Java Static Method to Parse Generic Enum Set from String

Question

I have a lot of classes which define some Enum whose values I want loaded from a user-entered String. So I find myself repeating the method:

public final class Status {
    public static enum TYPE { Slow, Haste, Sleep, Stop, Stone, Invis, Rage,
        Conf, Bleed, Weak, Dumb, Sil, Rot, Lev, Clumsy };

    public static Set<Status.TYPE> typesFromString(String string) { 
        EnumSet<Status.TYPE> set = EnumSet.noneOf(Status.TYPE.class);
        if (string == null)
            return set;
        String[] elements = string.split(",");
        for (String element : elements) {
            element = element.trim();
            for (TYPE type : EnumSet.allOf(Status.TYPE.class)) {
                if (type.toString().equalsIgnoreCase(element)) {
                    set.add(type);
                    break;
                }
            }
        }
        return set;
    }

Which is to say, given a string which contains comma-separated elements which match the enum entries, return a set populated with each match.

I'd love to make this generic, so I wouldn't have to maintain ten different copies of it, but I can't quite figure out how to make this generic while returning a set of enums. I think it would look vaguely like the following method:

public static Set<[Enum Class Specified in Argument]> setFromString(String string, [Class of Enum to Work With]) {
    Set<E extends Enum<E>> set = EnumSet.noneOf([Class of Enum]);
    if (string == null)
        return set;
    for (String element : string.split(",")) {
        element = element.trim().toLowerCase();
        for ([Element of Enum] type : EnumSet.allOf([Class of Enum])) {
            if (type.toString().trim().toLowerCase().equals(element))
                set.add(type);
        }
    }
}
return set;

Unrelated comment: the code convention to enum types is to use uppercase (i.e. SLOW, HASTE, SLEEP, STOP... instead of Slow, Haste, Sleep, Stop...) — Nir Alfasi
– Nir Alfasi, Commented Nov 16, 2014 at 17:57
Agreed, but I'd also like to display from these values directly, and I don't want the user seeing all-caps. If there's any way to have the best of both worlds, I'm all ears. — Hammer Bro.
– Hammer Bro., Commented Nov 16, 2014 at 18:01
Then here: use uppercase for the enum constants, and then use set.add(Enum.valueOf(myEnumClazz, element.toUpperCase())), which also avoids the loop. There's prebuilt utilities for getting an enum constant by name. — Louis Wasserman
– Louis Wasserman, Commented Nov 16, 2014 at 18:16
You misunderstand -- I want to later be able to call toString() on a Status.TYPE and display the results as I currently have them typed. If I convert the enum entries to upper-case, I'll get upper-case output. — Hammer Bro.
– Hammer Bro., Commented Nov 16, 2014 at 18:27

Tom Hawtin - tackline · Accepted Answer · 2014-11-16 17:21:10Z

6

You need to specify the generic parameter (<E extends Enum<E>) between the modifiers (public static) and the return value (Set<E>). And pass as an argument the Class of the enum (Class<E> clazz`). Which should look something like this:

    public static <E extends Enum<E>> Set<E> parseValues(
        String string, Class<E> clazz
    ) { 
        EnumSet<E> set = EnumSet.noneOf(clazz);
        if (string == null) {
            return set; // ( better, NPE )
        }
        String[] elements = string.split(",");
        for (String element : elements) {
            element = element.trim();
            for (E type : EnumSet.allOf(clazz)) {
                if (type.name().equalsIgnoreCase(element)) {
                    set.add(type);
                    break;
                }
            }
            // ( Do we really want to ignore spurious values? )
        }
        return set;
    }

(Not tested or compiled.)

answered Nov 16, 2014 at 17:21

Tom Hawtin - tackline

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1 Comment

Hammer Bro. Over a year ago

Yep, that did the trick. I guess I need to brush up on my generics, since I get the gist of what's going on when I read it but I was trying to define it with the signature public static Set<E extends Enum<E>> which was freaking Java out. Thanks. (Also, the comment about spurious values is great for posterity. I'm still thinking about it in my use case.)

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