Question

So in C++/C# you can create flags enums to hold multiple values, and storing a single meaningful integer in the database is, of course, trivial.

In Java you have EnumSets, which appear to be quite a nice way to pass enums around in memory, but how do you output the combined EnumSet to an integer for storage? Is there another way to approach this?

Answer 1

// From Adamski's answer
public static <E extends Enum<E>> int encode(EnumSet<E> set) {
    int ret = 0;

    for (E val : set) {
        ret |= 1 << val.ordinal();
    }

    return ret;
}

@SuppressWarnings("unchecked")
private static <E extends Enum<E>> EnumSet<E> decode(int code,
        Class<E> enumType) {
    try {
        E[] values = (E[]) enumType.getMethod("values").invoke(null);
        EnumSet<E> result = EnumSet.noneOf(enumType);
        while (code != 0) {
            int ordinal = Integer.numberOfTrailingZeros(code);
            code ^= Integer.lowestOneBit(code);
            result.add(values[ordinal]);
        }
        return result;
    } catch (IllegalAccessException ex) {
        // Shouldn't happen
        throw new RuntimeException(ex);
    } catch (InvocationTargetException ex) {
        // Probably a NullPointerException, caused by calling this method
        // from within E's initializer.
        throw (RuntimeException) ex.getCause();
    } catch (NoSuchMethodException ex) {
        // Shouldn't happen
        throw new RuntimeException(ex);
    }
}