Java-8:流如何将Map<K、List<D>>转换为Map<D、List<K>>

import java.util.*;
import java.util.stream.Collectors;

public class Main {
    public static void main(String[] args) {
        // 创建一个示例的Map<K, List<D>>
        Map<String, List<Integer>> map = new HashMap<>();
        map.put("A", Arrays.asList(1, 2, 3));
        map.put("B", Arrays.asList(4, 5, 6));
        map.put("C", Arrays.asList(7, 8, 9));

        // 使用流将Map<K, List<D>>转换为Map<D, List<K>>
        Map<Integer, List<String>> resultMap = map.entrySet().stream()
                .flatMap(entry -> entry.getValue().stream()
                        .map(value -> new AbstractMap.SimpleEntry<>(value, entry.getKey())))
                .collect(Collectors.groupingBy(AbstractMap.SimpleEntry::getKey,
                        Collectors.mapping(AbstractMap.SimpleEntry::getValue, Collectors.toList())));

        // 输出转换后的结果
        System.out.println(resultMap);
    }
}

{1=[A], 2=[A], 3=[A], 4=[B], 5=[B], 6=[B], 7=[C], 8=[C], 9=[C]}