code: public static synchronized void setDefault(Locale newLocale) {
          setDefault(Category.DISPLAY, newLocale);
          setDefault(Category.FORMAT, newLocale);
          defaultLocale = newLocale;
          ICU.setDefaultLocale(newLocale.toLanguageTag());
      }
docs: Sets the default locale for this instance of the Java Virtual Machine.
  This does not affect the host locale.
  <p>
  If there is a security manager, its <code>checkPermission</code>
  method is called with a <code>PropertyPermission("user.language", "write")</code>
  permission before the default locale is changed.
  <p>
  The Java Virtual Machine sets the default locale during startup
  based on the host environment. It is used by many locale-sensitive
  methods if no locale is explicitly specified.
  <p>
  Since changing the default locale may affect many different areas
  of functionality, this method should only be used if the caller
  is prepared to reinitialize locale-sensitive code running
  within the same Java Virtual Machine.
  <p>
  By setting the default locale with this method, all of the default
  locales for each Category are also set to the specified default locale.
  
  @throws SecurityException
  if a security manager exists and its
  <code>checkPermission</code> method doesn't allow the operation.
  @throws NullPointerException if <code>newLocale</code> is null
  @param newLocale the new default locale
  @see SecurityManager#checkPermission
  @see java.util.PropertyPermission