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本文通过示例介绍如何使用 Java 8 的 Stream API 对集合进行分组操作，包括按价格分组并收集相关商品信息，以及进一步将商品名称转换为 Set 集合以去除重复项。

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2.2 Group by Price – Collectors.groupingBy and Collectors.mapping example.

Java8Examples4.java

package com.mkyong.java8;

import java.math.BigDecimal;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.stream.Collectors;

public class Java8Examples4 {

    public static void main(String[] args) {

        //3 apple, 2 banana, others 1
        List<Item> items = Arrays.asList(
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 20, new BigDecimal("19.99")),
                new Item("orang", 10, new BigDecimal("29.99")),
                new Item("watermelon", 10, new BigDecimal("29.99")),
                new Item("papaya", 20, new BigDecimal("9.99")),
                new Item("apple", 10, new BigDecimal("9.99")),
                new Item("banana", 10, new BigDecimal("19.99")),
                new Item("apple", 20, new BigDecimal("9.99"))
                );

        //group by price
        Map<BigDecimal, List<Item>> groupByPriceMap =
            items.stream().collect(Collectors.groupingBy(Item::getPrice));

        System.out.println(groupByPriceMap);

        // group by price, uses 'mapping' to convert List<Item> to Set<String>
        Map<BigDecimal, Set<String>> result =
                items.stream().collect(
                        Collectors.groupingBy(Item::getPrice,
                                Collectors.mapping(Item::getName, Collectors.toSet())
                        )
                );

        System.out.println(result);

    }
}

output

{
    19.99=[
            Item{name='banana', qty=20, price=19.99}, 
            Item{name='banana', qty=10, price=19.99}
        ], 
    29.99=[
            Item{name='orang', qty=10, price=29.99}, 
            Item{name='watermelon', qty=10, price=29.99}
        ], 
    9.99=[
            Item{name='apple', qty=10, price=9.99}, 
            Item{name='papaya', qty=20, price=9.99}, 
            Item{name='apple', qty=10, price=9.99}, 
            Item{name='apple', qty=20, price=9.99}
        ]
}

//group by + mapping to Set
{
    19.99=[banana], 
    29.99=[orang, watermelon], 
    9.99=[papaya, apple]
}

GroupBy

Map<Integer, Integer> totalNoThings = somethings.stream()
    .collect(Collectors.groupingBy(Something::getParentKey,
        Collectors.summingInt(Something::getNoThings)));