CODEWITHSUNDEEP
javajava-stream
membersound
membersound

Reputation: 86727

How to divide a stream by object fields into two sets?

I have a stream, where each object is identified by a unique id. Moreover, each object has a either positive or negative Free values.

I want to divide this stream into two Sets, where one contains the ids whose Free values was positive, and one with the rest.

But I find the following not being the right way, as I'm collecting into lists outside of the stream.

class Foo {
    int free;
    long id;
}

public Tuple2<Set<Long>, Set<Long>> findPositiveAndNegativeIds() {
    Set<Long> positives = new HashSet<>();
    Set<Long> negatives = new HashSet<>();

    foos.stream()
            .forEach(f -> {
                if (f.free >= 0) positigves.add(f.id);
                else negatives.add(f.id);
            });
            
    return Tuple2.tuple(positives, negatives);
}

Could this be done better, somehow with partitionBy() or similar?

Upvotes: 1

Views: 667

Answers (1)

Sweeper
Sweeper

Reputation: 271135

You can indeed use partitioningBy. You can specify what to do with each partition in the second parameter.

var map = foos.stream().collect(Collectors.partitioningBy(
    foo -> foo.free >= 0, // assuming no 0
    // for each partition, map to id and collect to set
    Collectors.mapping(foo -> foo.id, Collectors.toSet())
));

map.get(true) will get you the set of ids with positive frees, and map.get(false) will get you the set of ids with negative frees.

Upvotes: 6

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