src/java.base/share/classes/java/util/Locale.java

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  import java.io.ObjectInputStream;
  import java.io.ObjectOutputStream;
  import java.io.ObjectStreamField;
  import java.io.Serializable;
  import java.text.MessageFormat;
+ import java.util.concurrent.ConcurrentHashMap;
  import java.util.spi.LocaleNameProvider;
  
  import sun.security.action.GetPropertyAction;
  import sun.util.locale.BaseLocale;
  import sun.util.locale.InternalLocaleBuilder;
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      /** serialization ID
       */
      static final long serialVersionUID = 9149081749638150636L;
  
      /**
+      * Enum for specifying the type defined in ISO 3166. This enum is used to
+      * retrieve the two-letter ISO3166-1 alpha-2, three-letter ISO3166-1
+      * alpha-3, four-letter ISO3166-3 country codes.
+      *
+      * @see #getISOCountries(Locale.IsoCountryCode)
+      * @since 9
+      */
+     public static enum IsoCountryCode {
+         /**
+          * PART1_ALPHA2 is used to represent the ISO3166-1 alpha-2 two letter
+          * country codes.
+          */
+         PART1_ALPHA2 {
+             @Override
+             Set<String> getISOCountriesImpl() {
+                 return Set.of(Locale.getISOCountries());
+             }
+         },
+         
+         /**
+          *
+          * PART1_ALPHA3 is used to represent the ISO3166-1 alpha-3 three letter
+          * country codes.
+          */
+         
+         PART1_ALPHA3 {
+             @Override
+             Set<String> getISOCountriesImpl() {
+                 return LocaleISOData.computeISO3166_1Alpha3Countries();
+             }
+         },
+         
+         /**
+          * PART3 is used to represent the ISO3166-3 four letter country codes.
+          */
+         
+         PART3 {
+             @Override
+             Set<String> getISOCountriesImpl() {
+                 return Set.of(LocaleISOData.ISO3166_3);
+             }
+         };
+ 
+         /**
+          * Concrete implementation of this mapping function attempts to compute value 
+          * for iso3166CodesMap for each IsoCountryCode type key.
+          */
+         abstract Set<String> getISOCountriesImpl();
+ 
+         /**
+          * Map to hold country codes for each ISO3166 part.
+          */
+         private static Map<IsoCountryCode, Set<String>> iso3166CodesMap = new ConcurrentHashMap<>();
+ 
+         /**
+          * This method is called from Locale class to retrieve country code set
+          * for getISOCountries(type)
+          */
+         static Set<String> retrieveISOCountryCodes(IsoCountryCode type) {
+             return iso3166CodesMap.computeIfAbsent(type, IsoCountryCode::getISOCountriesImpl);
+         }
+     }
+ 
+     /**
       * Display types for retrieving localized names from the name providers.
       */
      private static final int DISPLAY_LANGUAGE = 0;
      private static final int DISPLAY_COUNTRY  = 1;
      private static final int DISPLAY_VARIANT  = 2;
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      }
  
      /**
       * Returns a list of all 2-letter country codes defined in ISO 3166.
       * Can be used to create Locales.
       * <p>
       * <b>Note:</b> The <code>Locale</code> class also supports other codes for
       * country (region), such as 3-letter numeric UN M.49 area codes.
       * Therefore, the list returned by this method does not contain ALL valid
       * codes that can be used to create Locales.
!      *
       * @return An array of ISO 3166 two-letter country codes.
       */
      public static String[] getISOCountries() {
          if (isoCountries == null) {
              isoCountries = getISO2Table(LocaleISOData.isoCountryTable);
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      }
  
      /**
       * Returns a list of all 2-letter country codes defined in ISO 3166.
       * Can be used to create Locales.
+      * This method is equivalent to {@link #getISOCountries(Locale.IsoCountryCode type)}
+      * with {@code type}  {@link IsoCountryCode#PART1_ALPHA2}.
       * <p>
       * <b>Note:</b> The <code>Locale</code> class also supports other codes for
       * country (region), such as 3-letter numeric UN M.49 area codes.
       * Therefore, the list returned by this method does not contain ALL valid
       * codes that can be used to create Locales.
!      * <p>
!      * Note that this method does not return obsolete 2-letter country codes.
!      * ISO3166-3 codes which designate country codes for those obsolete codes,
!      * can be retrieved from {@link #getISOCountries(Locale.IsoCountryCode type)} with
!      * {@code type}  {@link IsoCountryCode#PART3}.
       * @return An array of ISO 3166 two-letter country codes.
       */
      public static String[] getISOCountries() {
          if (isoCountries == null) {
              isoCountries = getISO2Table(LocaleISOData.isoCountryTable);
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          System.arraycopy(isoCountries, 0, result, 0, isoCountries.length);
          return result;
      }
  
      /**
+      * Returns a {@code Set} of ISO3166 country codes for the specified type.
+      *
+      * @param type {@link Locale.IsoCountryCode} specified ISO code type.
+      * @see java.util.Locale.IsoCountryCode
+      * @throws NullPointerException if type is null
+      * @return a {@code Set} of ISO country codes for the specified type.
+      * @since 9
+      */
+     public static Set<String> getISOCountries(IsoCountryCode type) {
+         Objects.requireNonNull(type);
+         return IsoCountryCode.retrieveISOCountryCodes(type);
+     }
+ 
+     /**
       * Returns a list of all 2-letter language codes defined in ISO 639.
       * Can be used to create Locales.
       * <p>
       * <b>Note:</b>
       * <ul>