使用 Java Streams 按属性将对象列表组合在一起

Question

我有一个 SensorSample POJO 列表

public class SensorSample {

    private Device.SensorType sensorType; // This is an enum

    private double sample;

    private long timestamp;

    // Constructor

    // Setters

    // Getters

}

我需要按 对它们进行分组timestamp，以便SensorSample同一天的所有 s 都在一起。然后我需要减少它们，这样我SensorSample每天只有一个，它的值是当天所有对象sample的值的平均值。sample有没有办法用 Streams 做到这一点？

到目前为止，我将它们组合在一起：

Map<Long, List<SensorSample>> yearSamples = samples.stream()

                .collect(groupingBy(sample -> SECONDS_IN_A_DAY*Math.floorDiv(sample.getTimestamp(), SECONDS_IN_A_DAY)));

但我不知道如何更进一步。

Answer 1

像这样的东西，我想。要查找组的平均数：Map<Long, Double> averages = samples.stream()&nbsp; .collect(groupingBy(SensorSample::getTimestamp,&nbsp; &nbsp;averagingDouble(SensorSample::getSample)));我当天没有扩展您的公式，我认为如果我只是打电话getTimestamp并省略详细信息，它会更具可读性。如果您将getDay方法添加到SensorSample.如果您提供了 MCVE，这也将更容易测试，因为只有一个部分类可以继续测试上面的代码有点困难。

Answer 2

结果似乎您想要 aList<SensorSample>之后的每个组groupingBy都减少为单个SensorSample.List<SensorSample> result = samples.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .collect(groupingBy(sample -> SECONDS_IN_A_DAY*Math.floorDiv(sample.getTimestamp(), SECONDS_IN_A_DAY))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .map(e -> {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; SensorSample sensorSample = new SensorSample();&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; sensorSample.setTimestamp(e.getKey());&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; double average = e.getValue().stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .mapToDouble(SensorSample::getSample)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .average().orElse(0);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; sensorSample.setSample(average);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; sensorSample.setSensorType(e.getValue().get(0).getSensorType());&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; return sensorSample;&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }).collect(Collectors.toList());map逻辑似乎有点大，因此我会考虑将其重构为这样的方法：private static SensorSample apply(Map.Entry<Long, List<SensorSample>> e) {&nbsp; &nbsp; &nbsp; &nbsp; SensorSample sensorSample = new SensorSample();&nbsp; &nbsp; &nbsp; &nbsp; sensorSample.setTimestamp(e.getKey());&nbsp; &nbsp; &nbsp; &nbsp; double average = e.getValue().stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .mapToDouble(SensorSample::getSample)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .average().orElse(0);&nbsp; &nbsp; &nbsp; &nbsp; sensorSample.setSample(average);&nbsp; &nbsp; &nbsp; &nbsp; sensorSample.setSensorType(e.getValue().get(0).getSensorType());&nbsp; &nbsp; &nbsp; &nbsp; return sensorSample;}然后流管道将变为：List<SensorSample> result = samples.stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .collect(groupingBy(sample -> SECONDS_IN_A_DAY*Math.floorDiv(sample.getTimestamp(), SECONDS_IN_A_DAY))&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .entrySet()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .stream()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .map(Main::apply)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; .collect(Collectors.toList());Main包含该apply方法的类在哪里。