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?
