code: public Enumeration<?> propertyNames() {
          Hashtable<String,Object> h = new Hashtable<>();
          enumerate(h);
          return h.keys();
      }
docs: Returns an enumeration of all the keys in this property list,
  including distinct keys in the default property list if a key
  of the same name has not already been found from the main
  properties list.
  
  @return  an enumeration of all the keys in this property list, including
  the keys in the default property list.
  @throws  ClassCastException if any key in this property list
  is not a string.
  @see     java.util.Enumeration
  @see     java.util.Properties#defaults
  @see     #stringPropertyNames