code: private static List<String> toSingletonList(String propertyName,
                                           String value) {
          if ((null == value) || (value.length() == 0)) {
              return null;
          }
  
          if (value.charAt(0) == '@') {
              // These are very common, so we use shared copies
              // of these collections instead of re-creating.
              List<String> list;
  
              if (ALL.equals(value)) {
                  list = ALL_LIST;
              } else if (FORM.equals(value)){
                  list = FORM_LIST;
              } else if (THIS.equals(value)) {
                  list = THIS_LIST; 
              } else if (NONE.equals(value)) {
                  list = NONE_LIST;
              } else {
                  // RELEASE_PENDING i18n ;
                  throw new FacesException(value
                                       + " : Invalid id keyword specified for '"
                                       + propertyName
                                       + "' attribute");
              }
              
              return list;
          }
           
          return Collections.singletonList(value);
      }
docs: Converts a String with no spaces to a singleton list