如何在 Java 8 流中应用 groupby->mapping->reducing

Question

我有以下对象的列表。

public class Sample {
    private String driver;
    private String vehicle;
    private Double score;
    private Integer time;
}

我的数据如下所示。

我正在尝试 groupBy driver 然后应用 map 函数（在上表中提到），然后以 key 作为 driver 和 value 作为 map 函数的结果减少到 Map （类型为 Double）。 我在下面尝试过

list.stream().collect(Collectors.groupingBy(o-> o.getDriverId(),
            Collectors.mapping()
            ));

但无法应用地图功能。请提供一些意见以进一步进行。

Answer 1

试试这个：

Map<String, Double> res =
    list.stream()
        .collect(groupingBy(Sample::getDriver,
                            collectingAndThen(
                                reducing(new Sample("", "", 0.0,0),
                                         (a, b) -> new Sample("", "", a.score + b.score * b.time, a.time + b.time)), 
                                         s -> s.score / s.time)));

Answer 2

我希望这对您的问题陈述有所帮助。

import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class Test {
    public static void main(String[] args) {
        List<Sample> data = Arrays.asList(
        new Sample("D1","V1",1.0,10),
        new Sample("D1","V2",2.0,20),
        new Sample("D2","V3",3.0,30),
        new Sample("D2","V4",4.0,40));

        Map<String,Double> calculatedDataMapping = new HashMap<>();

        data.stream().collect(Collectors.groupingBy(Sample::getDriver)).forEach((k,v) -> {
            double value = 0.0;
           for(int i = 0 ; i < v.size()-1; i++){
               value = ((v.get(i).getScore() * v.get(i).getTime()) + (v.get(i+1).getScore() * v.get(i+1).getTime()))
                       / (v.get(i).getTime() + v.get(i + 1).getTime());
           }
           calculatedDataMapping.put(k,value);
        });
        System.out.println(calculatedDataMapping);
    }
}

Sample.java

public class Sample {

    private String driver;
    private String vehicle;
    private Double score;
    private Integer time;

    public Sample(String driver, String vehicle, Double score, Integer time) {
        this.driver = driver;
        this.vehicle = vehicle;
        this.score = score;
        this.time = time;
    }

    public String getDriver() {
        return driver;
    }

    public void setDriver(String driver) {
        this.driver = driver;
    }

    public String getVehicle() {
        return vehicle;
    }

    public void setVehicle(String vehicle) {
        this.vehicle = vehicle;
    }

    public Double getScore() {
        return score;
    }

    public void setScore(Double score) {
        this.score = score;
    }

    public Integer getTime() {
        return time;
    }

    public void setTime(Integer time) {
        this.time = time;
    }

    @Override
    public String toString() {
        return "Sample{" +
                "driver='" + driver + '\'' +
                ", vehicle='" + vehicle + '\'' +
                ", score=" + score +
                ", time=" + time +
                '}';
    }
}

输出

{D1=1.6666666666666667, D2=3.5714285714285716}

谢谢 格达尔