教务系统来说，选课是其所必备的功能，请实现一个选课模块，要求： 1、选择合适的集合 2、能够查询课程以及剩余的数量 3、能够查询学生所选的课，也可以通过课反向查询课下面的学生（显示学号+姓名） 4、选课需要记录具体学生。用java的hashmap完成

好的，下面是一个简单的选课模块的Java实现，使用HashMap来存储课程和学生的信息：

import java.util.*;

public class CourseSelection {
    // 用HashMap来存储课程和学生信息
    private Map<String, Course> courses = new HashMap<>();
    private Map<String, Student> students = new HashMap<>();

    // 添加课程
    public void addCourse(String courseName, int capacity) {
        Course course = new Course(courseName, capacity);
        courses.put(courseName, course);
    }

    // 添加学生
    public void addStudent(String studentId, String name) {
        Student student = new Student(studentId, name);
        students.put(studentId, student);
    }

    // 选课
    public boolean selectCourse(String studentId, String courseName) {
        Student student = students.get(studentId);
        Course course = courses.get(courseName);

        if (student == null || course == null) {
            return false;
        }

        if (student.getCourses().contains(course)) {
            return true; // 已选过该课程
        }

        if (course.getRemaining() <= 0) {
            return false; // 课程已满
        }

        student.addCourse(course);
        course.addStudent(student);
        return true;
    }

    // 查询课程信息
    public void printCourseInfo(String courseName) {
        Course course = courses.get(courseName);
        if (course == null) {
            System.out.println("该课程不存在！");
            return;
        }
        System.out.println("课程名称：" + course.getName());
        System.out.println("课程容量：" + course.getCapacity());
        System.out.println("剩余容量：" + course.getRemaining());
    }

    // 查询学生选课信息
    public void printStudentInfo(String studentId) {
        Student student = students.get(studentId);
        if (student == null) {
            System.out.println("该学生不存在！");
            return;
        }
        System.out.println("学生姓名：" + student.getName());
        System.out.println("学生编号：" + student.getId());
        System.out.println("所选课程：");
        List<Course> courses = student.getCourses();
        for (Course course : courses) {
            System.out.println(course.getName());
        }
    }

    // 查询某课程下的学生信息
    public void printCourseStudents(String courseName) {
        Course course = courses.get(courseName);
        if (course == null) {
            System.out.println("该课程不存在！");
            return;
        }
        System.out.println("课程名称：" + course.getName());
        System.out.println("选课学生：");
        List<Student> students = course.getStudents();
        for (Student student : students) {
            System.out.println(student.getId() + " " + student.getName());
        }
    }

    public static void main(String[] args) {
        CourseSelection selection = new CourseSelection();
        selection.addCourse("Java程序设计", 50);
        selection.addCourse("数据结构", 40);
        selection.addCourse("计算机组成原理", 30);
        selection.addStudent("10001", "张三");
        selection.addStudent("10002", "李四");
        selection.addStudent("10003", "王五");
        selection.selectCourse("10001", "Java程序设计");
        selection.selectCourse("10001", "数据结构");
        selection.selectCourse("10002", "Java程序设计");
        selection.selectCourse("10003", "Java程序设计");
        selection.printCourseInfo("Java程序设计");
        selection.printStudentInfo("10001");
        selection.printCourseStudents("Java程序设计");
    }
}

// 课程类
class Course {
    private String name;
    private int capacity;
    private List<Student> students = new ArrayList<>();

    public Course(String name, int capacity) {
        this.name = name;
        this.capacity = capacity;
    }

    public String getName() {
        return name;
    }

    public int getCapacity() {
        return capacity;
    }

    public int getRemaining() {
        return capacity - students.size();
    }

    public List<Student> getStudents() {
        return students;
    }

    public void addStudent(Student student) {
        students.add(student);
    }
}

// 学生类
class Student {
    private String id;
    private String name;
    private List<Course> courses = new ArrayList<>();

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

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public List<Course> getCourses() {
        return courses;
    }

    public void addCourse(Course course) {
        courses.add(course);
    }
}

以上代码演示了一个简单的选课系统，可以添加课程和学生信息，选课，并查询课程和学生的选课信息。其中，课程和学生信息以HashMap进行存储，选课时需要对课程和学生的存在性进行判断，同时需要记录学生选课信息和课程选课学生信息。