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；

好的，这是一个选课系统的代码设计，下面是一些代码示例：

1. CourseDB.java

public class CourseDB {
    private static CourseDB instance;

    private List<Course> courseList;

    private CourseDB() {
        courseList = new ArrayList<>();
        //初始化至少封装10门课程
        courseList.add(new Course("001", "Java程序设计", 0, 50, "张三"));
        courseList.add(new Course("002", "数据库原理", 0, 40, "李四"));
        courseList.add(new Course("003", "网络编程", 0, 30, "王五"));
        //...
    }

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

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

2. StudentDB.java

public class StudentDB {
    private static StudentDB instance;

    private List<Student> studentList;

    private StudentDB() {
        studentList = new ArrayList<>();
        //初始化至少封装5名学生信息
        studentList.add(new Student("001", "张三", "男", "大一", "123456"));
        studentList.add(new Student("002", "李四", "女", "大二", "234567"));
        //...
    }

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

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

3. Login.java

public class Login {
    private StudentDB studentDB;

    public Login(StudentDB studentDB) {
        this.studentDB = studentDB;
    }

    public boolean login(String studentId, String password) {
        List<Student> studentList = studentDB.getStudentList();
        for (Student student : studentList) {
            if (studentId.equals(student.getId()) && password.equals(student.getPassword())) {
                return true;
            }
        }
        return false;
    }
}

4. SelectCourse.java

public class SelectCourse {
    private CourseDB courseDB;
    private StudentDB studentDB;

    public SelectCourse(CourseDB courseDB, StudentDB studentDB) {
        this.courseDB = courseDB;
        this.studentDB = studentDB;
    }

    public boolean selectCourse(String studentId, String courseId) {
        List<Student> studentList = studentDB.getStudentList();
        List<Course> courseList = courseDB.getCourseList();
        Student student = null;
        Course course = null;
        for (Student s : studentList) {
            if (s.getId().equals(studentId)) {
                student = s;
                break;
            }
        }
        if (student == null) {
            return false;
        }
        for (Course c : courseList) {
            if (c.getId().equals(courseId)) {
                course = c;
                break;
            }
        }
        if (course == null) {
            return false;
        }
        if (student.getSelectedCourseList().contains(course)) {
            //已选课程，无法重复选择
            return false;
        }
        if (course.getSelectedNum() >= course.getTotalNum()) {
            //该课程已满
            return false;
        }
        //选课成功
        student.getSelectedCourseList().add(course);
        course.setSelectedNum(course.getSelectedNum() + 1);
        return true;
    }
}

5. MainProcess.java

public class MainProcess {
    private Login login;
    private SelectCourse selectCourse;

    public MainProcess(Login login, SelectCourse selectCourse) {
        this.login = login;
        this.selectCourse = selectCourse;
    }

    public void process() {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入学号：");
        String studentId = scanner.nextLine();
        System.out.println("请输入密码：");
        String password = scanner.nextLine();
        if (login.login(studentId, password)) {
            System.out.println("登录成功");
            System.out.println("请选择课程编号：");
            String courseId = scanner.nextLine();
            if (selectCourse.selectCourse(studentId, courseId)) {
                System.out.println("选课成功");
            } else {
                System.out.println("选课失败");
            }
        } else {
            System.out.println("登录失败");
        }
    }
}

这些代码实现了一个简单的选课系统，当然，还有很多细节和功能需要完善，但是这里提供的代码可以作为一个基础框架，供大家参考。