Java 8 grouping using custom collector?

Question

I have the following class.

class Person {

    String name;
    LocalDate birthday;
    Sex gender;
    String emailAddress;

    public int getAge() {
        return birthday.until(IsoChronology.INSTANCE.dateNow()).getYears();
    }

    public String getName() {
        return name;
    }
}

I'd like to be able to group by age and then collect the list of the persons names rather than the Person object itself; all in a single nice lamba expression.

To simplify all of this I am linking my current solution that store the result of the grouping by age and then iterates over it to collect the names.

ArrayList<OtherPerson> members = new ArrayList<>();

members.add(new OtherPerson("Fred", IsoChronology.INSTANCE.date(1980, 6, 20), OtherPerson.Sex.MALE, "[email protected]"));
members.add(new OtherPerson("Jane", IsoChronology.INSTANCE.date(1990, 7, 15), OtherPerson.Sex.FEMALE, "[email protected]"));
members.add(new OtherPerson("Mark", IsoChronology.INSTANCE.date(1990, 7, 15), OtherPerson.Sex.MALE, "[email protected]"));
members.add(new OtherPerson("George", IsoChronology.INSTANCE.date(1991, 8, 13), OtherPerson.Sex.MALE, "[email protected]"));
members.add(new OtherPerson("Bob", IsoChronology.INSTANCE.date(2000, 9, 12), OtherPerson.Sex.MALE, "[email protected]"));

Map<Integer, List<Person>> collect = members.stream().collect(groupingBy(Person::getAge));

Map<Integer, List<String>> result = new HashMap<>();

collect.keySet().forEach(key -> {
            result.put(key, collect.get(key).stream().map(Person::getName).collect(toList()));
});

Current solution

Not ideal and for the sake of learning I'd like to have a more elegant and performing solution.

Tunaki · Accepted Answer · 2016-04-16 11:05:36Z

17

When grouping a Stream with Collectors.groupingBy, you can specify a reduction operation on the values with a custom Collector. Here, we need to use Collectors.mapping, which takes a function (what the mapping is) and a collector (how to collect the mapped values). In this case the mapping is Person::getName, i.e. a method reference that returns the name of the Person, and we collect that into a List.

Map<Integer, List<String>> collect = 
    members.stream()
           .collect(Collectors.groupingBy(
               Person::getAge,
               Collectors.mapping(Person::getName, Collectors.toList()))
           );

edited Apr 16, 2016 at 11:05

answered Oct 26, 2015 at 12:16

Tunaki

138k46 gold badges370 silver badges443 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

lcardito Over a year ago

Ops, this was easy eh! Cheers!

Satish Patro Over a year ago

In this case, only String is there. what to do if members have List<String> names. And, all remaining same. What modification is required inside Collectors.mapping method. Also if any method to modify the object before returning it as value part. For example, Name one I need to modify to do some custom String operation.

Eran · Accepted Answer · 2015-10-26 12:16:18Z

3

You can use a mapping Collector to map the list of Person to a list of person names :

Map<Integer, List<String>> collect = 
    members.stream()
           .collect(Collectors.groupingBy(Person::getAge,
                                          Collectors.mapping(Person::getName, Collectors.toList())));

edited Oct 26, 2015 at 12:16

answered Oct 26, 2015 at 12:10

Eran

395k57 gold badges726 silver badges793 bronze badges

1 Comment

Satish Patro Over a year ago

In this case, only String is there. what to do if members have List<String> names. And, all remaining same. What modification is required inside Collectors.mapping method. Also if any method to modify the object before returning it as value part. For example, Name one I need to modify to do some custom String operation.

AnException · Accepted Answer · 2016-06-08 02:04:53Z

2

You can also use Collectors.toMap and provide mapping for key, value and merge function(if any).

Map<Integer, String> ageNameMap = 
    members.stream()
            .collect(Collectors.toMap(
              person -> person.getAge(), 
              person -> person.getName(), (pName1, pName2) -> pName1+"|"+pName2)
    );

answered Jun 8, 2016 at 2:04

AnException

211 bronze badge

Collectives™ on Stack Overflow

Java 8 grouping using custom collector?

3 Answers 3

2 Comments

1 Comment

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related