How to count items by enum value

Short answer

Take an EnumMap and increase the counter with merge(). Keys are printed in enum declaration order:

enum Status { NEW, PAID, SHIPPED }
 
void main() {
    List<Status> orders = List.of(Status.PAID, Status.NEW, Status.PAID, Status.SHIPPED, Status.PAID);
 
    Map<Status, Integer> counts = new EnumMap<>(Status.class);
    for (Status s : orders) {
        counts.merge(s, 1, Integer::sum);
    }
    IO.println(counts);
}

Output:

{NEW=1, PAID=3, SHIPPED=1}

With a stream

groupingBy() with an EnumMap factory and counting():

enum Status { NEW, PAID, SHIPPED }
 
record Order(int id, Status status) {}
 
void main() {
    List<Order> orders = List.of(
        new Order(1, Status.PAID), new Order(2, Status.NEW), new Order(3, Status.PAID));
 
    Map<Status, Long> counts = orders.stream()
        .collect(Collectors.groupingBy(Order::status,
            () -> new EnumMap<>(Status.class), Collectors.counting()));
    IO.println(counts);
}

Output:

{NEW=1, PAID=2}

With zeros for every value

Values that never occurred do not appear in the map. To show them too, fill the map with zeros first:

enum Status { NEW, PAID, SHIPPED }
 
void main() {
    List<Status> orders = List.of(Status.PAID, Status.NEW, Status.PAID);
 
    Map<Status, Integer> counts = new EnumMap<>(Status.class);
    for (Status s : Status.values()) {
        counts.put(s, 0);
    }
    for (Status s : orders) {
        counts.merge(s, 1, Integer::sum);
    }
    counts.forEach((status, n) -> IO.println(status + ": " + n));
}

Output:

NEW: 1
PAID: 2
SHIPPED: 0

Common mistake

Counting with a HashMap: the result is the same, but the key order depends on hash codes and may change from run to run. With an EnumMap the order always matches the order of the constants, and it is faster too. Another form of the mistake is counting by ordinal() in an int[] array: the code breaks when a new constant is added in the middle of the enum.

Thanks for reading!

Short answer

With a stream

With zeros for every value

Common mistake

Functions used in this recipe

Thanks for reading!