Java Stream API：将列表中的嵌套列表数据分组映射为Map

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.Date;
import java.util.List;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Trip {
    private Date startTime;
    private Date endTime;
    List<Employee> empList;
}

@Data
@NoArgsConstructor
@AllArgsConstructor
public class Employee {
    private String name;
    private String empId;
}

// 使用Java 16+ 的 record
public record TripEmployee(String empId, Trip trip) {}

// 对于Java 8-15，可以使用一个普通的类
/*
public class TripEmployee {
    private String empId;
    private Trip trip;

    public TripEmployee(String empId, Trip trip) {
        this.empId = empId;
        this.trip = trip;
    }

    public String getEmpId() { return empId; }
    public Trip getTrip() { return trip; }
    // 可以根据需要添加equals, hashCode, toString
}
*/

trips.stream()
    .flatMap(trip -> trip.getEmpList().stream() // 将每个Trip的empList转换为Stream<Employee>
        .map(emp -> new TripEmployee(emp.getEmpId(), trip)) // 将每个Employee映射为TripEmployee
    )
    // 此时流的类型为 Stream<TripEmployee>

    .collect(Collectors.groupingBy(
        TripEmployee::empId, // 根据empId进行分组
        Collectors.mapping(TripEmployee::trip, // 将TripEmployee映射为Trip
            Collectors.toList()) // 将映射后的Trip收集为List
    ));

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

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

// 假设 Trip 和 Employee 类已定义如上

// 辅助记录 (Java 16+)
public record TripEmployee(String empId, Trip trip) {}

public class TripGroupingExample {

    public static void main(String[] args) {
        // 示例数据
        Employee emp1 = new Employee("Alice", "E001");
        Employee emp2 = new Employee("Bob", "E002");
        Employee emp3 = new Employee("Charlie", "E003");

        Trip trip1 = new Trip(new Date(), new Date(), List.of(emp1, emp2));
        Trip trip2 = new Trip(new Date(), new Date(), List.of(emp1, emp3));
        Trip trip3 = new Trip(new Date(), new Date(), List.of(emp2));
        Trip trip4 = new Trip(new Date(), new Date(), List.of(emp3, emp1)); // 再次包含emp1

        List<Trip> trips = new ArrayList<>();
        trips.add(trip1);
        trips.add(trip2);
        trips.add(trip3);
        trips.add(trip4);

        // 使用Stream API生成Map<String, List<Trip>>
        Map<String, List<Trip>> empTripsMap = trips.stream()
            .flatMap(trip -> trip.getEmpList().stream() // 将每个Trip的empList扁平化为Stream<Employee>
                .map(emp -> new TripEmployee(emp.getEmpId(), trip)) // 将每个Employee映射为TripEmployee
            )
            .collect(Collectors.groupingBy(
                TripEmployee::empId, // 根据TripEmployee的empId进行分组
                Collectors.mapping(TripEmployee::trip, // 将TripEmployee映射回Trip
                    Collectors.toList()) // 将映射后的Trip收集为List
            ));

        // 打印结果
        empTripsMap.forEach((empId, tripList) -> {
            System.out.println("Employee ID: " + empId);
            tripList.forEach(trip -> System.out.println("  - Trip: " + trip));
            System.out.println("---");
        });

        /* 预期输出示例 (具体Trip对象内容取决于toString实现和日期)
        Employee ID: E001
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip1
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip2
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip4
        ---
        Employee ID: E002
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip1
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip3
        ---
        Employee ID: E003
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip2
          - Trip: Trip(startTime=..., endTime=..., empList=...) // trip4
        ---
        */
    }
}