收集器映射在Java中值为null时抛出NullPointerException，为什么？[副本]

Question

public static void main(String[] args) {
    Map<String, String> m = new HashMap<String, String>();
    m.put("a", "One");
    m.put("b", null);

    toUpperKeys1(m); // NullPointerException
    toUpperKeys2(m); // Working fine
    toUpperKeys3(m); // NullPointerException
}

static Map<String, String> toUpperKeys1(Map<String, String> map) {
    // Scenario 1 : Throwing NullPointerExpecption because Map.Entry::getValue is null
    return map.entrySet().stream().collect(Collectors.toMap(e -> e.getKey().toUpperCase(), Map.Entry::getValue));
}

static Map<String, String> toUpperKeys2(Map<String, String> map) {
    // Scenario 2 : Working fine
    Map<String, String> newMap = new HashMap<String, String>();
    map.entrySet().forEach(e -> newMap.put(e.getKey().toUpperCase(), e.getValue()));
    return newMap;
}

static Map<String, String> toUpperKeys3(Map<String, String> map) {
    // Scenario 3: Throwing NullPointerExpecption because e.getValue() is null
    return map.entrySet().stream().collect(Collectors.toMap(e -> e.getKey().toUpperCase(), e -> e.getValue()));
}

java.lang.NullPointerException
    at java.util.HashMap.merge(HashMap.java:1225)
    at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
    at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.util.HashMap$EntrySpliterator.forEachRemaining(HashMap.java:1699)
    at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
    at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
    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:566)
    at com.project.Demo.toUpperKeys1(Demo.java:25)
    at com.project.Demo.main(Demo.java:17)

因为场景2（for-each循环）运行良好，但是场景1（map.entry：：getValue）和场景3（e->e.getValue（））都抛出了NPE。

为什么toMap有这个问题，而在哈希映射的概念中，键和值都允许为空？

请解释一下。

Answer 1

正如收集器::tomap的文档所述

返回的收集器不允许空键和值。

在“b”=null中确实存在null值。你错了...不加操纵地设定价值。你当然在“操纵”。