Summary

Remove support for the java.locale.useOldISOCodes system property

Problem

The java.locale.useOldISOCodes system property, originally intended for compatibility, was deprecated in JDK25. Maintaining support adds unnecessary complexity to the codebase.

Solution

Eliminate the property definition from the java.util.Locale class, along with its handling in supporting classes. If the property is specified at runtime via the Java launcher, display a warning indicating that the value is ignored.

Specification

Modify the Legacy Language Codes section in the class description of java.util.Locale class as follows:

-  *
-  * <p>For backwards compatible behavior, the system property
-  * {@systemProperty java.locale.useOldISOCodes} reverts the behavior
-  * back to that of before Java SE 17. If the system property is set to
-  * {@code true}, those three current language codes are mapped to their
-  * backward compatible forms. The property is only read at Java runtime
-  * startup and subsequent calls to {@code System.setProperty()} will
-  * have no effect. <b>As of Java SE 25, the use of the
-  * {@code java.locale.useOldISOCodes} system property is deprecated.
-  * This backwards compatible behavior will be removed in a future release
-  * of the JDK.</b>
-  *
-  * <p>The APIs added in Java SE 7 map between the old and new language codes,
-  * maintaining the mapped codes internal to Locale (so that
-  * {@code getLanguage} and {@code toString} reflect the mapped
-  * code, which depends on the {@code java.locale.useOldISOCodes} system
-  * property), but using the new codes in the BCP 47 language tag APIs (so
-  * that {@code toLanguageTag} reflects the new one). This
-  * preserves the equivalence between Locales no matter which code or
-  * API is used to construct them. Java's default resource bundle
-  * lookup mechanism also implements this mapping, so that resources
-  * can be named using either convention, see {@link ResourceBundle.Control}.
+  * <p>The default resource bundle lookup mechanism also implements
+  * this mapping, so that resources can be named using either convention,
+  * see {@link ResourceBundle.Control}.