Java8流分组通过枚举和计数

Question

With the classes:

public class Person {

    private String name;
    private Color favouriteColor;
}

public enum Color {GREEN, YELLOW, BLUE, RED, ORANGE, PURPLE}

Having a List<Person> using the Java8 Stream API can I trasform it in a Map<Color, Long> having the count of each Color , also for the color that aren't included in the list. Example:

List<Person> list = List.of(
    new Person("Karl", Color.RED),
    new Person("Greg", Color.BLUE),
    new Person("Andrew", Color.GREEN)
);

Trasforming this list in a Map with all the colors of the enum with their count.

Thanks

SOLVED

Solved using a custom collector:

public static <T extends Enum<T>> Collector<T, ?, Map<T, Long>> counting(Class<T> type) {
    return Collectors.toMap(
        Function.<T>identity(),
        x -> 1l,
        Long::sum,
        () -> new HashMap(Stream.of(type.getEnumConstants()).collect(Collectors.toMap(Function.<T>identity(),t -> 0l)))
    );
}


list.stream()
    .map(Person::getFavouriteColor)
    .collect(counting(Color.class))

Answer 1

You can use the groupingBy collector to create a map but if you want to add default values for absent keys, you have to ensure that the returned map is mutable by providing a Supplier for the map. On the other hand, this adds the opportunity to create an EnumMap which is more suitable to this use case:

EnumMap<Color, Long> map = list.stream().collect(Collectors.groupingBy(
    Person::getFavouriteColor, ()->new EnumMap<>(Color.class), Collectors.counting()));
EnumSet.allOf(Color.class).forEach(c->map.putIfAbsent(c, 0L));

May be, you think it's cleaner to fill the map with default values within the supplier function:

EnumMap<Color, Long> map = list.stream().collect(Collectors.toMap(
    Person::getFavouriteColor, x->1L, Long::sum, ()->{
        EnumMap<Color, Long> em = new EnumMap<>(Color.class);
        EnumSet.allOf(Color.class).forEach(c->em.put(c, 0L));
        return em;
    }));

but of course, you can also use a stream to create that initial map:

EnumMap<Color, Long> map = list.stream().collect(Collectors.toMap(
    Person::getFavouriteColor, x->1L, Long::sum, () ->
        EnumSet.allOf(Color.class).stream().collect(Collectors.toMap(
        x->x, x->0L, Long::sum, ()->new EnumMap<>(Color.class)))));

But for completeness, you can do the same without the stream API, if you wish:

EnumMap<Color, Long> map = new EnumMap<>(Color.class);
list.forEach(p->map.merge(p.getFavouriteColor(), 1L, Long::sum));
EnumSet.allOf(Color.class).forEach(c->map.putIfAbsent(c, 0L));

Answer 2

You can try with:

Map<Color, Long> counted = list.stream()
        .collect(Collectors.groupingBy(Person::getFavouriteColor(), 
                                       Collectors.counting()));

Of course, this implies that you have a getter for the Person#favouriteColor member.

Then, in order to add the non-existing Color s to the Map, you can stream over all Color values, filter those who are not used as keys for the Map yet and put them with value of 0 :

Stream.of(Color.values())
      .filter(x -> !counted.containsKey(x))
      .forEach(x -> counted.put(x, 0L));