在 Java8 中收集统计信息

Question

我想在对一组数据进行分组后收集统计信息，但我不知道是否可以

Map<String, DoubleSummaryStatistics> menuStatistics =

                menuPrices.parallelStream()

                        .collect(Collectors.groupingBy(cp -> dt1.format(cp.getUpdateDate())),

                                Collector.of(DoubleSummaryStatistics::new));

因为我有一些编译问题：

Multiple markers at this line

    - The method of(Supplier<R>, BiConsumer<R,T>, BinaryOperator<R>, Collector.Characteristics...) in the type Collector is not applicable for the arguments 

     (DoubleSummaryStatistics::new)

    - The constructed object of type DoubleSummaryStatistics is incompatible with the descriptor's return type: R

Answer 1

您需要使用Collectors.summarizingDouble()，假设您的类（ 的元素类型Stream）有一些double您想要计算统计数据的属性：Map<String, DoubleSummaryStatistics>&nbsp; &nbsp; menuStatistics =&nbsp; &nbsp; &nbsp; &nbsp; menuPrices.parallelStream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .collect(Collectors.groupingBy(cp -> dt1.format(cp.getUpdateDate()),&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp;Collectors.summarizingDouble(cp -> cp.getSomeDoubleValue())));