JAVA

java stream group by multiple fields

Function<Item, List> classifier = (item) -> List.of(
    item.getFieldA(),
    item.getFieldB(),
    Optional.ofNullable(item.getFieldC())
);

Map<List, List<Item>> grouped = items.stream()
    .collect(Collectors.groupingBy(classifier));

java stream group by multiple fields

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