Group by multiple field names in java 8

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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.

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