使用Java Stream高效分组JPA实体并转换数据结构

{
  "New York": ["John", "Josh"],
  "Atlanta": ["Paul", "Charles"],
  "Los Angeles": ["Mark", "Susan"]
}

public Map<String, List<String>> findAllUsersImperative() {
    List<RegistryEntity> items = registryRepository.findAll(); // 假设从JPA Repository获取实体列表
    Map<String, List<String>> itemsGrouped = new HashMap<>();
    for (RegistryEntity s : items) {
        // 检查Map中是否已存在该城市对应的列表
        if (itemsGrouped.containsKey(s.getCity())) {
            itemsGrouped.get(s.getCity()).add(s.getName());
        } else {
            // 如果不存在，则创建新列表并添加姓名
            List<String> tempResults = new ArrayList<>();
            tempResults.add(s.getName());
            itemsGrouped.put(s.getCity(), tempResults);
        }
    }
    return itemsGrouped;
}

import jakarta.persistence.Entity;
import jakarta.persistence.Id; // 或者javax.persistence.Id
import jakarta.persistence.Table; // 或者javax.persistence.Table

@Entity
@Table(name = "registry_entity") // 假设表名为registry_entity
public class RegistryEntity {

    @Id
    private Long id;
    private String name;
    private String city;

    // 构造函数
    public RegistryEntity() {}

    public RegistryEntity(Long id, String name, String city) {
        this.id = id;
        this.name = name;
        this.city = city;
    }

    // Getter方法
    public Long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getCity() {
        return city;
    }

    // Setter方法 (如果需要)
    public void setId(Long id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setCity(String city) {
        this.city = city;
    }

    @Override
    public String toString() {
        return "RegistryEntity{" +
               "id=" + id +
               ", name='" + name + '\'' +
               ", city='" + city + '\'' +
               '}';
    }
}

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

public class RegistryService {

    // 假设 registryRepository 是一个JPA Repository实例
    // private RegistryRepository registryRepository; 

    public Map<String, List<String>> findAllUsersStream() {
        // 模拟从Repository获取数据
        List<RegistryEntity> items = List.of(
            new RegistryEntity(1L, "John", "New York"),
            new RegistryEntity(2L, "Paul", "Atlanta"),
            new RegistryEntity(3L, "Mark", "Los Angeles"),
            new RegistryEntity(4L, "Susan", "Los Angeles"),
            new RegistryEntity(5L, "Josh", "New York"),
            new RegistryEntity(6L, "Charles", "Atlanta")
        );
        // 实际应用中：
        // List<RegistryEntity> items = registryRepository.findAll();

        Map<String, List<String>> itemsGrouped =
            items.stream()
                 .collect(Collectors.groupingBy(
                     RegistryEntity::getCity, // 分组函数：按城市分组
                     Collectors.mapping(RegistryEntity::getName, Collectors.toList()) // 下游收集器：将每个组内的实体映射为姓名，并收集到列表中
                 ));

        return itemsGrouped;
    }

    public static void main(String[] args) {
        RegistryService service = new RegistryService();
        Map<String, List<String>> result = service.findAllUsersStream();
        System.out.println(result);
        // 预期输出: {New York=[John, Josh], Atlanta=[Paul, Charles], Los Angeles=[Mark, Susan]}
    }
}