java streams: grouping by and add fields

Question

I've a stream of MetricGroup, where:

public class MetricGroup {

    private String application;
    private int uploadedDocs;
    private long uploadedKbs;

    // getters and setters

}

I need to sumarize all application metrics. I mean, for each application (MetricGroup.application) 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 groupingBy

Stream.of(
    new MetricGroup("app1", 1,100),
    new MetricGroup("app1", 1,300),
    new MetricGroup("app2", 1,200)
)
.collect(Collector.groupingBy(MetricGroup::getApplication, accumulator??, combiner??)

The result would be a stream with:

< MetricGroup("app1", 2, 400) , MetricGroup("app2", 1, 200) >

Any ideas?

What's wrong with the answers you got on your similar question asked 30 mins ago? — Mark
– Mark, Commented Jan 8, 2019 at 11:02
I need to group by application, On the other post only I need to summarize all objects. Now I need to summarize by application. — Jordi
– Jordi, Commented Jan 8, 2019 at 11:03

Ousmane D. · Accepted Answer · 2019-01-08 11:07:06Z

4

You can group by application and then map the results to MetricGroup:

source.stream()
      .collect(groupingBy(MetricGroup::getApplication))
      .entrySet().stream()
      .map(s -> new MetricGroup(s.getKey(), 
                 s.getValue().stream().mapToInt(MetricGroup::getUploadedDocs).sum(),
                 s.getValue().stream().mapToLong(MetricGroup::getUploadedKbs).sum()))
      .collect(Collectors.toList());

answered Jan 8, 2019 at 11:07

Ousmane D.

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Comments

fps · Accepted Answer · 2019-01-08 13:00:40Z

If you could add the following copy constructor and merge method to the MetricGroup class:

public MetricGroup(MetricGroup another) {
    this.application = another.application;
    this.uploadedDocs = another.uploadedDocs;
    this.uploadedKbs = another.uploadedKbs;
}

public MetricGroup merge(MetricGroup another) {
    this.uploadedDocs += another.uploadedDocs;
    this.uploadedKbs += another.uploadedKbs;
    return this;
}

Then, you could use Collectors.toMap to group metrics by application:

Map<String, MetricGroup> result = Stream.of(
        new MetricGroup("app1", 1,100), 
        new MetricGroup("app1", 1,300), 
        new MetricGroup("app2", 1,200))
.collect(Collectors.toMap(
        MetricGroup::getApplication, 
        MetricGroup::new,      // use copy constructor
        MetricGroup::merge));  // use MetricGroup.merge method

Note that we need to use the copy constructor to not mutate the original MetricGroup elements of the stream.

rick · Accepted Answer · 2019-01-08 11:50:48Z

1

you can try something like this

private static final List<MetricGroup> mgArr = Arrays.asList(new MetricGroup("app1", 1,100),new MetricGroup("app1", 1,300),new MetricGroup("app2", 1,200));

Map<String, Long> groupedByAppKbs = mgArr.stream()
                .collect(groupingBy(MetricGroup::getApplication, summingLong(MetricGroup::getUploadedKbs)));

Than to access the vaues:

groupedByAppKbs.get("app1").longValue();

answered Jan 8, 2019 at 11:50

rick

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Comments

ETO · Accepted Answer · 2019-01-08 13:47:48Z

1

Use Collectors.reducing while grouping by MetricGroup.application:

Map<String, MetricGroup> collect = 
      source.stream()
            .collect(
                   groupingBy(MetricGroup::getApplication,
                              reducing(new MetricGroup(null, 0, 0),
                                       (m1, m2) -> new MetricGroup(
                                               m2.application,
                                               m1.getUploadedDocs() + m2.getUploadedDocs(),
                                               m1.getUploadedKbs() + m2.getUploadedKbs()))));

answered Jan 8, 2019 at 13:47

ETO

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