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When we work with WebFlux, we often end up handling the output of two Monos combined together with zip. The result is an object of type Tuple2, a structure that represents an immutable pair of values. Handling lots of Tuple2s, however, can make the code less readable and harder to maintain.

📌 The combinator variant

For this reason, there are variants of the zip, zipWith and zipWhen methods that take a combinator (an aggregation function) as a parameter, letting us get a well-typed object directly instead of a generic tuple.

// (1) Using zip methods to create a strongly-typed object instead of Tuple2
private record Car(String licensePlate, Integer numberOfDoors) {}

@Test
void test_combinator() {
    checkCar(Mono.zip(Mono.just("BMT 216A"), Mono.just(5), Car::new));
    checkCar(Mono.just("BMT 216A").zipWith(Mono.just(5), Car::new));
    checkCar(Mono.just("BMT 216A").zipWhen(t1 -> Mono.just(5), Car::new));
}

private void checkCar(final Mono<Car> carMono) {
    final Car actualCar = carMono.block();
    assertNotNull(actualCar);
    assertEquals("BMT 216A", actualCar.licensePlate());
    assertEquals(5, actualCar.numberOfDoors());
}

Thanks to this strategy, we can avoid working with generic tuples and get objects with clearly defined parameters, improving the readability and maintainability of the code.

📑 Note: beyond two Monos

The combinator methods are available in Mono only for two elements. If we need to handle Tuple3, Tuple4 or higher, we can define a custom method:

// (2) Custom zip method for combining three Monos into a strongly-typed object, avoiding Tuple3 usage
public static <T1, T2, T3, R> Mono<R> zip(
        final Mono<T1> m1, final Mono<T2> m2, final Mono<T3> m3, final Function3<T1, T2, T3, R> mapper) {
    return Mono.zip(m1, m2, m3).map(mapTuple(mapper));
}

private static <T1, T2, T3, R> Function<Tuple3<T1, T2, T3>, R> mapTuple(final Function3<T1, T2, T3, R> mapper) {
    return tuple -> mapper.apply(tuple.getT1(), tuple.getT2(), tuple.getT3());
}

Did you enjoy this Java tip? This idea came out of an interesting moment of sharing at the company!

See you at the next pill! ☕