Collectors

List<String> names = stream.collect(Collectors.toList());
Set<String>  nameSet = stream.collect(Collectors.toSet());
String csv = stream.collect(Collectors.joining(", "));
String delimited = stream.collect(Collectors.joining(", ", "[", "]"));
long count = stream.collect(Collectors.counting());

List<Person> people = ...;

// Group by one field
Map<String, List<Person>> byCity =
    people.stream().collect(Collectors.groupingBy(Person::getCity));

// Group and count (downstream collector)
Map<String, Long> countByCity =
    people.stream().collect(
        Collectors.groupingBy(Person::getCity, Collectors.counting()));

// Group and collect to specific collection type
Map<String, Set<String>> emailsByCity =
    people.stream().collect(
        Collectors.groupingBy(Person::getCity,
            Collectors.mapping(Person::getEmail, Collectors.toSet())));

// Multi-level grouping
Map<String, Map<String, List<Person>>> byCityThenGender =
    people.stream().collect(
        Collectors.groupingBy(Person::getCity,
            Collectors.groupingBy(Person::getGender)));

// Split into adults and minors
Map<Boolean, List<Person>> partitioned =
    people.stream().collect(
        Collectors.partitioningBy(p -> p.getAge() >= 18));

List<Person> adults = partitioned.get(true);
List<Person> minors = partitioned.get(false);

// Statistics in one pass
IntSummaryStatistics stats = people.stream()
    .collect(Collectors.summarizingInt(Person::getAge));

stats.getCount();   // 100
stats.getSum();     // 3500
stats.getMin();     // 18
stats.getMax();     // 75
stats.getAverage(); // 35.0

// Simpler aggregations
Optional<Integer> max = people.stream()
    .collect(Collectors.maxBy(Comparator.comparing(Person::getAge)));

Double average = people.stream()
    .collect(Collectors.averagingInt(Person::getAge));

Integer sum = people.stream()
    .collect(Collectors.summingInt(Person::getSalary));

// toMap(keyMapper, valueMapper)
Map<String, Person> byEmail =
    people.stream().collect(
        Collectors.toMap(Person::getEmail, p -> p));

// With merge function for duplicate keys
Map<String, String> cityToFirstName =
    people.stream().collect(
        Collectors.toMap(
            Person::getCity,
            Person::getName,
            (existing, replacement) -> existing // keep first
        ));

record Summary(long count, double average) {}

Summary summary = people.stream().collect(
    Collectors.teeing(
        Collectors.counting(),
        Collectors.averagingInt(Person::getAge),
        Summary::new
    )
);

List<String> immutable = stream.collect(Collectors.toUnmodifiableList());
Set<String>  immutableSet = stream.collect(Collectors.toUnmodifiableSet());