java 8 - Comparator.nullsFirst with null-safe-comparator

Question

I want to have a null-safe comparator, but it does not work:

Comparator<Item> sort_high = (i1, i2)-> Double.compare(i2.getUser().getValue(), i1.getUser().getValue());

items.sort(Comparator.nullsFirst(sort_high));

However, I get a NPE, if item.getUser().getValue() (or item.getUser()) is null.

at Item.lambda$1(Item.java:270)
    at java.util.Comparators$NullComparator.compare(Comparators.java:83)
    at java.util.TimSort.countRunAndMakeAscending(TimSort.java:355)
    at java.util.TimSort.sort(TimSort.java:220)
    at java.util.Arrays.sort(Arrays.java:1438)
    at java.util.List.sort(List.java:478)

What is wrong?

Answer 1

nullsFirst will take care of null Items ("i1"), but once two valid objects are found, your Comparator is invoked and you need to handle internal null references.

In your case, you could use something like:

items.sort(
  Comparator.nullsFirst(
    Comparator.comparing(Item::getUser,
      Comparator.nullsFirst(Comparator.comparingDouble(User::getValue))
    )
  )
);

(Assuming getValue() returns a double)

But I hardly recommend such convoluted code.