使用报NullPointerException

分析(Pair::getKey, Pair::getValue, (v1, v2) -> v1)

public static <T, K, U>
    Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                    Function<? super T, ? extends U> valueMapper,
                                    BinaryOperator<U> mergeFunction) {
    return toMap(keyMapper, valueMapper, mergeFunction, HashMap::new);
}

public static <T, K, U, M extends Map<K, U>>
    Collector<T, ?, M> toMap(Function<? super T, ? extends K> keyMapper,
                             Function<? super T, ? extends U> valueMapper,
                             BinaryOperator<U> mergeFunction,
                             Supplier<M> mapSupplier) {
    BiConsumer<M, T> accumulator
        = (map, element) -> map.merge(keyMapper.apply(element),
                                      valueMapper.apply(element), mergeFunction);
    return new CollectorImpl<>(mapSupplier, accumulator, mapMerger(mergeFunction), CH_ID);
}

看下Map接口的默认merge方法

default V merge(K key, V value,
                BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(remappingFunction);
    Objects.requireNonNull(value);
    V oldValue = get(key);
    V newValue = (oldValue == null) ? value :
    remappingFunction.apply(oldValue, value);
    if(newValue == null) {
        remove(key);
    } else {
        put(key, newValue);
    }
    return newValue;
}

该接口可以看出value不能为空，再看下我们使用的HashMap的重写的merge方法

@Override
public V merge(K key, V value,
               BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    if (value == null)
        throw new NullPointerException();
    if (remappingFunction == null)
        throw new NullPointerException();
    ...
        return value;
}

从源码可以看出value==null直接报空指针异常了