用Java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实现（Java语言）：

Course类：

public class Course {
    private String courseId;
    private String courseName;
    private int credit;
    private int maxNum;
    private int currentNum;

    public Course(String courseId, String courseName, int credit, int maxNum) {
        this.courseId = courseId;
        this.courseName = courseName;
        this.credit = credit;
        this.maxNum = maxNum;
        this.currentNum = 0;
    }

    public String getCourseId() {
        return courseId;
    }

    public void setCourseId(String courseId) {
        this.courseId = courseId;
    }

    public String getCourseName() {
        return courseName;
    }

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

    public int getCredit() {
        return credit;
    }

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

    public int getMaxNum() {
        return maxNum;
    }

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

    public int getCurrentNum() {
        return currentNum;
    }

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

    public boolean isFull() {
        return currentNum >= maxNum;
    }

    @Override
    public String toString() {
        return "Course{" +
                "courseId='" + courseId + '\'' +
                ", courseName='" + courseName + '\'' +
                ", credit=" + credit +
                ", maxNum=" + maxNum +
                ", currentNum=" + currentNum +
                '}';
    }
}

Student类：

public class Student {
    private String studentId;
    private String studentName;
    private String major;

    public Student(String studentId, String studentName, String major) {
        this.studentId = studentId;
        this.studentName = studentName;
        this.major = major;
    }

    public String getStudentId() {
        return studentId;
    }

    public void setStudentId(String studentId) {
        this.studentId = studentId;
    }

    public String getStudentName() {
        return studentName;
    }

    public void setStudentName(String studentName) {
        this.studentName = studentName;
    }

    public String getMajor() {
        return major;
    }

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

    @Override
    public String toString() {
        return "Student{" +
                "studentId='" + studentId + '\'' +
                ", studentName='" + studentName + '\'' +
                ", major='" + major + '\'' +
                '}';
    }
}

选课结果类：

public class Selection {
    private Course course;
    private Student student;

    public Selection(Course course, Student student) {
        this.course = course;
        this.student = student;
    }

    public Course getCourse() {
        return course;
    }

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

    public Student getStudent() {
        return student;
    }

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

    @Override
    public String toString() {
        return "Selection{" +
                "course=" + course +
                ", student=" + student +
                '}';
    }
}

测试类：

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

public class Main {
    public static void main(String[] args) {
        List<Course> courses = new ArrayList<>();
        List<Student> students = new ArrayList<>();
        List<Selection> selections = new ArrayList<>();

        Scanner scanner = new Scanner(System.in);

        System.out.print("请输入课程最多人数：");
        int maxNum = scanner.nextInt();

        Course course1 = new Course("001", "Java程序设计", 4, maxNum);
        Course course2 = new Course("002", "数据结构与算法", 3, maxNum);
        courses.add(course1);
        courses.add(course2);

        Student student1 = new Student("001", "张三", "计算机科学与技术");
        Student student2 = new Student("002", "李四", "软件工程");
        students.add(student1);
        students.add(student2);

        while (true) {
            System.out.println("请选择操作：1.添加课程 2.删除课程 3.修改课程 4.查询课程 5.添加学生 6.删除学生 7.修改学生 8.查询学生 9.选课 10.退选 11.退出");
            int choice = scanner.nextInt();
            switch (choice) {
                case 1:
                    System.out.print("请输入课程编号：");
                    String courseId = scanner.next();
                    System.out.print("请输入课程名称：");
                    String courseName = scanner.next();
                    System.out.print("请输入课程学分：");
                    int credit = scanner.nextInt();
                    Course course = new Course(courseId, courseName, credit, maxNum);
                    courses.add(course);
                    break;
                case 2:
                    System.out.print("请输入要删除的课程编号：");
                    String courseIdToDelete = scanner.next();
                    Course courseToDelete = null;
                    for (Course c : courses) {
                        if (c.getCourseId().equals(courseIdToDelete)) {
                            courseToDelete = c;
                            break;
                        }
                    }
                    if (courseToDelete != null) {
                        courses.remove(courseToDelete);
                        System.out.println("删除成功！");
                    } else {
                        System.out.println("未找到要删除的课程！");
                    }
                    break;
                case 3:
                    System.out.print("请输入要修改的课程编号：");
                    String courseIdToModify = scanner.next();
                    Course courseToModify = null;
                    for (Course c : courses) {
                        if (c.getCourseId().equals(courseIdToModify)) {
                            courseToModify = c;
                            break;
                        }
                    }
                    if (courseToModify != null) {
                        System.out.println("请输入修改后的课程名称（不修改请直接回车）：");
                        String newCourseName = scanner.next();
                        if (!newCourseName.equals("")) {
                            courseToModify.setCourseName(newCourseName);
                        }
                        System.out.println("请输入修改后的课程学分（不修改请直接回车）：");
                        String newCreditString = scanner.next();
                        if (!newCreditString.equals("")) {
                            int newCredit = Integer.parseInt(newCreditString);
                            courseToModify.setCredit(newCredit);
                        }
                        System.out.println("课程修改成功！");
                    } else {
                        System.out.println("未找到要修改的课程！");
                    }
                    break;
                case 4:
                    System.out.println("课程列表：");
                    for (Course c : courses) {
                        System.out.println(c);
                    }
                    break;
                case 5:
                    System.out.print("请输入学生学号：");
                    String studentId = scanner.next();
                    System.out.print("请输入学生姓名：");
                    String studentName = scanner.next();
                    System.out.print("请输入学生专业：");
                    String major = scanner.next();
                    Student student = new Student(studentId, studentName, major);
                    students.add(student);
                    break;
                case 6:
                    System.out.print("请输入要删除的学生学号：");
                    String studentIdToDelete = scanner.next();
                    Student studentToDelete = null;
                    for (Student s : students) {
                        if (s.getStudentId().equals(studentIdToDelete)) {
                            studentToDelete = s;
                            break;
                        }
                    }
                    if (studentToDelete != null) {
                        students.remove(studentToDelete);
                        System.out.println("删除成功！");
                    } else {
                        System.out.println("未找到要删除的学生！");
                    }
                    break;
                case 7:
                    System.out.print("请输入要修改的学生学号：");
                    String studentIdToModify = scanner.next();
                    Student studentToModify = null;
                    for (Student s : students) {
                        if (s.getStudentId().equals(studentIdToModify)) {
                            studentToModify = s;
                            break;
                        }
                    }
                    if (studentToModify != null) {
                        System.out.println("请输入修改后的学生姓名（不修改请直接回车）：");
                        String newStudentName = scanner.next();
                        if (!newStudentName.equals("")) {
                            studentToModify.setStudentName(newStudentName);
                        }
                        System.out.println("请输入修改后的学生专业（不修改请直接回车）：");
                        String newMajor = scanner.next();
                        if (!newMajor.equals("")) {
                            studentToModify.setMajor(newMajor);
                        }
                        System.out.println("学生修改成功！");
                    } else {
                        System.out.println("未找到要修改的学生！");
                    }
                    break;
                case 8:
                    System.out.println("学生列表：");
                    for (Student s : students) {
                        System.out.println(s);
                    }
                    break;
                case 9:
                    System.out.print("请输入要选课的学生学号：");
                    String studentIdToSelect = scanner.next();
                    Student studentToSelect = null;
                    for (Student s : students) {
                        if (s.getStudentId().equals(studentIdToSelect)) {
                            studentToSelect = s;
                            break;
                        }
                    }
                    if (studentToSelect == null) {
                        System.out.println("未找到要选课的学生！");
                        break;
                    }

                    System.out.println("可选课程列表：");
                    List<Course> availableCourses = new ArrayList<>();
                    for (Course c : courses) {
                        if (!c.isFull()) {
                            System.out.println(c);
                            availableCourses.add(c);
                        }
                    }
                    if (availableCourses.isEmpty()) {
                        System.out.println("当前没有可选课程！");
                        break;
                    }

                    System.out.print("请输入要选的课程编号：");
                    String courseIdToSelect = scanner.next();
                    Course courseToSelect = null;
                    for (Course c : availableCourses) {
                        if (c.getCourseId().equals(courseIdToSelect)) {
                            courseToSelect = c;
                            break;
                        }
                    }
                    if (courseToSelect == null) {
                        System.out.println("未找到要选的课程！");
                        break;
                    }

                    Selection selection = new Selection(courseToSelect, studentToSelect);
                    selections.add(selection);
                    courseToSelect.setCurrentNum(courseToSelect.getCurrentNum() + 1);
                    System.out.println("选课成功！");
                    break;
                case 10:
                    System.out.print("请输入要退选的学生学号：");
                    String studentIdToDrop = scanner.next();
                    System.out.print("请输入要退选的课程编号：");
                    String courseIdToDrop = scanner.next();
                    Selection selectionToDrop = null;
                    for (Selection s : selections) {
                        if (s.getStudent().getStudentId().equals(studentIdToDrop) && s.getCourse().getCourseId().equals(courseIdToDrop)) {
                            selectionToDrop = s;
                            break;
                        }
                    }
                    if (selectionToDrop == null) {
                        System.out.println("未找到要退选的选课记录！");
                        break;
                    }
                    selections.remove(selectionToDrop);
                    selectionToDrop.getCourse().setCurrentNum(selectionToDrop.getCourse().getCurrentNum() - 1);
                    System.out.println("退选成功！");
                    break;
                case 11:
                    System.out.println("谢谢使用！");
                    return;
                default:
                    System.out.println("输入不合法，请重新输入！");
                    break;
            }
        }
    }
}

以上是一个简单的学生选课系统的实现，可以完成添加、删除、修改、查询相关信息。