Finding enum value with Java 8 Stream API

Question

Suppose there is a simple enum called Type defined like this:

enum Type{
    X("S1"),
    Y("S2");

    private String s;

    private Type(String s) {
        this.s = s;
    }
}

Finding the correct enum for given s is trivially done with static method with for-loop (assume the method is defined inside enum), e.g.:

private static Type find(String val) {
        for (Type e : Type.values()) {
            if (e.s.equals(val))
                return e;
        }
        throw new IllegalStateException(String.format("Unsupported type %s.", val));
}

I think the functional equivalent of this expressed with Stream API would be something like this:

private static Type find(String val) {
     return Arrays.stream(Type.values())
            .filter(e -> e.s.equals(val))
            .reduce((t1, t2) -> t1)
            .orElseThrow(() -> {throw new IllegalStateException(String.format("Unsupported type %s.", val));});
}

How could we write this better and simpler? This code feels coerced and not very clear. The reduce() especially seems clunky and abused as it doesn't accumulate anything, performs no calculation and always simply returns t1 (provided the filter returns one value - if it doesn't that's clearly a disaster), not to mention t2 is there superfluous and confusing. Yet I couldn't find anything in Stream API that simply somehow returns directly a T from a Stream<T>.

Is there a better way?

question from:https://stackoverflow.com/questions/27807232/finding-enum-value-with-java-8-stream-api

Answer 1

I would use findFirst instead:

return Arrays.stream(Type.values())
            .filter(e -> e.s.equals(val))
            .findFirst()
            .orElseThrow(() -> new IllegalStateException(String.format("Unsupported type %s.", val)));

---

Though a Map could be better in this case:

enum Type{
    X("S1"),
    Y("S2");

    private static class Holder {
        static Map<String, Type> MAP = new HashMap<>();
    }

    private Type(String s) {
        Holder.MAP.put(s, this);
    }

    public static Type find(String val) {
        Type t = Holder.MAP.get(val);
        if(t == null) {
            throw new IllegalStateException(String.format("Unsupported type %s.", val));
        }
        return t;
    }
}

I learnt this trick from this answer. Basically the class loader initializes the static classes before the enum class, which allows you to fill the Map in the enum constructor itself. Very handy !

Hope it helps ! :)