Как решить задачу на группировку с помощью Stream API?

List<Person> people = List.of(
    new Person("Alice", "Moscow"),
    new Person("Bob", "Moscow"),
    new Person("Charlie", "London")
);

Map<String, List<Person>> byCity = people.stream()
    .collect(Collectors.groupingBy(Person::getCity));
// Результат: {"Moscow": [Alice, Bob], "London": [Charlie]}

Map<String, Long> countByCity = people.stream()
    .collect(Collectors.groupingBy(Person::getCity, Collectors.counting()));
// {"Moscow": 2, "London": 1}

Map<String, List<String>> namesByCity = people.stream()
    .collect(Collectors.groupingBy(
        Person::getCity,
        Collectors.mapping(Person::getName, Collectors.toList())
    ));
// {"Moscow": ["Alice", "Bob"], "London": ["Charlie"]}