使用Java Stream高效处理列表重复对象：按ID去重并保留最新记录

import java.time.LocalDateTime;
import java.util.Objects;

public class Student {
    private String id;
    private LocalDateTime startDatetime;

    public Student(String id, LocalDateTime startDatetime) {
        this.id = id;
        this.startDatetime = startDatetime;
    }

    public String getId() {
        return id;
    }

    public LocalDateTime getStartDatetime() {
        return startDatetime;
    }

    // 建议重写toString()方法，便于调试
    @Override
    public String toString() {
        return "Student{" +
               "id='" + id + '\'' +
               ", startDatetime=" + startDatetime +
               '}';
    }

    // 建议重写equals()和hashCode()，如果Student对象需要作为Set元素或Map的键
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return Objects.equals(id, student.id) && Objects.equals(startDatetime, student.startDatetime);
    }

    @Override
    public int hashCode() {
        return Objects.hash(id, startDatetime);
    }
}

import java.time.LocalDateTime;
import java.util.Comparator;
import java.util.List;
import java.util.function.Function;
import java.util.function.BinaryOperator;
import java.util.stream.Collectors;

public class StudentDeduplicator {

    public static void main(String[] args) {
        // 示例数据
        List<Student> students = List.of(
            new Student("1", LocalDateTime.of(2022, 11, 1, 14, 0)), // 最新
            new Student("1", LocalDateTime.of(2000, 2, 1, 1, 1)),   // 较旧
            new Student("1", LocalDateTime.of(1990, 2, 1, 1, 1)),   // 最旧
            new Student("2", LocalDateTime.of(1990, 2, 1, 1, 1)),   // 唯一
            new Student("3", LocalDateTime.of(2023, 1, 1, 10, 0)), // 最新
            new Student("3", LocalDateTime.of(2022, 1, 1, 10, 0))  // 较旧
        );

        System.out.println("原始学生列表：");
        students.forEach(System.out::println);

        // 使用Stream API去重并保留最新记录
        List<Student> uniqueStudents = students.stream()
            .collect(Collectors.toMap(
                Student::getId,                                 // keyMapper: 以ID作为Map的键
                Function.identity(),                            // valueMapper: 整个Student对象作为Map的值
                BinaryOperator.maxBy(Comparator.comparing(Student::getStartDatetime)) // mergeFunction: 当ID重复时，保留startDatetime更大的那个
            ))
            .values()                                           // 获取Map中所有去重后的Student对象
            .stream()                                           // 转换为新的Stream
            .sorted(Comparator.comparing(Student::getId))       // 可选：按ID排序，使结果有序
            .collect(Collectors.toList());                      // 收集为List (Java 16+ 可用 .toList())

        System.out.println("\n去重并保留最新记录后的学生列表：");
        uniqueStudents.forEach(System.out::println);
    }
}

原始学生列表：
Student{id='1', startDatetime=2022-11-01T14:00}
Student{id='1', startDatetime=2000-02-01T01:01}
Student{id='1', startDatetime=1990-02-01T01:01}
Student{id='2', startDatetime=1990-02-01T01:01}
Student{id='3', startDatetime=2023-01-01T10:00}
Student{id='3', startDatetime=2022-01-01T10:00}

去重并保留最新记录后的学生列表：
Student{id='1', startDatetime=2022-11-01T14:00}
Student{id='2', startDatetime=1990-02-01T01:01}
Student{id='3', startDatetime=2023-01-01T10:00}