How Can I Group Data by Multiple Fields Using Java 8 Streams?

Grouping by Multiple Fields in Java 8

When grouping objects by a field name, Java 8 provides the Collectors.groupingBy() method. However, what if we need to group by multiple fields?

Option 1: Chaining Collectors

One approach is to chain multiple Collectors.groupingBy() calls:

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

This creates a nested map where the first level is grouped by name and the second level is grouped by age. To retrieve a list of 18-year-old people named Fred:

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

Option 2: Defining a Grouping Record

Alternatively, we can create a class (e.g., NameAge) that represents the grouping fields:

class NameAge implements Comparable<NameAge> {
    String name;
    int age;
    
    // Constructor, getters, compareTo, etc.
}

Then, group by the NameAge record:

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

To retrieve:

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

Option 3: Using Pair Class

If implementing a custom grouping record is undesirable, we can utilize a pair class from libraries like Apache Commons or Java's new java.util.function.Pair. For example, with Apache Commons:

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

To retrieve:

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

Recommendation

The choice of approach depends on the specific requirements. Records offer a concise and clear solution, but libraries like Apache Commons may be preferred for more complex scenarios.

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