Group by multiple field names in java 8

Question

I found the code for grouping the objects by some field name from POJO. Below is the code for that:

public class Temp {

    static class Person {

        private String name;
        private int age;
        private long salary;

        Person(String name, int age, long salary) {

            this.name = name;
            this.age = age;
            this.salary = salary;
        }

        @Override
        public String toString() {
            return String.format("Person{name='%s', age=%d, salary=%d}", name, age, salary);
        }
    }

    public static void main(String[] args) {
        Stream<Person> people = Stream.of(new Person("Paul", 24, 20000),
                new Person("Mark", 30, 30000),
                new Person("Will", 28, 28000),
                new Person("William", 28, 28000));
        Map<Integer, List<Person>> peopleByAge;
        peopleByAge = people
                .collect(Collectors.groupingBy(p -> p.age, Collectors.mapping((Person p) -> p, toList())));
        System.out.println(peopleByAge);
    }
}

And the output is (which is correct):

{24=[Person{name='Paul', age=24, salary=20000}], 28=[Person{name='Will', age=28, salary=28000}, Person{name='William', age=28, salary=28000}], 30=[Person{name='Mark', age=30, salary=30000}]}

But what if I want to group by multiple fields? I can obviously pass some POJO in groupingBy() method after implementing equals() method in that POJO but is there any other option like I can group by more than one fields from the given POJO?

E.g. here in my case, I want to group by name and age.

Answer 1

You have a few options here. The simplest is to chain your collectors:

Map<String, Map<Integer, List<Person>>> map = people
    .collect(Collectors.groupingBy(Person::getName,
        Collectors.groupingBy(Person::getAge));

Then to get a list of 18 year old people called Fred you would use:

map.get("Fred").get(18);

A second option is to define a class that represents the grouping. This can be inside Person. This code uses a record but it could just as easily be a class (with equals and hashCode defined) in versions of Java before JEP 359 was added:

class Person {
    record NameAge(String name, int age) { }

    public NameAge getNameAge() {
        return new NameAge(name, age);
    }
}

Then you can use:

Map<NameAge, List<Person>> map = people.collect(Collectors.groupingBy(Person::getNameAge));

and search with

map.get(new NameAge("Fred", 18));

Finally if you don't want to implement your own group record then many of the Java frameworks around have a pair class designed for this type of thing. For example: apache commons pair If you use one of these libraries then you can make the key to the map a pair of the name and age:

Map<Pair<String, Integer>, List<Person>> map =
    people.collect(Collectors.groupingBy(p -> Pair.of(p.getName(), p.getAge())));

and retrieve with:

map.get(Pair.of("Fred", 18));

Personally I don't really see much value in generic tuples now that records are available in the language as records display intent better and require very little code.