Can EnumSet return values in written order?

Question

I have an EnumSet

public static Set<T> getTypes() {
    return EnumSet.of(
            TYPE_1,
            TYPE_2,
            TYPE_3
            );
}

And i want to get the values from it with a getter method

When i try to get the values in my program with a foreach loop they always come in wrong order. Is there a way to get them in correct order as written above? Thanks!

A Set usually doesn't have an order. A `TreeSet` has an order. — Johannes Kuhn, Dec 07 '22 at 11:57
@Sweeper I think enum-constants in the method were provided in the order that differs from the natural ordering. And OP was expected this custom order to be preserved. But `EnumSet` stores enum members ordered precisely as they were declared in the enum. — Alexander Ivanchenko, Dec 07 '22 at 12:22

score 2 · Answer 1 · answered Dec 07 '22 at 12:22

2

Whenever you want a Set to keep the order in which elements were added, there's one obvious choice: LinkedHashSet. That's the only Set implementation in the JDK that explicitly states that it preserves insertion order (unless you tell it to use last-access order).

answered Dec 07 '22 at 12:22

Rob Spoor

6,186
1
19
20

I don't know how I've overlooked this simple option. That's definitely a useful answer. – Alexander Ivanchenko Dec 07 '22 at 14:49

Alexander Ivanchenko · Answer 2 · 2022-12-07T12:16:50.710

The order of elements in the EnumSet is the same as the natural ordering of the enum constants (i.e. the order of their declaration).

EnumSet
    .of(DayOfWeek.SATURDAY, DayOfWeek.FRIDAY, DayOfWeek.THURSDAY)
    .forEach(System.out::println);

Output:

THURSDAY
FRIDAY
SATURDAY

If you need the order that differs from the natural ordering, you can switch to using a List instead of a Set:

List
    .of(DayOfWeek.SATURDAY, DayOfWeek.FRIDAY, DayOfWeek.THURSDAY)
    .forEach(System.out::println);

Output:

SATURDAY
FRIDAY
THURSDAY

Or if you need Set as a type, you can introduce a property responsible for ordering in your enum and make use of the TreeSet providing a Comparator which is based on this property.

Consider the following dummy enum:

public enum MyEnum {
    ONE(3), TWO(2), THREE(1);
    private int order;

    MyEnum(int order) {
        this.order = order;
    }

    public int getOrder() {
        return order;
    }
}

Example of storing the enum members into a TreeSet:

Stream.of(MyEnum.ONE, MyEnum.TWO, MyEnum.THREE)
    .collect(Collectors.toCollection(
        () -> new TreeSet<>(Comparator.comparing(MyEnum::getOrder))
    ))
    .forEach(System.out::println);

Output:

THREE
TWO
ONE

score 1 · Answer 3 · answered Dec 07 '22 at 12:27

1

Set is not keeping sort, so rather than it use List like:

public static List<T> getTypes() {
    return Arrays.asList(
            TYPE_1,
            TYPE_2,
            TYPE_3
    );
}

answered Dec 07 '22 at 12:27

utrucceh

1,076
6
11

Can EnumSet return values in written order?

3 Answers3