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?

Answer 1

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

)

Answer 2

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.