java-8 - Java 8 groupingby 返回多个字段

Question

在 Java 8 group by 如何对返回多个字段的单个字段进行分组。在下面的代码中，我传递了名称和要求和的字段，在这种情况下为“总计”。但是我想为客户列表中的每个“名称”返回“总计”和“余额”字段的总和(可以是一个键和值作为数组的映射)。
可以通过使用带有返回值的单个 groupingBy 来完成吗？

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class Sample {

    public static void main(String str[]){
        Customer custa = new Customer("A",1000,1500);
        Customer custa1 = new Customer("A",2000,2500);
        Customer custb = new Customer("B",3000,3500);
        Customer custc = new Customer("C",4000,4500);
        Customer custa2 = new Customer("A",1500,2500);

        List<Customer> listCust = new ArrayList<>();
        listCust.add(custa);
        listCust.add(custa1);
        listCust.add(custb);
        listCust.add(custc);
        listCust.add(custa2);

        Map<String, Double> retObj = 
            listCust.stream().collect(Collectors.groupingBy(Customer::getName,Collectors.summingDouble(Customer::getTotal)));


    System.out.println(retObj);
    }

    private static class Customer {
        private String name;
        private double total;
        private double balance;

        public Customer(String name, double total, double balance) {
            super();
            this.name = name;
            this.total = total;
            this.balance = balance;
        }
        public String getName() {
            return name;
        }
        public void setName(String name) {
            this.name = name;
        }
        public double getTotal() {
            return total;
        }
        public void setTotal(double total) {
            this.total = total;
        }
        public double getBalance() {
            return balance;
        }
        public void setBalance(double balance) {
            this.balance = balance;
        }
        @Override
        public String toString() {
            return "Customer [name=" + name + ", total=" + total + ", balance=" + balance + "]";
        }

    }
}

预期输出 -

{ 
  A = [4500,6500],
  B = [3000,3500] ,
  C = [4000,4500]
}

Answer 1

您可以编写自己的收集器来汇总和平衡

Collector<Customer, List<Double>, List<Double>> collector = Collector.of(
        () -> Arrays.asList(0.0, 0.0),
        (a, t) -> {
            a.set(0, a.get(0) + t.getTotal());
            a.set(1, a.get(1) + t.getBalance());
        },
        (a, b) -> {
            a.set(0, a.get(0) + b.get(0));
            a.set(1, a.get(1) + b.get(1));
            return a;
        }
);

Map<String, List<Double>> retObj = listCust
        .stream()
        .collect(Collectors.groupingBy(Customer::getName, collector));

System.out.println(retObj);

结果

{A=[4500.0, 6500.0], B=[3000.0, 3500.0], C=[4000.0, 4500.0]}