Convert a List to a Set with Java 8

Question

Is there a one-liner to convert a list of String to a set of enum? For instance, having:

public enum MyVal {
    ONE, TWO, THREE
}

and

List<String> myValues = Arrays.asList("ONE", "TWO", "TWO");

I'd like to convert myValues to a Set<MyVal> containing the same items as:

EnumSet.of(MyVal.ONE, MyVal.TWO)

http://stackoverflow.com/a/26731123/3641067 – buræquete Mar 07 '16 at 09:42 — buræquete, Mar 07 '16 at 09:42

Tunaki · Accepted Answer · 2016-03-07T10:28:34.537

21

Yes, you can make a Stream<String> of your elements, map each of them to the respective enum value with the mapper MyVal::valueOf and collect that into a new EnumSet with toCollection initialized by noneOf:

public static void main(String[] args) {
    List<String> myValues = Arrays.asList("ONE", "TWO", "TWO");
    EnumSet<MyVal> set =
        myValues.stream()
                .map(MyVal::valueOf)
                .collect(Collectors.toCollection(() -> EnumSet.noneOf(MyVal.class)));
    System.out.println(set); // prints "[ONE, TWO]"
}

If you are simply interested in having a Set as result, not an EnumSet, you can simply use the built-in collector Collectors.toSet().

edited Mar 07 '16 at 10:28

answered Mar 07 '16 at 09:23

Tunaki

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5

Does that count as a one-liner these days :-) ? – Thilo Mar 07 '16 at 09:26
Depends on the number of characters per line you allow. SO is stingy in that regard. – the8472 Mar 07 '16 at 09:31
1

Instead of `Collectors.toCollection(() -> EnumSet.noneOf(MyVal.class))` I'm doing `Collectors.toSet()` with the same result – pablo Mar 07 '16 at 09:37
5

@otonakav: you asked for an `EnumSet` explicitly. If any `Set` is sufficient, you can use `toSet()`, but then don’t ask for an `EnumSet`… – Holger Mar 07 '16 at 10:13
@Holger I've asked for a `Set` containing the same items as an `EnumSet` – pablo Mar 07 '16 at 10:17
4

@otonakav: maybe it’s just me, but the title of your question reads like “*Convert a List to an EnumSet with Java 8*”. Clearly “to an **EnumSet**” Believe me, Tunaki knows that there is `Collectors.toSet()`; the posted solution `toCollection(() -> EnumSet.noneOf(MyVal.class))` is just for your specific question. – Holger Mar 07 '16 at 10:20
1

@Holger Tunaki changed the title of my question but it is not a big deal. – pablo Mar 07 '16 at 10:22
2

@Holger Actually, I rephrased the title to that because initially it was "convert to a set of enum" and since the OP said "containing same items as EnumSet", I "clarified" the title "convert to an EnumSet"... Looks like it wasn't true. – Tunaki Mar 07 '16 at 10:23
3

I suggest fixing the title and expanding the answer as `toSet()` clearly is easier to use when the type of the `Set` doesn’t matter. – Holger Mar 07 '16 at 10:26

score 3 · Answer 2 · answered Mar 07 '16 at 18:04

3

Here's a two-liner (but shorter):

EnumSet<MyVal> myVals = EnumSet.allOf(MyVal.class);
myVals.removeIf(myVal -> !myValues.contains(myVal.name()));

Instead of adding elements present on the list, you could create an EnumSet with all possible values and remove the ones that are not present on the list.

answered Mar 07 '16 at 18:04

fps

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Interesting. It has the slightly different property that it doesn't throw an exception if any of the strings doesn't match an enum. Might or might not be desirable, depending on the situation. – slim Sep 30 '16 at 08:55

Convert a List to a Set with Java 8

2 Answers2