Question

我将Java 11流的结果放入Collectors.toMap(keyMapper, valueMapper)收集器中。碰巧我的值映射器（lambda）返回一个空值，结果是NullPointerException，这令人惊讶。

java.lang.NullPointerException
    at java.base/java.util.Objects.requireNonNull(Objects.java:221)
    at java.base/java.util.stream.Collectors.lambda$uniqKeysMapAccumulator$1(Collectors.java:178)
    at java.base/java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
    at java.base/java.util.TreeMap$EntrySpliterator.forEachRemaining(TreeMap.java:2962)
    at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:484)
    at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:474)
    at java.base/java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:913)
    at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
    at java.base/java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:578)
    at [my code]

对Objects.requireNonNull的致命呼叫位于Collectors.uniqKeysMapAccumulator()中：

    private static <T, K, V>
    BiConsumer<Map<K, V>, T> uniqKeysMapAccumulator(Function<? super T, ? extends K> keyMapper,
                                                    Function<? super T, ? extends V> valueMapper) {
        return (map, element) -> {
            K k = keyMapper.apply(element);
            V v = Objects.requireNonNull(valueMapper.apply(element));
            V u = map.putIfAbsent(k, v);
            if (u != null) throw duplicateKeyException(k, u, v);
        };
    }

关键表达为Objects.requireNonNull(valueMapper.apply(element))。

为什么收集器要求映射值不为空？这是实现中的错误吗？

Map.merge operation is required to reject null values。给Collectors.toMap()的{{1}}方法的文档说明mergeFunction将使用它。但是，2参数Map.merge方法的Collectors文档（没有toMap()）没有声明值必须为非空，也没有声明{{ 1}}完全被人感染。

为什么Collectors.toMap需要非null值

0 个答案: