Group By Array Object dengan aggregation sum dan avg

Case

saya ingin menggroup id yang sama dari list ini dan ingin sum dari parameter ke-3 dan nge avg parameter ke-4.

new Foo(1, "P1", 300.0, 400.0), 
new Foo(2, "P2", 600.0, 400.0),
new Foo(3, "P3", 30.0, 20.0),
new Foo(3, "P3", 70.0, 20.0),
new Foo(1, "P1", 360.0, 40.0),
new Foo(4, "P4", 320.0, 200.0),
new Foo(4, "P4", 500.0, 900.0)

expectednya ketika di group akan seperti ini

new Foo(1, "P1", 660.0, 220.0), 
new Foo(2, "P2", 600.0, 400.0),
new Foo(3, "P3", 100.0, 20.0),
new Foo(4, "P4", 820.0, 550.0)

How To

buat class Foo

class Foo{
        private int id;
        private String name;
        private double a;
        private double b;

        public Foo(int id, String name, double a, double b) {
            this.id = id;
            this.name = name;
            this.a = a;
            this.b = b;
        }

        @Override
        public String toString() {
            return "Foo{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", a=" + a +
                    ", b=" + b +
                    '}';
        }
    }

buat data listnya 

List<Foo> list = Arrays.asList(
                new Foo(1, "P1", 300, 400),
                new Foo(2, "P2", 600, 400),
                new Foo(3, "P3", 30, 20),
                new Foo(3, "P3", 70, 20),
                new Foo(1, "P1", 360, 40),
                new Foo(4, "P4", 320, 200),
                new Foo(4, "P4", 500, 900));

cara grouping perfield

list.stream().collect(Collectors.groupingBy(foo -> foo.id))
                .entrySet()
                .stream()
                .forEach(e -> System.out.println(e));

listnya akan digrouping per idnya, idnya akan jadi key lalu valuenya berisi list of Foo yang idnya sama. jika dirun hasilnya akan seperti ini. 

1=[Foo{id=1, name='P1', a=300, b=400.0}, Foo{id=1, name='P1', a=360, b=40.0}]
2=[Foo{id=2, name='P2', a=600, b=400.0}]
3=[Foo{id=3, name='P3', a=30, b=20.0}, Foo{id=3, name='P3', a=70, b=20.0}]
4=[Foo{id=4, name='P4', a=320, b=200.0}, Foo{id=4, name='P4', a=500, b=900.0}]

cara grouping per field dengan aggregation sum dan avg

list.stream().collect(Collectors.groupingBy(foo -> foo.id))
                .entrySet()
                .stream()
                .map(e -> {
                    double a = e.getValue().stream().mapToDouble(ax -> ax.a).sum();
                    double b = e.getValue().stream().mapToDouble(ax -> ax.b).average().getAsDouble();
                    return new Foo(e.getKey(), e.getValue().get(0).name, a, b);
                })
                .forEach(foo -> {
                    System.out.println(foo);
                });

kita sum field a dan avg field b lalu modife value dari Foo. jika dirun hasilnya akan seperti ini: 

Foo{id=1, name='P1', a=660, b=220.0}
Foo{id=2, name='P2', a=600, b=400.0}
Foo{id=3, name='P3', a=100, b=20.0}
Foo{id=4, name='P4', a=820, b=550.0}

berikut source lengkapnya: 

package com.example.percobaan;

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

public class Main {
    public static void main(String[] args) {
        List<Foo> list = Arrays.asList(
                new Foo(1, "P1", 300, 400),
                new Foo(2, "P2", 600, 400),
                new Foo(3, "P3", 30, 20),
                new Foo(3, "P3", 70, 20),
                new Foo(1, "P1", 360, 40),
                new Foo(4, "P4", 320, 200),
                new Foo(4, "P4", 500, 900));

        list.stream().collect(Collectors.groupingBy(foo -> foo.id))
                .entrySet()
                .stream()
                .forEach(e -> System.out.println(e));

        System.out.println("\n==============\n");

        list.stream().collect(Collectors.groupingBy(foo -> foo.id))
                .entrySet()
                .stream()
                .map(e -> {
                    double a = e.getValue().stream().mapToDouble(ax -> ax.a).sum();
                    double b = e.getValue().stream().mapToDouble(ax -> ax.b).average().getAsDouble();
                    return new Foo(e.getKey(), e.getValue().get(0).name, a, b);
                })
                .forEach(foo -> {
                    System.out.println(foo);
                });
    }
    static class Foo{
        private int id;
        private String name;
        private double a;
        private double b;

        public Foo(int id, String name, double a, double b) {
            this.id = id;
            this.name = name;
            this.a = a;
            this.b = b;
        }

        @Override
        public String toString() {
            return "Foo{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", a=" + a +
                    ", b=" + b +
                    '}';
        }
    }
}

Source: https://stackoverflow.com/questions/26340688/group-by-and-sum-objects-like-in-sql-with-java-lambdas