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.

just add a filter clause to remove nulls from stream before doing any other computation. — Sikorski, Aug 07 '18 at 11:46
[Why is Optional.get() without calling isPresent() bad](https://softwareengineering.stackexchange.com/questions/318912/why-is-optional-get-without-calling-ispresent-bad-but-not-iterator-next) — Soner from The Ottoman Empire, Aug 07 '18 at 11:49

Peter Lawrey · Accepted Answer · 2018-08-07T11:55:05.613

10

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.

edited Aug 07 '18 at 11:55

answered Aug 07 '18 at 11:49

Peter Lawrey

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very nice! was not aware of `LocalDate.MIN`, I wish I knew about it before - could have won a couple or arguments :) – Eugene Aug 07 '18 at 12:13
2

@Eugene although MIN is a large negative date, anything before about the 1750s is likely to be fraught. e.g. there was no 0 in the Roman number and no year 0 in the Julian calendar. – Peter Lawrey Aug 07 '18 at 14:59
I understand that- I looked at the implementation already, thank you anyway. It's good that I don't deal with such dates (like 99% of people), but when you need to reduce with an identity then definitely something like this needed – Eugene Aug 07 '18 at 15:01

Nikolas Charalambidis · Answer 2 · 2018-08-07T11:51:15.297

3

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.

edited Aug 07 '18 at 11:51

answered Aug 07 '18 at 11:45

Nikolas Charalambidis

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Maybe having nulls in list and looking for max is not the best idea, but provided comparator defines how to treat null values. – Bartek Aug 07 '18 at 11:49
I realized after writing my initial answer that max will still fill with `nullsFirst` when the list contains only nulls. `max` does not allow null as a value. – Malte Hartwig Aug 07 '18 at 11:49

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

2 Answers2