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Java compact1, compact2, compact3 java.util Class ResourceBundle（JDK8）

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Java™ Platform
Standard Ed. 8
compact1, compact2, compact3
java.util

Class 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().

    The implementation of a ResourceBundle subclass must be thread-safe if it's simultaneously used by multiple threads. The default implementations of the non-abstract methods in this class, and the methods in the direct known concrete subclasses ListResourceBundle and PropertyResourceBundle are thread-safe.

    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.

    For the getBundle factory methods that take no ResourceBundle.Control instance, their default behavior of resource bundle loading can be modified with installed ResourceBundleControlProvider implementations. Any installed providers are detected at the ResourceBundle class loading time. If any of the providers provides a ResourceBundle.Control for the given base name, that ResourceBundle.Control will be used instead of the default ResourceBundle.Control. If there is more than one service provider installed for supporting the same base name, the first one returned from ServiceLoader will be used.

    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

    • Field Detail

      • parent

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

    • Method Detail

      • getBaseBundleName

        public String getBaseBundleName()
        Returns the base name of this bundle, if known, or null if unknown. If not null, then this is the value of the baseName parameter that was passed to the ResourceBundle.getBundle(...) method when the resource bundle was loaded.
        Returns:
        The base name of the resource bundle, as provided to and expected by the ResourceBundle.getBundle(...) methods.
        Since:
        1.8
        See Also:
        getBundle(java.lang.String, java.util.Locale, java.lang.ClassLoader)

      • getString

        public final String getString(String key)
        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)
        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)
        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()
        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)
        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)
        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)
        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)
        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)
        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)
        Gets a resource bundle using the specified base name, locale, and class loader.

        This method behaves the same as calling getBundle(String, Locale, ClassLoader, Control) passing a default instance of ResourceBundle.Control unless another ResourceBundle.Control is provided with the ResourceBundleControlProvider SPI. Refer to the description of modifying the default behavior.

        The following describes the default behavior.

        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, script, country, and variant are all empty strings, then the base name is the only candidate bundle name. Otherwise, a list of candidate locales is generated from the attribute values of the specified locale (language, script, country and variant) and appended to the base name. Typically, this will look like the following:

             baseName + "_" + language + "_" + script + "_" + country + "_" + variant
     baseName + "_" + language + "_" + script + "_" + country
     baseName + "_" + language + "_" + script
     baseName + "_" + language + "_" + country + "_" + variant
     baseName + "_" + language + "_" + country
     baseName + "_" + language

        Candidate bundle names where the final component is an empty string are omitted, along with the underscore. For example, if country is an empty string, the second and the fifth candidate bundle names above would be omitted. Also, if script is an empty string, the candidate names including script are omitted. For example, a locale with language "de" and variant "JAVA" will produce candidate names with base name "MyResource" below.

             MyResource_de__JAVA
     MyResource_de
        In the case that the variant contains one or more underscores ('_'), a sequence of bundle names generated by truncating the last underscore and the part following it is inserted after a candidate bundle name with the original variant. For example, for a locale with language "en", script "Latn, country "US" and variant "WINDOWS_VISTA", and bundle base name "MyResource", the list of candidate bundle names below is generated: 
         MyResource_en_Latn_US_WINDOWS_VISTA
 MyResource_en_Latn_US_WINDOWS
 MyResource_en_Latn_US
 MyResource_en_Latn
 MyResource_en_US_WINDOWS_VISTA
 MyResource_en_US_WINDOWS
 MyResource_en_US
 MyResource_en
        Note: For some Locales, the list of candidate bundle names contains extra names, or the order of bundle names is slightly modified. See the description of the default implementation of getCandidateLocales for details.

        getBundle then iterates over the candidate bundle names to find the first one for which it can instantiate an actual resource bundle. It uses the default controls' getFormats method, which generates two bundle names for each generated name, the first a class name and the second a properties file name. For each candidate bundle name, it attempts to create a resource bundle:

        • First, it attempts to load a class using the generated class 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 using the generated properties file name. 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.

        This continues until a result resource bundle is instantiated or the list of candidate bundle names is exhausted. If no matching resource bundle is found, the default control's getFallbackLocale method is called, which returns the current default locale. A new sequence of candidate locale names is generated using this locale and and searched again, as above.

        If still no result bundle is found, the base name alone is looked up. If this still fails, a MissingResourceException is thrown.

        Once a result resource bundle has been found, its parent chain is instantiated. If the result bundle already has a parent (perhaps because it was returned from a cache) the chain is complete.

        Otherwise, getBundle examines the remainder of the candidate locale list that was used during the pass that generated the result resource bundle. (As before, candidate bundle names where the final component is an empty string are omitted.) When it comes to the end of the candidate list, it tries the plain bundle name. With each of the candidate bundle names it attempts to instantiate a resource bundle (first looking for a class and then a properties file, as described above).

        Whenever it succeeds, it calls the previously instantiated resource bundle's setParent method with the new resource bundle. This continues until the list of names is exhausted or the current bundle already has a non-null parent.

        Once the parent chain is complete, the bundle is returned.

        Note: getBundle caches instantiated resource bundles and might return the same resource bundle instance multiple times.

        Note: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 locale arguments below will instantiate resource bundles as follows:

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

        The file MyResources_fr_CH.properties is never used because it is hidden by the 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)
        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 candidate 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

      • handleGetObject

        protected abstract Object handleGetObject(String key)
        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()
        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)
        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()
        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()
        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 subsequent calls. Subclasses can override this method for faster handling.

        Returns:
        a Set of the keys contained only in this ResourceBundle
        Since:
        1.6
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