如果每个键有多个列表项，则将Java 8 lambdas组列表映射为不同的键

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

public class Main {
    public static void main(String[] args) {
        // 创建一个包含多个键和列表项的Map
        Map<String, List<String>> map = new HashMap<>();
        map.put("key1", new ArrayList<>(List.of("item1", "item2")));
        map.put("key2", new ArrayList<>(List.of("item3", "item4")));
        map.put("key3", new ArrayList<>(List.of("item5", "item6")));

        // 使用Java 8 lambdas将每个键下的多个列表项映射为不同的键
        Map<String, List<String>> result = map.entrySet().stream()
                .flatMap(entry -> entry.getValue().stream().map(item -> new AbstractMap.SimpleEntry<>(item, entry.getKey())))
                .collect(Collectors.groupingBy(Map.Entry::getKey, Collectors.mapping(Map.Entry::getValue, Collectors.toList())));

        // 打印结果
        result.forEach((key, value) -> System.out.println(key + ": " + value));
    }
}

item1: [key1]
item2: [key1]
item3: [key2]
item4: [key2]
item5: [key3]
item6: [key3]