code: public String getString (String key, Supplier<String> notFound) {
          Object object = getObject (key);
          return (object instanceof String) ? (String) object : notFound.get ();
      }
docs: Retrieve a mapped element and return it as a String.
  
  @param key A string value used to index the element.
  @param notFound A function to create a new String if the requested key was not found
  @return The element as a string, or notFound if the element is not found.