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

Question

I am having List of below object.

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

My data would look like below.

I am trying to groupBy driver and then apply map function(mentioned in the above table) and then reduce to Map with key as driver and value as map function's result(which is of type Double ). I have tried below

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

But not able to apply the map function. Please give some inputs to proceed further.

Answer 1

Try this:

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

I hope this will help you for your problem statement.

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 +
                '}';
    }
}

Output

{D1=1.6666666666666667, D2=3.5714285714285716}

Thanks Girdhar