NullPointerException: element cannot be mapped to a null key

Question

I have read the topic:

Collectors.groupingBy doesn't accept null keys

But I don't understand how can I apply it for my issue:

my code:

Map<String, List<MappingEntry>> mappingEntryMap = mapping.getMappingEntries()
                .stream()
                .collect(Collectors.groupingBy(MappingEntry::getMilestone, Collectors.mapping(e -> e, Collectors.toList())));

For me MappingEntry::getMilestone sometimes can return null. It is ok for my situation but I see:

Caused by: java.lang.NullPointerException: element cannot be mapped to a null key
    at java.util.Objects.requireNonNull(Objects.java:228)
    at java.util.stream.Collectors.lambda$groupingBy$45(Collectors.java:907)
    at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
    at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
    at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)

How can I avoid this exception?

@Eugene it is not allowable for my case. I must not lose the data. This approach described in the linked topic — gstackoverflow, Dec 18 '17 at 12:53
@gstackoverflow what about applying a default key for all `null` keys? — Lino, Dec 18 '17 at 12:53
Also `Collectors.mapping(e -> e, Collectors.toList())` can be simplified to `Collectors.toList()` — Lino, Dec 18 '17 at 12:54
@Marvin, I undetstand, that I can, but i prefere to use streams. And it looks like stream should have this option — gstackoverflow, Dec 18 '17 at 12:59
@Lino … and `Collectors.toList()` can be omitted when using `Collectors.groupingBy`. — Holger, Dec 19 '17 at 10:00
Please checking my answer, solution using Optional instead of raw nulls — Mateusz, Nov 05 '21 at 11:59

score 20 · Accepted Answer · answered Dec 18 '17 at 13:04

20

Use Collectors.toMap instead and specify that a HashMap is used (as it allows one null key)

 Collectors.toMap(
       MappingEntry::getMilestone,
       x -> {
           List<MappingEntry> list = new ArrayList<>();
           list.add(x);
           return list;
       },
       (left, right) -> {
            left.addAll(right);
            return left;
       },
       HashMap::new

)

answered Dec 18 '17 at 13:04

Eugene

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Not a beautiful solution. You changed the data structure as well as the logic. Doesn't deserve to be the best answer. – Rajesh Paul May 24 '21 at 07:14
@RajeshPaul there are al least 17 people that disagree with you. – Eugene May 24 '21 at 09:30

Ousmane D. · Answer 2 · 2017-12-18T15:01:29.483

Given that you want to retain the MappingEntry objects regardless of when getMilestone() is null or non-null and knowing that a NullPointerException will be thrown when a particular contract in not met then we can avoid that by using a replacement key to group the MappingEntry objects which have a null milestone and yet group the other MappingEntry objects as they're supposed to be.

Map<String, List<MappingEntry>> mappingEntryMap = 
             mapping.getMappingEntries()
                    .stream()
                    .collect(groupingBy(m -> m.getMilestone() == null ?
                                  "absentMilestone" : m.getMilestone()));

The trick here is to use a ternary operator which provides a key to group all the MappingEntry objects that have a absent milestone into a single group and if the milestone is not absent then we can group by its value as you'd expect.

This is absolutely correct answer and deserves to be chosen as the best answer. Thanks @Ousmane D. — Rajesh Paul, May 24 '21 at 07:13

Lino · Answer 3 · 2017-12-18T13:18:48.150

You can use the Collector.of method for this to specify a custom collector and use a HashMap for the resulting map because according to the Java-8 docs of HashMap

[...] permits null values and the null key. [...]

Collector.of(
    HashMap::new, 
    (map, e) -> map.computeIfAbsent(e.getMileStone(), k -> new ArrayList<>()).add(e), 
    (left, right) -> {
         right.forEach((key,list) -> left.computeIfAbsent(key, k -> new ArrayList<>()).addAll(list));
         return left;
     })
)

score 0 · Answer 4 · answered Nov 05 '21 at 10:10

In my opinion the best solution:

Use Optional.empty() instead of pasing null value.

Map<Optional<String>, List<MappingEntry>> mappingEntryMap = mapping.getMappingEntries()
            .stream()
            .collect(Collectors.groupingBy(Optional::ofNullable, Collectors.mapping(e -> e, Collectors.toList())));

score 0 · Answer 5 · answered Nov 19 '21 at 12:10

use filter and get only not null data.

like

            Map<String, List<Entity>> map = list
                .stream()
                .filter(entity -> entity.getZoneRefId()!=null)
                .collect(
                        Collectors.
                                groupingBy(
                                        Entity::getZoneName));

NullPointerException: element cannot be mapped to a null key

5 Answers5