java代码：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）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”； （7）学生选课成功后，选课人数相应发生变化；

以下是一个简单的 Java 代码示例，实现了上述要求：

CourseDB.java

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

public class CourseDB {
    private static CourseDB instance = null;
    private List<Course> courses;

    private CourseDB() {
        courses = new ArrayList<>();
        // 初始化课程信息
        courses.add(new Course(1, "Java程序设计", 0, 50, "张三"));
        courses.add(new Course(2, "数据结构", 0, 60, "李四"));
        courses.add(new Course(3, "计算机网络", 0, 40, "王五"));
        courses.add(new Course(4, "数据库原理", 0, 45, "赵六"));
        courses.add(new Course(5, "操作系统", 0, 55, "钱七"));
        courses.add(new Course(6, "软件工程", 0, 30, "刘八"));
        courses.add(new Course(7, "人工智能", 0, 35, "孙九"));
        courses.add(new Course(8, "计算机组成原理", 0, 40, "周十"));
        courses.add(new Course(9, "编译原理", 0, 25, "吴十一"));
        courses.add(new Course(10, "离散数学", 0, 30, "郑十二"));
    }

    public static CourseDB getInstance() {
        if (instance == null) {
            instance = new CourseDB();
        }
        return instance;
    }

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

StudentDB.java

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

public class StudentDB {
    private static StudentDB instance = null;
    private List<Student> students;

    private StudentDB() {
        students = new ArrayList<>();
        // 初始化学生信息
        students.add(new Student(1001, "张三", "男", 1, "123456"));
        students.add(new Student(1002, "李四", "女", 2, "123456"));
        students.add(new Student(1003, "王五", "男", 3, "123456"));
        students.add(new Student(1004, "赵六", "女", 2, "123456"));
        students.add(new Student(1005, "钱七", "男", 1, "123456"));
    }

    public static StudentDB getInstance() {
        if (instance == null) {
            instance = new StudentDB();
        }
        return instance;
    }

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

Login.java

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

public class Login {
    public static Student doLogin() {
        Scanner in = new Scanner(System.in);
        StudentDB studentDB = StudentDB.getInstance();
        List<Student> students = studentDB.getStudents();
        while (true) {
            System.out.print("请输入学号：");
            int studentId = in.nextInt();
            in.nextLine();
            System.out.print("请输入密码：");
            String password = in.nextLine();

            for (Student student : students) {
                if (student.getStudentId() == studentId && student.getPassword().equals(password)) {
                    System.out.println("登陆成功，欢迎 " + student.getName() + " 同学！");
                    return student;
                }
            }

            System.out.println("学号或密码错误，请重新输入！");
        }
    }
}

SelectCourse.java

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

public class SelectCourse {
    public static void doSelectCourse(Student student) {
        Scanner in = new Scanner(System.in);
        CourseDB courseDB = CourseDB.getInstance();
        List<Course> courses = courseDB.getCourses();
        System.out.println("可选课程列表：");
        for (Course course : courses) {
            if (course.getTotalNum() > course.getSelectedNum()) { // 可选
                System.out.println(course.getCourseId() + "\t" + course.getCourseName() + "\t" + course.getTeacher());
            }
        }
        while (true) {
            System.out.print("请选择要选的课程编号（输入0退出）：");
            int courseId = in.nextInt();
            in.nextLine();
            if (courseId == 0) {
                return;
            }
            for (Course course : courses) {
                if (course.getCourseId() == courseId) {
                    // 检查是否已选
                    if (student.getCourseIds().contains(courseId)) {
                        System.out.println("该课程已经选过了！");
                        break;
                    }
                    // 检查是否可选
                    if (course.getTotalNum() <= course.getSelectedNum()) {
                        System.out.println("该课程已满！");
                        break;
                    }
                    // 选课成功
                    student.getCourseIds().add(courseId);
                    course.setSelectedNum(course.getSelectedNum() + 1);
                    System.out.println("选课成功！");
                    return;
                }
            }
            System.out.println("课程编号不存在，请重新输入！");
        }
    }
}

MainProcess.java

public class MainProcess {
    public static void main(String[] args) {
        Student student = Login.doLogin();
        while (true) {
            System.out.println("请选择操作：");
            System.out.println("1. 选课");
            System.out.println("2. 查看已选课程");
            System.out.println("0. 退出");
            Scanner in = new Scanner(System.in);
            int choice = in.nextInt();
            switch (choice) {
                case 1:
                    SelectCourse.doSelectCourse(student);
                    break;
                case 2:
                    System.out.println("已选课程列表：");
                    for (int courseId : student.getCourseIds()) {
                        Course course = CourseDB.getInstance().getCourses().get(courseId - 1);
                        System.out.println(course.getCourseId() + "\t" + course.getCourseName() + "\t" + course.getTeacher());
                    }
                    break;
                case 0:
                    System.out.println("退出系统！");
                    return;
                default:
                    System.out.println("输入错误，请重新输入！");
            }
        }
    }
}

注意：为了简化代码，这里假设每个学生只能选一门课程，如果需要支持多门课程选修，请调整代码。