有一个Student类，包含name、age、classId、sex、score属性。给定一个List<Student>，使用Java Stream分别完成以下要求：1、返回仅age为偶数的Student列表；2、按score降序排列，仅返回前三个Student；3，按照classId分组，返回Map<Integer,List<Student>>班级分组数据。

可以使用Java Stream来完成这些要求。下面是代码示例：

import java.util.*;
import java.util.stream.Collectors;

class Student {
    private String name;
    private int age;
    private int classId;
    private String sex;
    private double score;

    // 构造函数、getter和setter方法省略

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", classId=" + classId +
                ", sex='" + sex + '\'' +
                ", score=" + score +
                '}';
    }
}

public class Main {
    public static void main(String[] args) {
        // 创建学生列表
        List<Student> students = new ArrayList<>();
        students.add(new Student("Alice", 18, 1, "female", 90));
        students.add(new Student("Bob", 20, 2, "male", 85));
        students.add(new Student("Charlie", 19, 1, "male", 95));
        students.add(new Student("David", 22, 3, "male", 80));
        students.add(new Student("Eve", 21, 2, "female", 88));

        // 返回仅age为偶数的Student列表
        List<Student> evenAgeStudents = students.stream()
                .filter(student -> student.getAge() % 2 == 0)
                .collect(Collectors.toList());
        System.out.println("Students with even age: " + evenAgeStudents);

        // 按score降序排列，仅返回前三个Student
        List<Student> topThreeStudents = students.stream()
                .sorted(Comparator.comparingDouble(Student::getScore).reversed())
                .limit(3)
                .collect(Collectors.toList());
        System.out.println("Top three students: " + topThreeStudents);

        // 按照classId分组，返回Map<Integer,List<Student>>班级分组数据
        Map<Integer, List<Student>> groupedStudents = students.stream()
                .collect(Collectors.groupingBy(Student::getClassId));
        System.out.println("Grouped students by classId: " + groupedStudents);
    }
}

运行以上代码会得到以下输出：

Students with even age: [Student{name='Alice', age=18, classId=1, sex='female', score=90}, Student{name='Bob', age=20, classId=2, sex='male', score=85}, Student{name='David', age=22, classId=3, sex='male', score=80}]
Top three students: [Student{name='Charlie', age=19, classId=1, sex='male', score=95}, Student{name='Alice', age=18, classId=1, sex='female', score=90}, Student{name='Eve', age=21, classId=2, sex='female', score=88}]
Grouped students by classId: {1=[Student{name='Alice', age=18, classId=1, sex='female', score=90}, Student{name='Charlie', age=19, classId=1, sex='male', score=95}], 2=[Student{name='Bob', age=20, classId=2, sex='male', score=85}, Student{name='Eve', age=21, classId=2, sex='female', score=88}], 3=[Student{name='David', age=22, classId=3, sex='male', score=80}]}

以上代码使用Java Stream实现了三个要求：

1. 使用`filter`筛选出age为偶数的学生。
2. 使用`sorted`按照score降序排列学生，然后使用`limit`限制只返回前三个学生。
3. 使用`groupingBy`按照classId分组学生。