Java Stream : multiple grouping

Question

I can group by one field but i want to group by both together

//persons grouped by gender
Map<String, Long> personsGroupedByGender = persons.stream().collect(Collectors.groupingBy(Person::getGender, Collectors.counting()));

//persons grouped by age
Map<Int, Long> personsGroupedByAge = persons.stream().collect(Collectors.groupingBy(Person::getAge, Collectors.counting()));

//persons grouped by gender and age 

    ???

.getAge() returns a String?

Robin Topper
– Robin Topper

2017-05-10 19:56:54 +00:00
Commented May 10, 2017 at 19:56 — Robin Topper
– Robin Topper, Commented May 10, 2017 at 19:56
I corrrected that one

Marvy
– Marvy

2017-05-11 06:28:03 +00:00
Commented May 11, 2017 at 6:28 — Marvy
– Marvy, Commented May 11, 2017 at 6:28
@Marvy may be Map<Integer, Long>? Probably a typo.

Eugene
– Eugene

2017-05-11 06:46:21 +00:00
Commented May 11, 2017 at 6:46 — Eugene
– Eugene, Commented May 11, 2017 at 6:46

Eugene · Accepted Answer · 2017-05-10 20:00:09Z

7

You can group twice, the result will be what you expect, but in a slightly different wrapping.

persons.stream().collect(Collectors.groupingBy(Person::getGender,  
              Collectors.groupingBy(Person::getAge, Collectors.counting())));

answered May 10, 2017 at 20:00

Eugene

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Eugene Over a year ago

@Marvy if it helped might as well accept it. meta.stackexchange.com/questions/5234/…

Joe C · Accepted Answer · 2017-05-10 19:53:51Z

1

My suggestion would be to create a helper class which represents your grouping key (gender and age, in this case), with appropriate equals and hashCode implementations. You can then create a mapper function from your Person to this key, and group by that.

answered May 10, 2017 at 19:53

Joe C

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1 Comment

Eugene Over a year ago

that's not a bad suggestion overall and while I did not downvote - you probably got it because you should have showed some code also...

gvashchenko · Accepted Answer · 2021-04-22 13:22:21Z

0

import org.apache.commons.lang3.tuple.Pair;

Map<Pair<String, String>, Long> personsGroupedByGenderAge = persons.stream()
    .collect(Collectors.groupingBy(
         person -> Pair.of(person.getGender, person.getAge),
         Collectors.counting()));

answered Apr 22, 2021 at 13:22

gvashchenko

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