java代码：学生选课系统至少应包括以下信息： 学生信息：学号、姓名、性别、年级、密码 课程信息：课程编号、课程名称、已选人数、可选人数、授课教师。 (1) 文件CourseDB.java：单例类，模拟数据库中的课程，初始化后至少封装10门课程； （2）文件StudentDB.java：单列类，模拟数据库中的课程，初始化后至少封装5名学生信息； （3）文件Login.java:负责登录的业务逻辑； （4）文件SelectCourse.java：负责选课的业务逻辑； （5）文件MainProcess.java：负责整体业务流程； （6）文件Course.java: 创建课程类； （7）文件Student.java：创建学生类； （8）学生选课成功后，选课状态变更为“已选”，否则，课程状态变更为“可选”；学生选择“已选”课程，提示“该课程无法重复选择”； （9）学生选课成功后，选课人数相应发生变化；

好的，下面是一个简单的示例代码，实现了学生选课系统的基本功能：

Course.java：

public class Course {
    private String id;
    private String name;
    private int selectedNum;
    private int totalNum;
    private String teacher;

    public Course(String id, String name, int totalNum, String teacher) {
        this.id = id;
        this.name = name;
        this.totalNum = totalNum;
        this.teacher = teacher;
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getSelectedNum() {
        return selectedNum;
    }

    public int getTotalNum() {
        return totalNum;
    }

    public String getTeacher() {
        return teacher;
    }

    public boolean select() {
        if (selectedNum < totalNum) {
            selectedNum++;
            return true;
        } else {
            return false;
        }
    }

    public boolean cancel() {
        if (selectedNum > 0) {
            selectedNum--;
            return true;
        } else {
            return false;
        }
    }

    public String getStatus() {
        if (selectedNum == totalNum) {
            return "已满";
        } else {
            return "可选";
        }
    }
}

Student.java：

public class Student {
    private String id;
    private String name;
    private String gender;
    private String grade;
    private String password;

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

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getGender() {
        return gender;
    }

    public String getGrade() {
        return grade;
    }

    public boolean checkPassword(String password) {
        return this.password.equals(password);
    }
}

CourseDB.java：

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

public class CourseDB {
    private static CourseDB instance = new CourseDB();
    private List<Course> courseList;

    private CourseDB() {
        courseList = new ArrayList<>();
        courseList.add(new Course("001", "Java基础", 50, "张三"));
        courseList.add(new Course("002", "Web开发", 50, "李四"));
        courseList.add(new Course("003", "Python基础", 50, "王五"));
        courseList.add(new Course("004", "算法设计与分析", 50, "赵六"));
        courseList.add(new Course("005", "数据库系统原理", 50, "钱七"));
        courseList.add(new Course("006", "计算机组成原理", 50, "孙八"));
        courseList.add(new Course("007", "计算机网络", 50, "周九"));
        courseList.add(new Course("008", "操作系统原理", 50, "吴十"));
        courseList.add(new Course("009", "机器学习", 50, "张三"));
        courseList.add(new Course("010", "深度学习", 50, "李四"));
    }

    public static CourseDB getInstance() {
        return instance;
    }

    public List<Course> getCourseList() {
        return courseList;
    }

    public Course getCourseById(String courseId) {
        for (Course course : courseList) {
            if (course.getId().equals(courseId)) {
                return course;
            }
        }
        return null;
    }
}

StudentDB.java：

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

public class StudentDB {
    private static StudentDB instance = new StudentDB();
    private List<Student> studentList;

    private StudentDB() {
        studentList = new ArrayList<>();
        studentList.add(new Student("001", "张三", "男", "大一", "123456"));
        studentList.add(new Student("002", "李四", "女", "大一", "123456"));
        studentList.add(new Student("003", "王五", "男", "大二", "123456"));
        studentList.add(new Student("004", "赵六", "女", "大二", "123456"));
        studentList.add(new Student("005", "钱七", "男", "大三", "123456"));
    }

    public static StudentDB getInstance() {
        return instance;
    }

    public List<Student> getStudentList() {
        return studentList;
    }

    public Student getStudentById(String studentId) {
        for (Student student : studentList) {
            if (student.getId().equals(studentId)) {
                return student;
            }
        }
        return null;
    }
}

Login.java：

import java.util.Scanner;

public class Login {
    private static Scanner scanner = new Scanner(System.in);

    public static Student login() {
        System.out.println("请输入学号：");
        String studentId = scanner.nextLine();
        System.out.println("请输入密码：");
        String password = scanner.nextLine();
        StudentDB studentDB = StudentDB.getInstance();
        Student student = studentDB.getStudentById(studentId);
        if (student != null && student.checkPassword(password)) {
            System.out.println("欢迎您，" + student.getName() + "！");
            return student;
        } else {
            System.out.println("学号或密码错误！");
            return null;
        }
    }
}

SelectCourse.java：

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

public class SelectCourse {
    private static Scanner scanner = new Scanner(System.in);

    public static void select(Student student) {
        CourseDB courseDB = CourseDB.getInstance();
        List<Course> courseList = courseDB.getCourseList();
        System.out.println("可选课程列表：");
        for (Course course : courseList) {
            System.out.println(course.getId() + "\t" + course.getName() + "\t" + course.getTeacher() + "\t" + course.getStatus());
        }
        System.out.println("请输入课程编号：");
        String courseId = scanner.nextLine();
        Course course = courseDB.getCourseById(courseId);
        if (course == null) {
            System.out.println("课程编号不存在！");
        } else {
            if (course.select()) {
                System.out.println("选课成功！");
                studentSelected(student, course);
            } else {
                System.out.println("选课失败，该课程已满！");
            }
        }
    }

    public static void cancel(Student student) {
        CourseDB courseDB = CourseDB.getInstance();
        List<Course> courseList = courseDB.getCourseList();
        System.out.println("已选课程列表：");
        for (Course course : courseList) {
            if (course.getSelectedNum() > 0) {
                System.out.println(course.getId() + "\t" + course.getName() + "\t" + course.getTeacher());
            }
        }
        System.out.println("请输入课程编号：");
        String courseId = scanner.nextLine();
        Course course = courseDB.getCourseById(courseId);
        if (course == null) {
            System.out.println("课程编号不存在！");
        } else {
            if (course.cancel()) {
                System.out.println("退课成功！");
                studentCanceled(student, course);
            } else {
                System.out.println("退课失败，您未选择该课程！");
            }
        }
    }

    private static void studentSelected(Student student, Course course) {
        student.setSelected(course.getId());
        System.out.println("选课状态已更新！");
    }

    private static void studentCanceled(Student student, Course course) {
        student.setCanceled(course.getId());
        System.out.println("退课状态已更新！");
    }
}

MainProcess.java：

public class MainProcess {
    private static Student currentStudent = null;

    public static void main(String[] args) {
        while (true) {
            System.out.println("1.登录 2.选课 3.退课 4.退出");
            String choice = Login.scanner.nextLine();
            switch (choice) {
                case "1":
                    currentStudent = Login.login();
                    break;
                case "2":
                    if (currentStudent == null) {
                        System.out.println("请先登录！");
                    } else {
                        SelectCourse.select(currentStudent);
                    }
                    break;
                case "3":
                    if (currentStudent == null) {
                        System.out.println("请先登录！");
                    } else {
                        SelectCourse.cancel(currentStudent);
                    }
                    break;
                case "4":
                    System.exit(0);
                default:
                    System.out.println("无效的选项！");
            }
        }
    }
}

注意：以上代码仅作为示例代码，可能存在不完善或错误之处，仅供学习参考。