CODEWITHSUNDEEP
javajava-8functional-programmingjava-streamcollectors
James
James

Reputation: 597

partitioningBy with a downstream collector produced an unexpected result

I have a test program:

public class App {

    public static void main(String[] args) {

        List<Integer> a = Arrays.asList(1, 11);
        List<Integer> b = Arrays.asList(2, 22);
        List<Integer> c = Arrays.asList(3, 33);

        Map<String, List<Integer>> map = new HashMap<>();
        map.put("a", a);
        map.put("b", b);
        map.put("c", c);

        Set<String> valid = new HashSet<>();
        valid.add("a");

        Map<Boolean, List<Map.Entry<String, List<Integer>>>> partitions =
            map.entrySet().stream()
                .collect(Collectors.partitioningBy(
                    entry -> valid.contains(entry.getKey())));

        System.out.println(partitions);

        // partition by the key of the map
        // then reduce the values into a single collection

        Map<Boolean, List<Integer>> result = map.entrySet().stream()
            .collect(Collectors.partitioningBy(
                entry -> valid.contains(entry.getKey()),
                Collectors.mapping(Map.Entry::getValue,
                                   Collectors.reducing(new ArrayList<>(),
                                                       (l1, l2) -> {
                                                           l1.addAll(l2);
                                                           return l1;
                                                       }))));

        System.out.println(result);
    }
}

I'm expecting the final result to be

{false=[b=[2, 22], c=[3, 33]], true=[a=[1, 11]]}
{false=[2, 22, 3, 33], true=[1, 11]}

But in the actual result, both true and false keys have all 6 integers:

{false=[b=[2, 22], c=[3, 33]], true=[a=[1, 11]]}
{false=[1, 11, 2, 22, 3, 33], true=[1, 11, 2, 22, 3, 33]}

Notice the 2 partitioning functions are exactly the same. But the downstream mixed up the values in the separate partitions. How can that be? I assume the downstream would only operate on each partition...

What did I miss here?

Thanks.

Upvotes: 1

Views: 1381

Answers (2)

Nikolas
Nikolas

Reputation: 44398

To complete already accepted answer, you can use Collectors.groupingBy using your classifier with Collectors.flatMapping as of Java 9 as a downstream collector.

Map<Boolean, List<Integer>> result = map.entrySet().stream()
    .collect(Collectors.groupingBy(
         e -> valid.contains(e.getKey()), 
         Collectors.flatMapping(e -> e.getValue().stream(), Collectors.toList())));

Upvotes: 0

Eklavya
Eklavya

Reputation: 18440

For reducing same ArrayList reference is used in both partition. You can use Collectors.toMap and create a new instance merging two lists.

Map<Boolean, List<Integer>> result = 
        map.entrySet()
        .stream()
        .collect(Collectors.toMap(e -> valid.contains(e.getKey()), Map.Entry::getValue, 
        (l1, l2) -> {
          List<Integer> l3 = new ArrayList<>(l1);
          l3.addAll(l2);
          return l3;
        }));

And if you want to use the same taste

Map<Boolean, List<Integer>> result = 
        map.entrySet()
        .stream()
        .collect(Collectors.toMap(e-> valid.contains(e.getKey()), Map.Entry::getValue,
            (l1, l2) -> Stream.concat(l1.stream(), l2.stream())
                              .collect(Collectors.toList())));

Upvotes: 1

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