使用Lambda和Stream从嵌套列表构建Map

Map<String, String> entityGroup = new HashMap<>();
groups.forEach(g -> g.getEntities()
                .forEach(e -> entityGroup.put(e.getKey(), g.getKey()))
);

import java.util.AbstractMap;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

// 假设 Group 和 Entity 模型定义如下
class Group {
    private String key;
    private List<Entity> entities;

    public Group(String key, List<Entity> entities) {
        this.key = key;
        this.entities = entities;
    }

    public String getKey() { return key; }
    public List<Entity> getEntities() { return entities; }
}

class Entity {
    private String key;
    private String value; // 示例，实际可能不需要

    public Entity(String key, String value) {
        this.key = key;
        this.value = value;
    }

    public String getKey() { return key; }
    public String getValue() { return value; }
}

public class MapFromNestedList {
    public static void main(String[] args) {
        // 示例数据
        List<Group> groups = List.of(
            new Group("GroupA", List.of(new Entity("Entity1", "Val1"), new Entity("Entity2", "Val2"))),
            new Group("GroupB", List.of(new Entity("Entity3", "Val3"), new Entity("Entity4", "Val4")))
        );

        Map<String, String> entityGroup = groups.stream()
            // 1. 展平嵌套列表：将每个Group的List<Entity>流合并成一个单一的Entity流
            .flatMap(group -> group.getEntities().stream()
                // 2. 映射为Map.Entry：为每个Entity创建键值对
                //    键是Entity的key，值是其所属Group的key
                .map(entity -> Map.entry(entity.getKey(), group.getKey()))) // Java 9+ 使用 Map.entry
            // 3. 收集为Map：将所有的Map.Entry收集成一个Map
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

        System.out.println(entityGroup); // 输出: {Entity1=GroupA, Entity2=GroupA, Entity3=GroupB, Entity4=GroupB}

        // Java 8 兼容方案 (使用 AbstractMap.SimpleEntry)
        Map<String, String> entityGroupJava8 = groups.stream()
            .flatMap(group -> group.getEntities().stream()
                .map(entity -> new AbstractMap.SimpleEntry<>(entity.getKey(), group.getKey())))
            .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

        System.out.println(entityGroupJava8);
    }
}