How to sort an array of ints using a custom comparator?

How about using streams (Java 8)?

int[] ia = {99, 11, 7, 21, 4, 2};
ia = Arrays.stream(ia).
    boxed().
    sorted((a, b) -> b.compareTo(a)). // sort descending
    mapToInt(i -> i).
    toArray();

Or in-place:

int[] ia = {99, 11, 7, 21, 4, 2};
System.arraycopy(
        Arrays.stream(ia).
            boxed().
            sorted((a, b) -> b.compareTo(a)). // sort descending
            mapToInt(i -> i).
            toArray(),
        0,
        ia,
        0,
        ia.length
    );

You can use IntArrays.quickSort(array, comparator) from fastutil library.

If you can't change the type of your input array the following will work:

final int[] data = new int[] { 5, 4, 2, 1, 3 };
final Integer[] sorted = ArrayUtils.toObject(data);
Arrays.sort(sorted, new Comparator<Integer>() {
    public int compare(Integer o1, Integer o2) {
        // Intentional: Reverse order for this demo
        return o2.compareTo(o1);
    }
});
System.arraycopy(ArrayUtils.toPrimitive(sorted), 0, data, 0, sorted.length);

This uses ArrayUtils from the commons-lang project to easily convert between int[] and Integer[], creates a copy of the array, does the sort, and then copies the sorted data over the original.