Comparable
contract specifies that e.compareTo(null)
must throw NullPointerException
.
From the API:
Note that
null
is not an instance of any class, ande.compareTo(null)
should throw aNullPointerException
even thoughe.equals(null)
returnsfalse
.
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:
Comparator
, or is it violating an unwritten rule regarding comparing null
and throwing NullPointerException
?List
containing null
elements, or is that a sure sign of a design error?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.