3.其他功能需求： (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；代码

很抱歉，我的回答中没有包含代码，以下是一个可供参考的Java实现：

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<Course>();
        courses.add(new Course("CSE101", "Introduction to Computer Science", 50, 0));
        courses.add(new Course("CSE102", "Data Structures and Algorithms", 40, 0));
        courses.add(new Course("CSE201", "Operating Systems", 30, 0));
        courses.add(new Course("CSE202", "Database Systems", 20, 0));
        courses.add(new Course("CSE301", "Artificial Intelligence", 10, 0));
        courses.add(new Course("CSE302", "Computer Networks", 50, 0));
        courses.add(new Course("CSE401", "Software Engineering", 40, 0));
        courses.add(new Course("CSE402", "Computer Graphics", 30, 0));
        courses.add(new Course("CSE501", "Advanced Topics in Computer Science", 20, 0));
        courses.add(new Course("CSE502", "Parallel and Distributed Computing", 10, 0));
    }

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

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

    public Course getCourseById(String id) {
        for (Course c : courses) {
            if (c.getId().equals(id)) {
                return c;
            }
        }
        return null;
    }
}

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<Student>();
        students.add(new Student("S001", "John"));
        students.add(new Student("S002", "Mary"));
        students.add(new Student("S003", "Tom"));
        students.add(new Student("S004", "Kate"));
        students.add(new Student("S005", "Mike"));
    }

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

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

    public Student getStudentById(String id) {
        for (Student s : students) {
            if (s.getId().equals(id)) {
                return s;
            }
        }
        return null;
    }
}

Login.java:

import java.util.Scanner;

public class Login {
    private static final String USERNAME = "admin";
    private static final String PASSWORD = "123456";

    public boolean doLogin() {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your username: ");
        String username = scanner.nextLine();
        System.out.print("Enter your password: ");
        String password = scanner.nextLine();
        if (username.equals(USERNAME) && password.equals(PASSWORD)) {
            System.out.println("Login success!");
            return true;
        } else {
            System.out.println("Username or password incorrect!");
            return false;
        }
    }
}

SelectCourse.java:

import java.util.Scanner;

public class SelectCourse {
    public void doSelect() {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your student ID: ");
        String studentId = scanner.nextLine();
        System.out.print("Enter the course ID you want to select: ");
        String courseId = scanner.nextLine();
        Student student = StudentDB.getInstance().getStudentById(studentId);
        Course course = CourseDB.getInstance().getCourseById(courseId);
        if (student == null) {
            System.out.println("Student not found!");
        } else if (course == null) {
            System.out.println("Course not found!");
        } else if (student.hasSelectedCourse(course)) {
            System.out.println("This course has been selected by you before!");
        } else if (course.isFull()) {
            System.out.println("This course is already full!");
        } else {
            student.selectCourse(course);
            course.selectByStudent();
            System.out.println("Select course success!");
        }
    }
}

MainProcess.java:

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

public class MainProcess {
    private static final int MENU_LOGIN = 1;
    private static final int MENU_VIEW_COURSES = 2;
    private static final int MENU_SELECT_COURSE = 3;
    private static final int MENU_VIEW_SELECTED_COURSES = 4;
    private static final int MENU_EXIT = 5;

    public static void main(String[] args) {
        boolean isLogin = false;
        while (true) {
            if (!isLogin) {
                System.out.println("Please login first!");
            }
            int choice = printMenu(isLogin);
            switch (choice) {
                case MENU_LOGIN:
                    isLogin = new Login().doLogin();
                    break;
                case MENU_VIEW_COURSES:
                    viewCourses();
                    break;
                case MENU_SELECT_COURSE:
                    if (isLogin) {
                        new SelectCourse().doSelect();
                    } else {
                        System.out.println("Please login first!");
                    }
                    break;
                case MENU_VIEW_SELECTED_COURSES:
                    if (isLogin) {
                        viewSelectedCourses();
                    } else {
                        System.out.println("Please login first!");
                    }
                    break;
                case MENU_EXIT:
                    System.out.println("Bye!");
                    return;
                default:
                    System.out.println("Invalid choice, please try again!");
                    break;
            }
        }
    }

    private static int printMenu(boolean isLogin) {
        System.out.println("============ Menu ============");
        System.out.println("1. Login");
        System.out.println("2. View courses");
        System.out.println("3. Select course");
        System.out.println("4. View selected courses");
        System.out.println("5. Exit");
        System.out.print("Enter your choice: ");
        Scanner scanner = new Scanner(System.in);
        int choice = scanner.nextInt();
        return choice;
    }

    private static void viewCourses() {
        List<Course> courses = CourseDB.getInstance().getCourses();
        System.out.println("============ Courses ============");
        System.out.println("ID\t\tName\t\t\t\tCapacity\tSelected");
        for (Course course : courses) {
            System.out.println(course.getId() + "\t" + course.getName() + "\t\t" + course.getCapacity() + "\t\t" + course.getSelected());
        }
        System.out.println("=================================");
    }

    private static void viewSelectedCourses() {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter your student ID: ");
        String studentId = scanner.nextLine();
        Student student = StudentDB.getInstance().getStudentById(studentId);
        if (student == null) {
            System.out.println("Student not found!");
        } else {
            List<Course> courses = student.getSelectedCourses();
            System.out.println("============ Selected Courses ============");
            System.out.println("ID\t\tName");
            for (Course course : courses) {
                System.out.println(course.getId() + "\t" + course.getName());
            }
            System.out.println("==========================================");
        }
    }
}

Course.java:

public class Course {
    private String id;
    private String name;
    private int capacity;
    private int selected;

    public Course(String id, String name, int capacity, int selected) {
        this.id = id;
        this.name = name;
        this.capacity = capacity;
        this.selected = selected;
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getCapacity() {
        return capacity;
    }

    public int getSelected() {
        return selected;
    }

    public boolean isFull() {
        return selected >= capacity;
    }

    public void selectByStudent() {
        selected++;
    }
}

Student.java:

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

public class Student {
    private String id;
    private String name;
    private List<Course> selectedCourses;

    public Student(String id, String name) {
        this.id = id;
        this.name = name;
        selectedCourses = new ArrayList<Course>();
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public List<Course> getSelectedCourses() {
        return selectedCourses;
    }

    public boolean hasSelectedCourse(Course course) {
        return selectedCourses.contains(course);
    }

    public void selectCourse(Course course) {
        selectedCourses.add(course);
    }
}

这个选课系统还有很多改进的空间，比如可以将用户输入的代码进行校验，防止输入非法字符导致程序崩溃，也可以增加一些异常处理机制来提高程序的健壮性。