Java Collectors.Stream：POJO构建和设置多个聚合值[重复]

Question

public class Item{
   String name;
   Double quantity;
   Double price;
   Double totalDollarAmount;

    public Item(String name, Double quantity, Double price) {
        this.name = name;
        this.quantity= quantity;
        this.price = price;
    }

   //Basic Getters and setters

   public Double getTotalDollarAmount(){
      return getQuantity()*getPrice();
   }
}

从列表 中，我希望能够快速计算我购买的每件商品的数量、平均价格以及为该商品花费的总金额。假设对于这个场景，我有以下列表：

        List<Item> itemsOnly = Arrays.asList(
                new Item("apple", 10.0, 9.99),
                new Item("banana", 20.0, 19.99),
                new Item("orange", 10.0, 29.99),
                new Item("watermelon", 10.0, 29.99),
                new Item("papaya", 20.0, 9.99),
                new Item("apple", 100.0, 9.99),
                new Item("apple", 20.0, 9.99)
        );

如果我想获得列表中每个唯一项目的总数量、平均价格和总美元金额，我可以这样做：

System.out.println("Total Quantity for each Item: " + itemsOnly.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.summingDouble(Item::getQuantity))));
System.out.println("Average Price for each Item: " + itemsOnly.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.averagingDouble(Item::getPrice))));
System.out.println("Total Dollar Amount for each Item: " + itemsOnly.stream().collect(
                Collectors.groupingBy(Item::getName, Collectors.summingDouble(Item::getTotalDollarAmount))));

这将返回以下内容：

Total Quantity for each Item: {papaya=20.0, orange=10.0, banana=20.0, apple=130.0, watermelon=10.0}
Average Price for each Item: {papaya=9.99, orange=29.99, banana=19.99, apple=9.99, watermelon=29.99}
Total Dollar Amount for each Item: {papaya=199.8, orange=299.9, banana=399.79999999999995, apple=1298.7, watermelon=299.9}

Answer 1

你快到了。若要将分组项合并到单个项中，可以使用Reduction收集器。

下面是一个方法：

首先，定义一种合并两项的方式：

public static Item merge (Item i1, Item i2) {
    final double count = i1.quantity + i2.quantity;
    final double avgPrice = (i1.quantity * i1.price + i2.quantity * i2.price) / count;
    return new Item(i1.name, count, avgPrice);
}

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

public class Main
{
    public static void main(String[] args) {        
        List<Item> itemsOnly = Arrays.asList(
                new Item("apple", 10.0, 9.99),
                new Item("banana", 20.0, 19.99),
                new Item("orange", 10.0, 29.99),
                new Item("watermelon", 10.0, 29.99),
                new Item("papaya", 20.0, 9.99),
                new Item("apple", 100.0, 9.99),
                new Item("apple", 20.0, 9.99)
        );
        
        Map<String, Item> groupedItems = itemsOnly.stream().collect(
                Collectors.groupingBy(
                     item -> item.name,
                     Collectors.collectingAndThen(
                         Collectors.<Item>reducing(Main::merge),
                         Optional::get // No need for null check: grouping should send at least one element to the reducer
                    )
                )
        );

        for (Item i : groupedItems.values()) System.out.println(i);                 
    }
    
    public static Item merge (Item i1, Item i2) {
        final double count = i1.quantity + i2.quantity;
        final double avgPrice = (i1.quantity * i1.price + i2.quantity * i2.price) / count;
        return new Item(i1.name, count, avgPrice);
    }
    
    public static class Item {
        public final String name;
        public final double quantity;
        public final double price;

        public Item(String name, double quantity, double price) {
            this.name = name;
            this.quantity= quantity;
            this.price = price;
        }

        public double getTotalDollarAmount(){
          return quantity*price;
        }
        
        public String toString() { return String.format("%s: quantity: %d, price: %f, total: %f", name, (int) quantity, price, getTotalDollarAmount()); }
    }
}
 

Map<String, Item> groupedItems = itemsOnly.stream().collect(
            Collectors.toMap(
                item -> item.name,
                Function.identity(),
                Main::merge
            )
);