How to get max of dates list with stream, when having nulls in list

Question

I have following code:

List<LocalDate> dates = Arrays.asList(null, null, 
    LocalDate.now(), LocalDate.now().minusDays(9));

LocalDate max = dates.stream()
    .max(Comparator.nullsLast(Comparator.naturalOrder())).get();

Which produce null pointer exception when I try to get the max, but when trying to get min it works ok. It seems unclear cause max takes Comparator which handles null values. How to sort such array with stream preserving null values.

By the way, I checked JavaDoc of max, which states:

@throws NullPointerException if the maximum element is null

EDIT:

LocalDate max = dates.stream()
    .max(Comparator.nullsLast(Comparator.naturalOrder())).orElse(null);

Throws null pointer as well.

Answer 1

If you expect the answer to be null because it contains one you can do your own reduction.

List<LocalDate> dates = Arrays.asList(null, null,
        LocalDate.now(), LocalDate.now().minusDays(9));

LocalDate max = dates.stream()
        .reduce(LocalDate.MIN,
                BinaryOperator.maxBy(Comparator.nullsLast(Comparator.naturalOrder())));
System.out.println(max);

prints

null

NOTE: This is different in the sense that an empty list returns LocalDate.MIN.

A less obvious option is to replace null with LocalDate.MAX assuming this is never used.

LocalDate max = dates.stream()
        .map(d -> d == null ? LocalDate.MAX : d)
        .max(Comparator.naturalOrder())
        .map(d -> LocalDate.MAX.equals(d) ? null : d)
        .orElse(null);

This returns null for an empty list.

Answer 2

Having null values in the List and looking for the max value? Firstly get rid of the null values using Stream::filter. Then the job is easy:

LocalDate max = dates.stream()
                     .filter(Objects::nonNull)
                     .max(Comparator.naturalOrder()).get();

If you insist on using the Comparator only, use Comparator::nullsFirst instead:

LocalDate max = dates.stream().max(Comparator.nullsFirst(Comparator.naturalOrder())).get();

• Comparator::nullsFirst returns a null-friendly comparator that considers null to be less than non-null.
• Comparator::nullsLast returns a null-friendly comparator that considers null to be greater than non-null.