Nice general way to sort nulls to the bottom, regardless?

Question

I\'m writing some custom Comparators, and I\'d like them to push null items to the bottom of the list, regardless of whether I\'m sorting ascending or descending. What\'s a

Answer 1

I agree with Jon Skeet (it's so easy :). I tried to implement a very simple decorator:

class NullComparators {

    static  Comparator atEnd(final Comparator comparator) {
        return new Comparator() {

            public int compare(T o1, T o2) {
                if (o1 == null && o2 == null) {
                    return 0;
                }

                if (o1 == null) {
                    return 1;
                }

                if (o2 == null) {
                    return -1;
                }

                return comparator.compare(o1, o2);
            }
        };
    }

    static  Comparator atBeginning(final Comparator comparator) {
        return Collections.reverseOrder(atEnd(comparator));
    }
}

given a Comparator:

Comparator wrapMe = new Comparator() {
      public int compare(String o1, String o2) {
          return o1.compareTo(o2);
      }
};

and some test data:

List strings = Arrays.asList(null, "aaa", null, "bbb", "ccc", null);

you can sort with nulls at end:

Collections.sort(strings, NullComparators.atEnd(wrapMe));

[aaa, bbb, ccc, null, null, null]

or at beginning:

Collections.sort(strings, NullComparators.atBeginning(wrapMe));

[null, null, null, ccc, bbb, aaa]