Java streams adding multiple values conditionally

Question

I have a List of objects like this, where amount can be negative or positive:

class Sale {
   String country;
   BigDecimal amount;
}

And I would like to end up with a pair of sums of all negative values, and all positive values, by country.

With these values:

country | amount
nl      | 9
nl      | -3
be      | 7.9
be      | -7

Is there a way to end up with Map<String, Pair<BigDecimal, BigDecimal>> using a single stream?

It's easy to do this with two separate streams, but I can't figure it out with just one.

Answer 1

It should be using Collectors.toMap with a merge function to sum pairs.

Assuming that a Pair is immutable and has only getters for the first and second elements, the code may look like this:

static Map<String, Pair<BigDecimal, BigDecimal>> sumUp(List<Sale> list) {
    return list.stream()
               .collect(Collectors.toMap(
                   Sale::getCountry,
                   sale -> sale.getAmount().signum() >= 0 
                       ? new Pair<>(sale.getAmount(), BigDecimal.ZERO)
                       : new Pair<>(BigDecimal.ZERO, sale.getAmount()),
                   (pair1, pair2) -> new Pair<>(
                       pair1.getFirst().add(pair2.getFirst()),
                       pair1.getSecond().add(pair2.getSecond())
                   )
                   // , LinkedHashMap::new // optional parameter to keep insertion order
               ));
}

Test

List<Sale> list = Arrays.asList(
    new Sale("us", new BigDecimal(100)),
    new Sale("uk", new BigDecimal(-10)),
    new Sale("us", new BigDecimal(-50)),
    new Sale("us", new BigDecimal(200)),
    new Sale("uk", new BigDecimal(333)),
    new Sale("uk", new BigDecimal(-70))
);

Map<String, Pair<BigDecimal, BigDecimal>> map = sumUp(list);

map.forEach((country, pair) -> 
    System.out.printf("%-4s|%s%n%-4s|%s%n", 
        country, pair.getFirst(), country, pair.getSecond()
));

Output

uk  |333
uk  |-80
us  |300
us  |-50