Java Streams：将嵌套Map扁平化并映射为DTO列表的实战指南

import java.time.LocalDate;
import java.time.format.DateTimeFormatter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.stream.Collectors;

// 定义Value枚举
public enum Value {
    VALUE1, VALUE2, VALUE3
}

// 源数据模型：Person类
class Person {
    private String id;
    private Value tag;
    private Object specificValue; // 可以是Integer, String, LocalDate等
    private LocalDate eventDate;
    private String message;

    public Person(String id, Value tag, Object specificValue, LocalDate eventDate, String message) {
        this.id = id;
        this.tag = tag;
        this.specificValue = specificValue;
        this.eventDate = eventDate;
        this.message = message;
    }

    // Getters
    public String getId() { return id; }
    public Value getTag() { return tag; }
    public Object getSpecificValue() { return specificValue; }
    public LocalDate getEventDate() { return eventDate; }
    public String getMessage() { return message; }

    @Override
    public String toString() {
        return "Person{" +
               "id='" + id + '\'' +
               ", tag=" + tag +
               ", specificValue=" + specificValue +
               ", eventDate=" + eventDate +
               ", message='" + message + '\'' +
               '}';
    }
}

// 目标数据模型：PersonDto类
class PersonDto {
    private Value tag;
    private String id;
    private String date; // 格式化后的日期字符串
    private Object result; // 对应Person的specificValue

    public PersonDto(Value tag, String id, String date, Object result) {
        this.tag = tag;
        this.id = id;
        this.date = date;
        this.result = result;
    }

    // Getters
    public Value getTag() { return tag; }
    public String getId() { return id; }
    public String getDate() { return date; }
    public Object getResult() { return result; }

    @Override
    public String toString() {
        return "PersonDto{" +
               "tag=" + tag +
               ", id='" + id + '\'' +
               ", date='" + date + '\'' +
               ", result=" + result +
               '}';
    }
}

public class StreamTransformationExample {

    // 定义日期格式化器
    private static final DateTimeFormatter formatter = DateTimeFormatter.ofPattern("dd-MM-yyyy");

    public static void main(String[] args) {
        // 1. 准备源数据
        Map<String, List<Person>> personsData = new HashMap<>();

        // 模拟数据
        LocalDate date1 = LocalDate.of(2000, 10, 10);
        LocalDate date2 = LocalDate.of(2000, 10, 10);
        LocalDate date3 = LocalDate.of(2000, 10, 10);
        LocalDate date4 = LocalDate.of(2000, 10, 10);
        LocalDate date5 = LocalDate.of(2000, 10, 10);
        LocalDate date6 = LocalDate.of(2000, 10, 10);


        personsData.put("p1", Arrays.asList(
                new Person("p1", Value.VALUE1, 10, date1, "Message"),
                new Person("p1", Value.VALUE2, "Text", date3, "Message"),
                new Person("p1", Value.VALUE3, LocalDate.of(2000, 11, 11), date5, "Message")
        ));
        personsData.put("p2", Arrays.asList(
                new Person("p2", Value.VALUE1, 20, date2, "Message"),
                new Person("p2", Value.VALUE2, "Text", date4, "Message"),
                new Person("p2", Value.VALUE3, LocalDate.of(2000, 11, 12), date6, "Message")
        ));

        System.out.println("--- 原始数据 ---");
        personsData.forEach((key, value) -> System.out.println(key + ": " + value));
        System.out.println("\n--- 转换后的DTO列表 ---");

        // 2. 使用Stream API进行转换
        List<PersonDto> resultDtos = personsData.values().stream()
                .flatMap(List::stream) // 扁平化Stream<List<Person>> 为 Stream<Person>
                .map(person -> new PersonDto(
                        person.getTag(),                     // PersonDto的tag
                        person.getId(),                      // PersonDto的id
                        formatter.format(person.getEventDate()), // 格式化日期为字符串
                        person.getSpecificValue()            // PersonDto的result (Object类型)
                ))
                .collect(Collectors.toList()); // 收集为List<PersonDto>

        // 3. 打印结果
        resultDtos.forEach(System.out::println);
    }
}

--- 原始数据 ---
p1: [Person{id='p1', tag=VALUE1, specificValue=10, eventDate=2000-10-10, message='Message'}, Person{id='p1', tag=VALUE2, specificValue=Text, eventDate=2000-10-10, message='Message'}, Person{id='p1', tag=VALUE3, specificValue=2000-11-11, eventDate=2000-10-10, message='Message'}]
p2: [Person{id='p2', tag=VALUE1, specificValue=20, eventDate=2000-10-10, message='Message'}, Person{id='p2', tag=VALUE2, specificValue=Text, eventDate=2000-10-10, message='Message'}, Person{id='p2', tag=VALUE3, specificValue=2000-11-12, eventDate=2000-10-10, message='Message'}]

--- 转换后的DTO列表 ---
PersonDto{tag=VALUE1, id='p1', date='10-10-2000', result=10}
PersonDto{tag=VALUE2, id='p1', date='10-10-2000', result=Text}
PersonDto{tag=VALUE3, id='p1', date='10-10-2000', result=2000-11-11}
PersonDto{tag=VALUE1, id='p2', date='10-10-2000', result=20}
PersonDto{tag=VALUE2, id='p2', date='10-10-2000', result=Text}
PersonDto{tag=VALUE3, id='p2', date='10-10-2000', result=2000-11-12}