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为什么 Collectors.groupingBy 会抛出 NullPointerException？

Caused by: java.lang.NullPointerException
        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)
        at com.evry.integrator.terpcrm.schedular.CustomerTeamsJmsServiceImpl.importDataFromTerpAndNotifyTopic(CustomerTeamsJmsServiceImpl.java:90)

复合键：

 //Entity with Getter and setters
public String getCompositeKey() {
    return getRegistryId() + "_" + getPersonPartyId() + "_" + getSource()+ "_" + getRoleName();
}

//Earlier following was composite key and it works fine; when added another field...NPE but when i revert to following no issues
//public String getCompositeKey() {
//  return getRegistryId() + "_" + getPersonPartyId() + "_" + getSource();
//}

根据之前工作正常的复合键在表中查找重复项的代码，但现在添加另一个字段会引发 NPE：（DB 中不存在空值）

Set<String> duplicates  = xxedgeCrtV.stream()
                    .collect(Collectors.groupingBy(XxedgeCrtV::getCompositeKey, Collectors.counting()))
                    .entrySet().stream()
                    .filter(e -> e.getValue() > 1L)
                    .map(e -> e.getKey())
                    .collect(Collectors.toSet());

更多数据：

在 40,000 条记录中；有 10 条记录，getRoleName 为空，但是当我通过全部四个分组查询计数时，它不返回任何记录，这意味着所有记录。

在 void main 中，当我尝试以下操作时，没有 NullPointerException：

        String a = null;
        String b = "gigi";
        String c = "migi";
        String d = "figi";
        System.out.println(d+ "_" + c+ "_" +b + "_"+a);
         Output: figi_migi_gigi_null

笔记：

我已经处理了 getRoleName null 案例，但仍然是同样的问题。

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