Comparable and Comparator contract with regards to null

Question

Comparable contract specifies that e.compareTo(null) must throw NullPointerException.

From the API:

Note that null is not an instance of any class, and e.compareTo(null) should throw a NullPointerException even though e.equals(null) returns false.

On the other hand, Comparator API mentions nothing about what needs to happen when comparing null. Consider the following attempt of a generic method that takes a Comparable, and return a Comparator for it that puts null as the minimum element.

static <T extends Comparable<? super T>> Comparator<T> nullComparableComparator() {
   return new Comparator<T>() {
      @Override public int compare(T el1, T el2) {
         return
            el1 == null ? -1 :
            el2 == null ? +1 :
            el1.compareTo(el2);
      }
   };
}

This allows us to do the following:

List<Integer> numbers = new ArrayList<Integer>(
   Arrays.asList(3, 2, 1, null, null, 0)
);
Comparator<Integer> numbersComp = nullComparableComparator();
Collections.sort(numbers, numbersComp);
System.out.println(numbers);
// "[null, null, 0, 1, 2, 3]"

List<String> names = new ArrayList<String>(
   Arrays.asList("Bob", null, "Alice", "Carol")
);
Comparator<String> namesComp = nullComparableComparator();
Collections.sort(names, namesComp);
System.out.println(names);
// "[null, Alice, Bob, Carol]"

So the questions are:

• Is this an acceptable use of a Comparator, or is it violating an unwritten rule regarding comparing null and throwing NullPointerException?
• Is it ever a good idea to even have to sort a List containing null elements, or is that a sure sign of a design error?

Answer 1

Comparable doesn't allow null simply because:

a.compareTo(b) == -b.compareTo(a)

for all objects a and b where !a.equals(b). More specifically:

a.equals(b) ? b.equals(a) && a.compareTo(b) == 0 &&
                  b.compareTo(a) == 0 && a.hashCode() == b.hashCode()
            : !b.equals(a) && a.compareTo(b) != 0 &&
                  a.compareTo(b) == -b.compareTo(a)

must evaluate to true to satisfy the relevant contracts.

So null isn't allowed because you can't do:

null.compareTo(a)

Comparator is more flexible so handling of null is an implementation-specific issue. Support it or not depending on what you want your Comparator to do.

Answer 2

Is it ever a good idea to even have to sort a List containing null elements, or is that a sure sign of a design error?

Conceptually, null means "nothing", and placing nothing in a list seems weird to me. Also, the Java List contract states that

Some list implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements

so a List implementation in Java is not even required to support null elements at all. To sum up, if you do not have a good reason to put null into a list, don't, and if you do, test that it actually works as expected.

Answer 3

Is it ever a good idea to even have to sort a List containing null elements, or is that a sure sign of a design error?

Well, it probably doesn't make sense for the list to contain a null Object, but maybe your List contains a "business object" and you can sort on different properties of the business object, some of which may contain nulls.

Is this an acceptable use of a Comparator

The BeanComparator allows you to sort on a propery in a business object even if the property contains null, so I would have to say it is an acceptable use of a Comparator.