How to Group by Multiple Fields in Java 8 Using `Collectors.groupingBy()`?

Grouping by Multiple Fields in Java 8

In Java 8, the Collectors.groupingBy() method is commonly used to group objects based on a specific field. However, what if you need to group by multiple fields from the same POJO?

There are several options available:

Chaining Collectors

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

This code groups the Person objects first by name and then by age, resulting in a nested map structure.

Defining a Grouping Class

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

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

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

By creating a record or class that encapsulates the grouping criteria (e.g., NameAge), you can directly group by the desired fields.

Using a Pair Class

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

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

If you're using a third-party library, you can leverage its pair class to represent the grouping criteria.

Choosing the Best Option

The choice of approach depends on the specific requirements of your project:

• If you need a simple and straightforward way to group by multiple fields, chaining collectors is a pragmatic solution.
• If you prefer a more encapsulated approach that makes the grouping intention explicit, defining a grouping class or using a pair class is recommended.
• The availability of records in Java versions >= 16 simplifies the creation of grouping classes, making it a preferred choice for clarity and maintainability.

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