Java Stream 进阶：优雅地移除重复对象并保留最新记录

package org.example;

import java.time.LocalDateTime;
import java.util.Objects; // 导入 Objects 类

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

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    public LocalDateTime getStartDatetime() {
        return startDatetime;
    }

    public void setStartDatetime(LocalDateTime startDatetime) {
        this.startDatetime = startDatetime;
    }

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

    @Override
    public String toString() {
        return "Student{id='" + id + "', startDatetime=" + startDatetime + '}';
    }

    // 建议重写 equals 和 hashCode，尽管本例中不是必需的，但对于集合操作是良好实践
    @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);
    }
}

List<Student> students = List.of(
    new Student("1", LocalDateTime.now()),
    new Student("1", LocalDateTime.of(2000, 2, 1, 1, 1)),
    new Student("1", LocalDateTime.of(1990, 2, 1, 1, 1)),
    new Student("2", LocalDateTime.of(1990, 2, 1, 1, 1))
);

public static <T, K, U> Collector<T, ?, Map<K, U>> toMap(
    Function<? super T, ? extends K> keyMapper,
    Function<? super T, ? extends U> valueMapper,
    BinaryOperator<U> mergeFunction
)

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

public class Main {

    // Student 类定义（与上面保持一致）
    public static class Student {
        private String id;
        private LocalDateTime startDatetime;

        public String getId() {
            return id;
        }

        public void setId(String id) {
            this.id = id;
        }

        public LocalDateTime getStartDatetime() {
            return startDatetime;
        }

        public void setStartDatetime(LocalDateTime startDatetime) {
            this.startDatetime = startDatetime;
        }

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

        @Override
        public String toString() {
            return "Student{id='" + id + "', startDatetime=" + startDatetime + '}';
        }
    }

    public static void main(String[] args) {
        // 原始学生列表
        List<Student> students = new ArrayList<>() {{
            add(new Student("1", LocalDateTime.now())); // 最新的ID为1的记录
            add(new Student("1", LocalDateTime.of(2000, 2, 1, 1, 1)));
            add(new Student("1", LocalDateTime.of(1990, 2, 1, 1, 1)));
            add(new Student("2", LocalDateTime.of(1990, 2, 1, 1, 1)));
            add(new Student("3", LocalDateTime.of(2020, 1, 1, 0, 0))); // 新增一个不重复的记录
            add(new Student("3", LocalDateTime.of(2019, 1, 1, 0, 0))); // 较旧的ID为3的记录
        }};

        System.out.println("原始学生列表:");
        students.forEach(System.out::println);
        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作为键
                Function.identity(),                                    // valueMapper: 保留原始Student对象作为值
                BinaryOperator.maxBy(Comparator.comparing(Student::getStartDatetime)) // mergeFunction: 冲突时保留startDatetime最新的
            ))
            .values()                                                   // 获取Map中所有的值（去重后的Student对象）
            .stream()                                                   // 将值集合转换为新的Stream
            .sorted(Comparator.comparing(Student::getStartDatetime))    // 可选：根据startDatetime排序结果
            .toList(); // Java 16+ 新特性，等价于 .collect(Collectors.toList())

        System.out.println("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=2023-10-27T10:30:45.123456}
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=2020-01-01T00:00}
Student{id='3', startDatetime=2019-01-01T00:00}
--------------------
去重并保留最新记录后的学生列表:
Student{id='2', startDatetime=1990-02-01T01:01}
Student{id='1', startDatetime=2023-10-27T10:30:45.123456} // 此处的日期时间会是运行时的当前时间
Student{id='3', startDatetime=2020-01-01T00:00}