code: public String[] subMapKeysAsStrings(PropertyMap map) {
          String[] result = null;
          Set<?> keySet = map.subMapKeySet();
          int keySize = keySet.size();
          if (keySize > 0) {          
              result = keySet.toArray(new String[keySize]);
          }
          return result;
      }
docs: Get the array of all keys that provide valid sub-maps and descendents.
  For example, a property map containing "test.one", "users.nick",
  "users.john" will result in two keys: "test" and "users".
  
  @param map The property map to retrieve from
  
  @return The array of all unique keys that provide sub maps as strings
  
  @see PropertyMap#subMapKeySet()