change the search strategy, or define caching parameters. Refer to the descriptions of the class and the getBundle factory method for details.

Cache Management

Resource bundle instances created by the getBundle factory methods are cached by default, and the factory methods return the same resource bundle instance multiple times if it has been cached. getBundle clients may clear the cache, manage the lifetime of cached resource bundle instances using time-to-live values, or specify not to cache resource bundle instances. Refer to the descriptions of the getBundle factory method, clearCache, ResourceBundle.Control.getTimeToLive, and ResourceBundle.Control.needsReload for details.

Example

The following is a very simple example of a ResourceBundle subclass, MyResources, that manages two resources (for a larger number of resources you would probably use a Map). Notice that you don't need to supply a value if a "parent-level" ResourceBundle handles the same key with the same value (as for the okKey below).

 // default (English language, United States)
 public class MyResources extends ResourceBundle {
     public Object handleGetObject(String key) {
         if (key.equals("okKey")) return "Ok";
         if (key.equals("cancelKey")) return "Cancel";
         return null;
     }

     public Enumeration<String> getKeys() {
         return Collections.enumeration(keySet());
     }

     // Overrides handleKeySet() so that the getKeys() implementation
     // can rely on the keySet() value.
     protected Set<String> handleKeySet() {
         return new HashSet<String>(Arrays.asList("okKey", "cancelKey"));
     }
 }

 // German language
 public class MyResources_de extends MyResources {
     public Object handleGetObject(String key) {
         // don't need okKey, since parent level handles it.
         if (key.equals("cancelKey")) return "Abbrechen";
         return null;
     }

     protected Set<String> handleKeySet() {
         return new HashSet<String>(Arrays.asList("cancelKey"));
     }
 }
 

You do not have to restrict yourself to using a single family of ResourceBundles. For example, you could have a set of bundles for exception messages, ExceptionResources (ExceptionResources_fr, ExceptionResources_de, ...), and one for widgets, WidgetResource (WidgetResources_fr, WidgetResources_de, ...); breaking up the resources however you like.

Since:

JDK1.1

See Also:

ListResourceBundle, PropertyResourceBundle, MissingResourceException

Nested Class Summary

static class

ResourceBundle.Control ResourceBundle.Control defines a set of callback methods that are invoked by the ResourceBundle.getBundle factory methods during the bundle loading process.

Field Summary

protected ResourceBundle

parent Suppressed. The parent bundle of this bundle.

Constructor Summary

ResourceBundle()
Suppressed.
Sole constructor.

Method Summary

static void	clearCache() Suppressed. Removes all resource bundles from the cache that have been loaded using the caller's class loader.
static void	clearCache(ClassLoader loader) Suppressed. Removes all resource bundles from the cache that have been loaded using the given class loader.
boolean	containsKey(String key) Suppressed. Determines whether the given key is contained in this ResourceBundle or its parent bundles.
static ResourceBundle	getBundle(String baseName) Suppressed. Gets a resource bundle using the specified base name, the default locale, and the caller's class loader.
static ResourceBundle	getBundle(String baseName, Locale locale) Suppressed. Gets a resource bundle using the specified base name and locale, and the caller's class loader.
static ResourceBundle	getBundle(String baseName, Locale locale, ClassLoader loader) Suppressed. Gets a resource bundle using the specified base name, locale, and class loader.
static ResourceBundle	getBundle(String baseName, Locale targetLocale, ClassLoader loader, ResourceBundle.Control control) Suppressed. Returns a resource bundle using the specified base name, target locale, class loader and control.
static ResourceBundle	getBundle(String baseName, Locale targetLocale, ResourceBundle.Control control) Suppressed. Returns a resource bundle using the specified base name, target locale and control, and the caller's class loader.
static ResourceBundle	getBundle(String baseName, ResourceBundle.Control control) Suppressed. Returns a resource bundle using the specified base name, the default locale and the specified control.
abstract Enumeration<String>	getKeys() Suppressed. Returns an enumeration of the keys.
Locale	getLocale() Suppressed. Returns the locale of this resource bundle.
Object	getObject(String key) Suppressed. Gets an object for the given key from this resource bundle or one of its parents.
String	getString(String key) Suppressed. Gets a string for the given key from this resource bundle or one of its parents.
String[]	getStringArray(String key) Suppressed. Gets a string array for the given key from this resource bundle or one of its parents.
protected abstract Object	handleGetObject(String key) Suppressed. Gets an object for the given key from this resource bundle.
protected Set<String>	handleKeySet() Suppressed. Returns a Set of the keys contained only in this ResourceBundle.
Set<String>	keySet() Suppressed. Returns a Set of all keys contained in this ResourceBundle and its parent bundles.
protected void	setParent(ResourceBundle parent) Suppressed. Sets the parent bundle of this bundle.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

parent

protected ResourceBundle parent

Suppressed. default deny

The parent bundle of this bundle. The parent bundle is searched by getObject when this bundle does not contain a particular resource.

Constructor Detail

ResourceBundle

public ResourceBundle()

Suppressed. default deny

Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Detail

getString

public final String getString(String key)

Suppressed. default deny

Gets a string for the given key from this resource bundle or one of its parents. Calling this method is equivalent to calling

(String) getObject(key).

Parameters:

key - the key for the desired string

Returns:

the string for the given key

Throws:

NullPointerException - if key is null

MissingResourceException - if no object for the given key can be found

ClassCastException - if the object found for the given key is not a string

getStringArray

public final String[] getStringArray(String key)

Suppressed. default deny

Gets a string array for the given key from this resource bundle or one of its parents. Calling this method is equivalent to calling

(String[]) getObject(key).

Parameters:

key - the key for the desired string array

Returns:

the string array for the given key

Throws:

NullPointerException - if key is null

MissingResourceException - if no object for the given key can be found

ClassCastException - if the object found for the given key is not a string array

getObject

public final Object getObject(String key)

Suppressed. default deny

Gets an object for the given key from this resource bundle or one of its parents. This method first tries to obtain the object from this resource bundle using handleGetObject. If not successful, and the parent resource bundle is not null, it calls the parent's getObject method. If still not successful, it throws a MissingResourceException.

Parameters:

key - the key for the desired object

Returns:

the object for the given key

Throws:

NullPointerException - if key is null

MissingResourceException - if no object for the given key can be found

getLocale

public Locale getLocale()

Suppressed. default deny

Returns the locale of this resource bundle. This method can be used after a call to getBundle() to determine whether the resource bundle returned really corresponds to the requested locale or is a fallback.

Returns:

the locale of this resource bundle

setParent

protected void setParent(ResourceBundle parent)

Suppressed. default deny

Sets the parent bundle of this bundle. The parent bundle is searched by getObject when this bundle does not contain a particular resource.

Parameters:

parent - this bundle's parent bundle.

getBundle

public static final ResourceBundle getBundle(String baseName)

Suppressed. default deny

Gets a resource bundle using the specified base name, the default locale, and the caller's class loader. Calling this method is equivalent to calling

getBundle(baseName, Locale.getDefault(), this.getClass().getClassLoader()),

except that getClassLoader() is run with the security privileges of ResourceBundle. See getBundle for a complete description of the search and instantiation strategy.

Parameters:

baseName - the base name of the resource bundle, a fully qualified class name

Returns:

a resource bundle for the given base name and the default locale

Throws:

NullPointerException - if baseName is null

MissingResourceException - if no resource bundle for the specified base name can be found

getBundle

public static final ResourceBundle getBundle(String baseName,
                                             ResourceBundle.Control control)

Policy unspecified!

Returns a resource bundle using the specified base name, the default locale and the specified control. Calling this method is equivalent to calling

 getBundle(baseName, Locale.getDefault(),
           this.getClass().getClassLoader(), control),
 

except that getClassLoader() is run with the security privileges of ResourceBundle. See getBundle for the complete description of the resource bundle loading process with a ResourceBundle.Control.

Parameters:

baseName - the base name of the resource bundle, a fully qualified class name

control - the control which gives information for the resource bundle loading process

Returns:

a resource bundle for the given base name and the default locale

Throws:

NullPointerException - if baseName or control is null

MissingResourceException - if no resource bundle for the specified base name can be found

IllegalArgumentException - if the given control doesn't perform properly (e.g., control.getCandidateLocales returns null.) Note that validation of control is performed as needed.

Since:

1.6

getBundle

public static final ResourceBundle getBundle(String baseName,
                                             Locale locale)

Suppressed. default deny

Gets a resource bundle using the specified base name and locale, and the caller's class loader. Calling this method is equivalent to calling

getBundle(baseName, locale, this.getClass().getClassLoader()),

except that getClassLoader() is run with the security privileges of ResourceBundle. See getBundle for a complete description of the search and instantiation strategy.

Parameters:

baseName - the base name of the resource bundle, a fully qualified class name

locale - the locale for which a resource bundle is desired

Returns:

a resource bundle for the given base name and locale

Throws:

NullPointerException - if baseName or locale is null

MissingResourceException - if no resource bundle for the specified base name can be found

getBundle

public static final ResourceBundle getBundle(String baseName,
                                             Locale targetLocale,
                                             ResourceBundle.Control control)

Policy unspecified!

Returns a resource bundle using the specified base name, target locale and control, and the caller's class loader. Calling this method is equivalent to calling

 getBundle(baseName, targetLocale, this.getClass().getClassLoader(),
           control),
 

except that getClassLoader() is run with the security privileges of ResourceBundle. See getBundle for the complete description of the resource bundle loading process with a ResourceBundle.Control.

Parameters:

baseName - the base name of the resource bundle, a fully qualified class name

targetLocale - the locale for which a resource bundle is desired

control - the control which gives information for the resource bundle loading process

Returns:

a resource bundle for the given base name and a Locale in locales

Throws:

NullPointerException - if baseName, locales or control is null

MissingResourceException - if no resource bundle for the specified base name in any of the locales can be found.

IllegalArgumentException - if the given control doesn't perform properly (e.g., control.getCandidateLocales returns null.) Note that validation of control is performed as needed.

Since:

1.6

getBundle

public static ResourceBundle getBundle(String baseName,
                                       Locale locale,
                                       ClassLoader loader)

Suppressed. default deny

Gets a resource bundle using the specified base name, locale, and class loader.

Conceptually, getBundle uses the following strategy for locating and instantiating resource bundles:

getBundle uses the base name, the specified locale, and the default locale (obtained from Locale.getDefault) to generate a sequence of candidate bundle names. If the specified locale's language, country, and variant are all empty strings, then the base name is the only candidate bundle name. Otherwise, the following sequence is generated from the attribute values of the specified locale (language1, country1, and variant1) and of the default locale (language2, country2, and variant2):

• baseName + "_" + language1 + "_" + country1 + "_" + variant1
• baseName + "_" + language1 + "_" + country1
• baseName + "_" + language1
• baseName + "_" + language2 + "_" + country2 + "_" + variant2
• baseName + "_" + language2 + "_" + country2
• baseName + "_" + language2
• baseName

Candidate bundle names where the final component is an empty string are omitted. For example, if country1 is an empty string, the second candidate bundle name is omitted.

getBundle then iterates over the candidate bundle names to find the first one for which it can instantiate an actual resource bundle. For each candidate bundle name, it attempts to create a resource bundle:

• First, it attempts to load a class using the candidate bundle name. If such a class can be found and loaded using the specified class loader, is assignment compatible with ResourceBundle, is accessible from ResourceBundle, and can be instantiated, getBundle creates a new instance of this class and uses it as the result resource bundle.
• Otherwise, getBundle attempts to locate a property resource file. It generates a path name from the candidate bundle name by replacing all "." characters with "/" and appending the string ".properties". It attempts to find a "resource" with this name using ClassLoader.getResource. (Note that a "resource" in the sense of getResource has nothing to do with the contents of a resource bundle, it is just a container of data, such as a file.) If it finds a "resource", it attempts to create a new PropertyResourceBundle instance from its contents. If successful, this instance becomes the result resource bundle.

If no result resource bundle has been found, a MissingResourceException is thrown.

Once a result resource bundle has been found, its parent chain is instantiated. getBundle iterates over the candidate bundle names that can be obtained by successively removing variant, country, and language (each time with the preceding "_") from the bundle name of the result resource bundle. As above, candidate bundle names where the final component is an empty string are omitted. With each of the candidate bundle names it attempts to instantiate a resource bundle, as described above. Whenever it succeeds, it calls the previously instantiated resource bundle's setParent method with the new resource bundle, unless the previously instantiated resource bundle already has a non-null parent.

getBundle caches instantiated resource bundles and may return the same resource bundle instance multiple times.

The baseName argument should be a fully qualified class name. However, for compatibility with earlier versions, Sun's Java SE Runtime Environments do not verify this, and so it is possible to access PropertyResourceBundles by specifying a path name (using "/") instead of a fully qualified class name (using ".").

Example:
The following class and property files are provided:

     MyResources.class
     MyResources.properties
     MyResources_fr.properties
     MyResources_fr_CH.class
     MyResources_fr_CH.properties
     MyResources_en.properties
     MyResources_es_ES.class
 

The contents of all files are valid (that is, public non-abstract subclasses of ResourceBundle for the ".class" files, syntactically correct ".properties" files). The default locale is Locale("en", "GB").

Calling getBundle with the shown locale argument values instantiates resource bundles from the following sources:

• Locale("fr", "CH"): result MyResources_fr_CH.class, parent MyResources_fr.properties, parent MyResources.class
• Locale("fr", "FR"): result MyResources_fr.properties, parent MyResources.class
• Locale("de", "DE"): result MyResources_en.properties, parent MyResources.class
• Locale("en", "US"): result MyResources_en.properties, parent MyResources.class
• Locale("es", "ES"): result MyResources_es_ES.class, parent MyResources.class

The file MyResources_fr_CH.properties is never used because it is hidden by MyResources_fr_CH.class. Likewise, MyResources.properties is also hidden by MyResources.class.

Parameters:

baseName - the base name of the resource bundle, a fully qualified class name

locale - the locale for which a resource bundle is desired

loader - the class loader from which to load the resource bundle

Returns:

a resource bundle for the given base name and locale

Throws:

NullPointerException - if baseName, locale, or loader is null

MissingResourceException - if no resource bundle for the specified base name can be found

Since:

1.2

getBundle

public static ResourceBundle getBundle(String baseName,
                                       Locale targetLocale,
                                       ClassLoader loader,
                                       ResourceBundle.Control control)

Policy unspecified!

Returns a resource bundle using the specified base name, target locale, class loader and control. Unlike the getBundle factory methods with no control argument, the given control specifies how to locate and instantiate resource bundles. Conceptually, the bundle loading process with the given control is performed in the following steps.

1. This factory method looks up the resource bundle in the cache for the specified baseName, targetLocale and loader. If the requested resource bundle instance is found in the cache and the time-to-live periods of the instance and all of its parent instances have not expired, the instance is returned to the caller. Otherwise, this factory method proceeds with the loading process below.
2. The control.getFormats method is called to get resource bundle formats to produce bundle or resource names. The strings "java.class" and "java.properties" designate class-based and property-based resource bundles, respectively. Other strings starting with "java." are reserved for future extensions and must not be used for application-defined formats. Other strings designate application-defined formats.
3. The control.getCandidateLocales method is called with the target locale to get a list of candidate Locales for which resource bundles are searched.
4. The control.newBundle method is called to instantiate a ResourceBundle for the base bundle name, a candidate locale, and a format. (Refer to the note on the cache lookup below.) This step is iterated over all combinations of the candidate locales and formats until the newBundle method returns a ResourceBundle instance or the iteration has used up all the combinations. For example, if the candidate locales are Locale("de", "DE"), Locale("de") and Locale("") and the formats are "java.class" and "java.properties", then the following is the sequence of locale-format combinations to be used to call control.newBundle.

   Locale	format
   Locale("de", "DE")	java.class
   Locale("de", "DE")	java.properties
   Locale("de")	java.class
   Locale("de")	java.properties
   Locale("")	java.class
   Locale("")	java.properties

5. If the previous step has found no resource bundle, proceed to Step 6. If a bundle has been found that is a base bundle (a bundle for Locale("")), and the candidate locale list only contained Locale(""), return the bundle to the caller. If a bundle has been found that is a base bundle, but the candidate locale list contained locales other than Locale(""), put the bundle on hold and proceed to Step 6. If a bundle has been found that is not a base bundle, proceed to Step 7.
6. The control.getFallbackLocale method is called to get a fallback locale (alternative to the current target locale) to try further finding a resource bundle. If the method returns a non-null locale, it becomes the next target locale and the loading process starts over from Step 3. Otherwise, if a base bundle was found and put on hold in a previous Step 5, it is returned to the caller now. Otherwise, a MissingResourceException is thrown.
7. At this point, we have found a resource bundle that's not the base bundle. If this bundle set its parent during its instantiation, it is returned to the caller. Otherwise, its parent chain is instantiated based on the list of candidate locales from which it was found. Finally, the bundle is returned to the caller.

During the resource bundle loading process above, this factory method looks up the cache before calling the control.newBundle method. If the time-to-live period of the resource bundle found in the cache has expired, the factory method calls the control.needsReload method to determine whether the resource bundle needs to be reloaded. If reloading is required, the factory method calls control.newBundle to reload the resource bundle. If control.newBundle returns null, the factory method puts a dummy resource bundle in the cache as a mark of nonexistent resource bundles in order to avoid lookup overhead for subsequent requests. Such dummy resource bundles are under the same expiration control as specified by control.

All resource bundles loaded are cached by default. Refer to control.getTimeToLive for details.

The following is an example of the bundle loading process with the default ResourceBundle.Control implementation.

Conditions:

• Base bundle name: foo.bar.Messages
• Requested Locale: Locale.ITALY
• Default Locale: Locale.FRENCH
• Available resource bundles: foo/bar/Messages_fr.properties and foo/bar/Messages.properties

First, getBundle tries loading a resource bundle in the following sequence.

• class foo.bar.Messages_it_IT
• file foo/bar/Messages_it_IT.properties
• class foo.bar.Messages_it
• file foo/bar/Messages_it.properties
• class foo.bar.Messages
• file foo/bar/Messages.properties

At this point, getBundle finds foo/bar/Messages.properties, which is put on hold because it's the base bundle. getBundle calls control.getFallbackLocale("foo.bar.Messages", Locale.ITALY) which returns Locale.FRENCH. Next, getBundle tries loading a bundle in the following sequence.

• class foo.bar.Messages_fr
• file foo/bar/Messages_fr.properties
• class foo.bar.Messages
• file foo/bar/Messages.properties

getBundle finds foo/bar/Messages_fr.properties and creates a ResourceBundle instance. Then, getBundle sets up its parent chain from the list of the candiate locales. Only foo/bar/Messages.properties is found in the list and getBundle creates a ResourceBundle instance that becomes the parent of the instance for foo/bar/Messages_fr.properties.

Parameters:

baseName - the base name of the resource bundle, a fully qualified class name

targetLocale - the locale for which a resource bundle is desired

loader - the class loader from which to load the resource bundle

control - the control which gives information for the resource bundle loading process

Returns:

a resource bundle for the given base name and locale

Throws:

NullPointerException - if baseName, targetLocale, loader, or control is null

MissingResourceException - if no resource bundle for the specified base name can be found

IllegalArgumentException - if the given control doesn't perform properly (e.g., control.getCandidateLocales returns null.) Note that validation of control is performed as needed.

Since:

1.6

clearCache

public static final void clearCache()

Suppressed. default deny

Removes all resource bundles from the cache that have been loaded using the caller's class loader.

Since:

1.6

See Also:

ResourceBundle.Control.getTimeToLive(String,Locale)

clearCache

public static final void clearCache(ClassLoader loader)

Suppressed. default deny

Removes all resource bundles from the cache that have been loaded using the given class loader.

Parameters:

loader - the class loader

Throws:

NullPointerException - if loader is null

Since:

1.6

See Also:

ResourceBundle.Control.getTimeToLive(String,Locale)

handleGetObject

protected abstract Object handleGetObject(String key)

Suppressed. default deny

Gets an object for the given key from this resource bundle. Returns null if this resource bundle does not contain an object for the given key.

Parameters:

key - the key for the desired object

Returns:

the object for the given key, or null

Throws:

NullPointerException - if key is null

getKeys

public abstract Enumeration<String> getKeys()

Suppressed. default deny

Returns an enumeration of the keys.

Returns:

an Enumeration of the keys contained in this ResourceBundle and its parent bundles.

containsKey

public boolean containsKey(String key)

Suppressed. default deny

Determines whether the given key is contained in this ResourceBundle or its parent bundles.

Parameters:

key - the resource key

Returns:

true if the given key is contained in this ResourceBundle or its parent bundles; false otherwise.

Throws:

NullPointerException - if key is null

Since:

1.6

keySet

public Set<String> keySet()

Suppressed. default deny

Returns a Set of all keys contained in this ResourceBundle and its parent bundles.

Returns:

a Set of all keys contained in this ResourceBundle and its parent bundles.

Since:

1.6

handleKeySet

protected Set<String> handleKeySet()

Suppressed. default deny

Returns a Set of the keys contained only in this ResourceBundle.

The default implementation returns a Set of the keys returned by the getKeys method except for the ones for which the handleGetObject method returns null. Once the Set has been created, the value is kept in this ResourceBundle in order to avoid producing the same Set in the next calls. Override this method in subclass implementations for faster handling.

Returns:

a Set of the keys contained only in this ResourceBundle

Since:

1.6