Java Optional usage for multi-map

Question

Continent is a composite Object. Structure is :

Continent
--Country
----State
------Town

so in this notation:

town= Optional.of(continent)
            .map(Continent::getCountry)
            .map(Country::getState)
            .map(State::getTown)
            .orElse(null);

this works fine, but when I try to write a general mapper,

  public static <T, R> T getFromMapping(R source,
                                      T defaultValue,
                                      Function<?,?>... functions) {
      Optional sourceWrapper = Optional.ofNullable(source);
      for (Function function : functions) {
         sourceWrapper.map(function);
      }
    return (T) sourceWrapper.orElse(defaultValue);
  }

and invoke it by

 portfolio = getFromMapping(continent, null,
            ((Function<Continent, Country>) Continent::getCountry)
            ((Function<Country, State>) Country::getState),
            ((Function<State, Town>) State::getTown));

it compile just fine but don't work. The mapper jump to second step to and said Continent could not cast to country, why? there supposed to be no cast while doing mapping, how to fix it?

You might want to swap your T and R to avoid confusion. java.util.function has the loose naming convention that T and U are inputs and R is a return value. You are essentially building a Function<R,T> — Hank D
– Hank D, Commented May 5, 2016 at 14:15
Suggestion: if you create variants of getFromMapping with fixed numbers of functions, you'll have a typesafe solution and do not need to cast the arguments anymore. — stholzm
– stholzm, Commented May 5, 2016 at 14:34
@stholzm yeah, but I just want to make the function flexible to extend. — Martin T Y Ho
– Martin T Y Ho, Commented May 5, 2016 at 15:01

stholzm · Accepted Answer · 2016-05-05 10:33:50Z

5

Optional#map does not modify itself, but returns a new Optional instead, so sourceWrapper still contains your continent. You should reassign the variable in the for loop:

sourceWrapper = sourceWrapper.map(function);

edited May 5, 2016 at 10:33

answered May 5, 2016 at 9:44

stholzm

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