java streams: straightforward reduce

Question

I've a stream of MetricGroup, where:

public class MetricGroup {

    private int uploadedDocs;
    private long uploadedKbs;

    // getters and setters

}

I need to sumarize all metrics in one single metric. I mean, I need to add all metric.uploadedDocs into a sumMetric.uploadedDocs and metric.uploadedKds into a sumMetric.uploadedKbs.

I figure out I need some kind of reduce

Stream.of(new MetricGroup(1,100), new MetricGroup(1,200))
    .reduce(????);

Any ideas?

Answer 1

You can use this overload of reduce:

T reduce(T identity,
     BinaryOperator<T> accumulator)

like this:

.reduce(new MetricGroup(0, 0),
        (x, y) -> new MetricGroup(
                      x.getUploadedDocs() + y.getUploadedDocs()
                      x.getUploadedKbs() + y.getUploadedKbs()
                  )
        )

You can also use the collect method:

private static MetricGroup combine(MetricGroup x, MetricGroup y) {
    x.setUploadedDocs(x.getUploadedDocs() + y.getUploadedDocs());
    x.setUploadedKbs(x.getUploadedKbs() + y.getUploadedKbs());
    return x;
}

// ....

.collect(() -> new MetricGroup(0, 0),
    YourClass::combine,
    YourClass::combine
)

Answer 2

To avoid the creation of several/many MetricGroup objects during the reduce call, you can make two separate calls to sum the UploadedDocs and UploadedKbs respectively and then construct a new MetricGroup representing the result.

int uploadedDocsSum = source.stream().mapToInt(MetricGroup::getUploadedDocs).sum();
long uploadedKbsSum = source.stream().mapToLong(MetricGroup::getUploadedKbs).sum();
MetricGroup result = new MetricGroup(uploadedDocsSum, uploadedKbsSum);

Arguably more readable as well...