Java 8分组与返回多个字段 [英] Java 8 groupingby with returning multiple field
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问题描述
在Java 8组中,如何在一个返回多个字段的字段上进行分组。在下面的代码中,我传递名称和要求的字段,在这种情况下是总计。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(可以是键和值为数组的映射)。
可以通过使用带有返回值的单个groupingBy来完成吗?
In Java 8 group by how to groupby on a single field which returns more than one field. In the below code by I am passing name and the field to be summed which is 'total' in this scenario. however I would like to return sum of 'total' and 'balance' field for every 'name' in the Customer list (can be a map with key and value as array). Can it be done by using a single groupingBy with the return values?
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]
}
推荐答案
您可以编写自己的收集器来计算总和余额
You can write your own collector to sum total and balance
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]}
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