Creating String from a bitmask

In my Android app I store days of week in an int variable by using bit masking.

I'm using the following SparseIntArray to get the day flags for the corresponding days:

public static final SparseIntArray DAY_FLAGS = new SparseIntArray(7);
static {
    DAYS.put(Calendar.MONDAY, 1);
    DAYS.put(Calendar.TUESDAY, 2);
    DAYS.put(Calendar.WEDNESDAY, 4);
    DAYS.put(Calendar.THURSDAY, 8);
    DAYS.put(Calendar.FRIDAY, 16);
    DAYS.put(Calendar.SATURDAY, 32);
    DAYS.put(Calendar.SUNDAY, 64);
}

I'd like to be able to create String from a mask, for example:

int value - String

---

1 - "Mon"

42 - "Tue Thu Sat"

127 - "Mon Tue Wed Thu Fri Sat Sun"

For this I'm using the following method:

public static String maskToString(Context context, int mask, int firstDayOfWeek) {
    List<Integer> dayList = new ArrayList<>(7);

    // The idea is to iterate through the SparseIntArray and check
    // if the bit in the mask is set for the particular day.
    // If it is, add that day to the dayList.
    for (int i = 0; i < DAY_FLAGS.size(); i++) {
        if (isFlagSet(DAY_FLAGS.valueAt(i), mask)) {
            dayList.add(DAY_FLAGS.keyAt(i));
        }
    }

    // Sort the days by taking the first day of the week into account
    Collections.sort(dayList, new DayComparator(firstDayOfWeek));

    // This List will contain the individual Strings for the days
    List<String> dayStringList = new ArrayList<>(dayList.size());

    // dayToString() simply returns a String resource for a given day
    for (Integer day : dayList) {
        dayStringList.add(dayToString(context, day));
    }

    // join the individual Strings into a single String
    return TextUtils.join(" ", dayStringList);
}

The isFlagSet() method simply checks if a given flag is set in a given mask:

private static boolean isFlagSet(int flag, int mask) {
    return ((mask & flag) == flag);
}

My maskToString() method seems clumsy (mostly in terms of unnecessary(?) object creation). I believe it can be optimized, I just don't know how.

(I can use only Java 7 features, no Java 8)

I'd appreciate any advice.

EDIT:

One thing I came up with regarding optimization is to create a static final DayComparator instance and pass the firstDayOfWeek variable in a setter (instead of the constructor):

private static final DayComparator DAY_COMPARATOR = new DayComparator();

public static String maskToString(Context context, int mask, int firstDayOfWeek) {
    List<Integer> dayList = new ArrayList<>(7);

    for (int i = 0; i < DAY_FLAGS.size(); i++) {
        if (isFlagSet(DAY_FLAGS.valueAt(i), mask)) {
            dayList.add(DAY_FLAGS.keyAt(i));
        }
    }

    // so we do not create a new object here every time the method is called
    DAY_COMPARATOR.setFirstDayOfWeek(firstDayOfWeek);
    Collections.sort(days, DAY_COMPARATOR);

    List<String> dayStringList = new ArrayList<>(dayList.size());

    for (Integer day : dayList) {
        dayStringList.add(dayToString(context, day));
    }

    return TextUtils.join(" ", dayStringList);
}

What do you think? Any other suggestions?