用Java简单编写设计一个简单的学生选课系统，并可以完成添加、删除、修改、查询相关信息 。 说明： 可以设计以下类： 描述课程基本信息类(课程编号，课程名，学分 )； 描述学生基本信息类(学号，姓名，专业方向 )； 记录选课结果的类(课程，选课的学生，课程最多人数，当前人数 )； 测试类(设定或输入最多人数，创建课程对象，创建选课学生对象，创建选课对象，显示选课情况 )

好的，以下是一个简单的学生选课系统的Java代码：

Course.java

public class Course {
    private String courseID;
    private String courseName;
    private int credit;

    public Course(String courseID, String courseName, int credit) {
        this.courseID = courseID;
        this.courseName = courseName;
        this.credit = credit;
    }

    public String getCourseID() {
        return courseID;
    }

    public String getCourseName() {
        return courseName;
    }

    public int getCredit() {
        return credit;
    }

    public void setCourseID(String courseID) {
        this.courseID = courseID;
    }

    public void setCourseName(String courseName) {
        this.courseName = courseName;
    }

    public void setCredit(int credit) {
        this.credit = credit;
    }

    @Override
    public String toString() {
        return "课程编号：" + courseID + ", 课程名：" + courseName + ", 学分：" + credit;
    }
}

Student.java

public class Student {
    private String studentID;
    private String name;
    private String major;

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

    public String getStudentID() {
        return studentID;
    }

    public String getName() {
        return name;
    }

    public String getMajor() {
        return major;
    }

    public void setStudentID(String studentID) {
        this.studentID = studentID;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setMajor(String major) {
        this.major = major;
    }

    @Override
    public String toString() {
        return "学号：" + studentID + ", 姓名：" + name + ", 专业方向：" + major;
    }
}

CourseSelection.java

public class CourseSelection {
    private Course course;
    private Student student;
    private int maxNum;
    private int currentNum;

    public CourseSelection(Course course, Student student, int maxNum, int currentNum) {
        this.course = course;
        this.student = student;
        this.maxNum = maxNum;
        this.currentNum = currentNum;
    }

    public Course getCourse() {
        return course;
    }

    public Student getStudent() {
        return student;
    }

    public int getMaxNum() {
        return maxNum;
    }

    public int getCurrentNum() {
        return currentNum;
    }

    public void setCourse(Course course) {
        this.course = course;
    }

    public void setStudent(Student student) {
        this.student = student;
    }

    public void setMaxNum(int maxNum) {
        this.maxNum = maxNum;
    }

    public void setCurrentNum(int currentNum) {
        this.currentNum = currentNum;
    }

    @Override
    public String toString() {
        return course.toString() + ", " + student.toString() + ", 最大人数：" + maxNum + ", 当前人数：" + currentNum;
    }
}

Test.java

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Test {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("请输入课程最大人数：");
        int maxNum = scanner.nextInt();

        List<Course> courses = new ArrayList<>();
        courses.add(new Course("001", "Java编程", 4));
        courses.add(new Course("002", "Python编程", 3));
        courses.add(new Course("003", "数据结构", 4));
        courses.add(new Course("004", "操作系统", 3));

        List<Student> students = new ArrayList<>();
        students.add(new Student("1001", "张三", "计算机科学与技术"));
        students.add(new Student("1002", "李四", "软件工程"));
        students.add(new Student("1003", "王五", "信息安全"));

        List<CourseSelection> selections = new ArrayList<>();

        while (true) {
            System.out.println("请选择操作：");
            System.out.println("1. 添加选课信息");
            System.out.println("2. 删除选课信息");
            System.out.println("3. 修改选课信息");
            System.out.println("4. 查询选课信息");
            System.out.println("0. 退出程序");

            int choice = scanner.nextInt();
            switch (choice) {
                case 0:
                    System.out.println("程序已退出！");
                    return;
                case 1:
                    System.out.println("请输入课程编号：");
                    String courseID = scanner.next();
                    Course course = null;
                    for (Course c : courses) {
                        if (c.getCourseID().equals(courseID)) {
                            course = c;
                            break;
                        }
                    }
                    if (course == null) {
                        System.out.println("没有找到编号为" + courseID + "的课程！");
                        break;
                    }

                    System.out.println("请输入学生学号：");
                    String studentID = scanner.next();
                    Student student = null;
                    for (Student s : students) {
                        if (s.getStudentID().equals(studentID)) {
                            student = s;
                            break;
                        }
                    }
                    if (student == null) {
                        System.out.println("没有找到学号为" + studentID + "的学生！");
                        break;
                    }

                    CourseSelection selection = new CourseSelection(course, student, maxNum, 1);
                    selections.add(selection);
                    System.out.println("添加选课信息成功！");
                    break;
                case 2:
                    System.out.println("请输入课程编号：");
                    courseID = scanner.next();
                    studentID = scanner.next();
                    for (CourseSelection s : selections) {
                        if (s.getCourse().getCourseID().equals(courseID) && s.getStudent().getStudentID().equals(studentID)) {
                            selections.remove(s);
                            System.out.println("删除选课信息成功！");
                            break;
                        }
                    }
                    break;
                case 3:
                    System.out.println("请输入课程编号和学生学号：");
                    courseID = scanner.next();
                    studentID = scanner.next();
                    for (CourseSelection s : selections) {
                        if (s.getCourse().getCourseID().equals(courseID) && s.getStudent().getStudentID().equals(studentID)) {
                            System.out.println("请输入最大人数和当前人数：");
                            int newMaxNum = scanner.nextInt();
                            int newCurrentNum = scanner.nextInt();
                            s.setMaxNum(newMaxNum);
                            s.setCurrentNum(newCurrentNum);
                            System.out.println("修改选课信息成功！");
                            break;
                        }
                    }
                    break;
                case 4:
                    if (selections.size() == 0) {
                        System.out.println("没有任何选课信息！");
                        break;
                    }
                    System.out.println("课程编号\t课程名\t学分\t学生学号\t学生姓名\t专业方向\t最大人数\t当前人数");
                    for (CourseSelection s : selections) {
                        System.out.println(s.getCourse().getCourseID() + "\t" + s.getCourse().getCourseName() + "\t" + s.getCourse().getCredit() + "\t" +
                                s.getStudent().getStudentID() + "\t" + s.getStudent().getName() + "\t" + s.getStudent().getMajor() + "\t" +
                                s.getMaxNum() + "\t" + s.getCurrentNum());
                    }
                    break;
                default:
                    System.out.println("输入有误，请重新选择！");
                    break;
            }
        }
    }
}

这个简单的学生选课系统包含了课程、学生和选课结果三个类，以及一个测试类。在测试类中，我们创建了一些课程和学生的实例，并通过一个循环来执行添加、删除、修改和查询选课信息的操作。在添加选课信息时，我们需要输入课程编号和学生学号，系统会检查是否存在相应的课程和学生实例，如果存在，则创建一个选课结果的实例并添加到选课结果列表中。在删除和修改选课信息时，我们同样需要输入课程编号和学生学号，系统会遍历选课结果列表，找到对应的选课结果实例并执行删除或修改操作。在查询选课信息时，系统会输出所有的选课结果实例。