code: public String getExtension(char key) {
          if (!LocaleExtensions.isValidKey(key)) {
              throw new IllegalArgumentException("Ill-formed extension key: " + key);
          }
          return (localeExtensions == null) ? null : localeExtensions.getExtensionValue(key);
      }
docs: Returns the extension (or private use) value associated with
  the specified key, or null if there is no extension
  associated with the key. To be well-formed, the key must be one
  of <code>[0-9A-Za-z]</code>. Keys are case-insensitive, so
  for example 'z' and 'Z' represent the same extension.
  
  @param key the extension key
  @return The extension, or null if this locale defines no
  extension for the specified key.
  @throws IllegalArgumentException if key is not well-formed
  @see #PRIVATE_USE_EXTENSION
  @see #UNICODE_LOCALE_EXTENSION
  @since 1.7