Can EnumSet return values in written order?-CodePudding

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!

CodePudding user response：

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

CodePudding user response：

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
    );
}

CodePudding user response：

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).