利用java代码输出下面7个文件学生选课系统实现： 1. 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。 2.基本功能需求： （1）使用命令行完成系统的功能设计； （2）启动程序，输出“学生选课管理——登录”。 （2）提示客户输入学号、密码。 （3）登录后显示操作菜单，效果如下图所示： （4）根据用户选择的功能，进行相关的展示： 查看全部课程： 选课功能： 查看已选课程： 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；

这里提供一个简单的学生选课系统的Java代码，包括上述的基本功能和其他功能需求。代码中涉及的类包括CourseDB、StudentDB、Login、SelectCourse和MainProcess。请注意，这只是一个简单的示例，实际的实现可能需要更复杂的代码和更多的功能。

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("001", "Java Programming", 0, 2, "John Smith"));
        courses.add(new Course("002", "Database Systems", 0, 2, "Mary Johnson"));
        courses.add(new Course("003", "Web Development", 0, 2, "Tom Brown"));
        courses.add(new Course("004", "Data Structures", 0, 2, "Lisa Chen"));
        courses.add(new Course("005", "Algorithms", 0, 2, "David Lee"));
        courses.add(new Course("006", "Computer Networks", 0, 2, "Susan Wang"));
        courses.add(new Course("007", "Operating Systems", 0, 2, "Kevin Wu"));
        courses.add(new Course("008", "Software Engineering", 0, 2, "Emily Liu"));
        courses.add(new Course("009", "Artificial Intelligence", 0, 2, "Brian Zhang"));
        courses.add(new Course("010", "Computer Graphics", 0, 2, "Cathy Chen"));
    }

    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<Student>();
        students.add(new Student("2018001", "Alice", "Female", "Freshman", "123456"));
        students.add(new Student("2018002", "Bob", "Male", "Sophomore", "234567"));
        students.add(new Student("2018003", "Charlie", "Male", "Junior", "345678"));
        students.add(new Student("2018004", "David", "Male", "Senior", "456789"));
        students.add(new Student("2018005", "Emily", "Female", "Freshman", "567890"));
    }

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

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

Login.java

import java.util.Scanner;

public class Login {
    private StudentDB studentDB;

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

    public Student login() {
        Scanner scanner = new Scanner(System.in);
        System.out.println("学生选课管理——登录");
        System.out.print("请输入学号：");
        String id = scanner.nextLine();
        System.out.print("请输入密码：");
        String password = scanner.nextLine();
        for (Student student : studentDB.getStudents()) {
            if (student.getId().equals(id) && student.getPassword().equals(password)) {
                System.out.println("登录成功，欢迎 " + student.getName() + "！");
                return student;
            }
        }
        System.out.println("登录失败，请检查学号和密码是否正确。");
        return null;
    }
}

SelectCourse.java

import java.util.Scanner;

public class SelectCourse {
    private CourseDB courseDB;
    private Student student;

    public SelectCourse(Student student) {
        courseDB = CourseDB.getInstance();
        this.student = student;
    }

    public void select() {
        Scanner scanner = new Scanner(System.in);
        System.out.println("选课功能");
        System.out.println("课程编号\t课程名称\t授课教师\t已选人数\t可选人数");
        for (Course course : courseDB.getCourses()) {
            System.out.println(course.getId() + "\t" + course.getName() + "\t" + course.getTeacher() + "\t" + course.getSelectedNum() + "\t" + course.getAvailableNum());
        }
        System.out.print("请输入要选择的课程编号：");
        String courseId = scanner.nextLine();
        Course selectedCourse = null;
        for (Course course : courseDB.getCourses()) {
            if (course.getId().equals(courseId)) {
                selectedCourse = course;
                break;
            }
        }
        if (selectedCourse == null) {
            System.out.println("选择失败，无此课程编号！");
            return;
        }
        if (selectedCourse.getSelectedNum() >= selectedCourse.getAvailableNum()) {
            System.out.println("选择失败，该课程已经选满！");
            return;
        }
        if (student.hasCourse(selectedCourse)) {
            System.out.println("选择失败，您已经选择了该课程！");
            return;
        }
        student.addCourse(selectedCourse);
        selectedCourse.setSelectedNum(selectedCourse.getSelectedNum() + 1);
        System.out.println("选择成功！");
    }

    public void listSelectedCourses() {
        System.out.println("查看已选课程");
        System.out.println("课程编号\t课程名称\t授课教师");
        for (Course course : student.getSelectedCourses()) {
            System.out.println(course.getId() + "\t" + course.getName() + "\t" + course.getTeacher());
        }
    }
}

MainProcess.java

import java.util.Scanner;

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

    public MainProcess() {
        login = new Login();
    }

    public void run() {
        student = login.login();
        if (student != null) {
            selectCourse = new SelectCourse(student);
            Scanner scanner = new Scanner(System.in);
            while (true) {
                System.out.println("请选择操作：");
                System.out.println("1. 查看全部课程");
                System.out.println("2. 选课功能");
                System.out.println("3. 查看已选课程");
                System.out.println("4. 退出系统");
                int choice = scanner.nextInt();
                scanner.nextLine();
                switch (choice) {
                    case 1:
                        listAllCourses();
                        break;
                    case 2:
                        selectCourse.select();
                        break;
                    case 3:
                        selectCourse.listSelectedCourses();
                        break;
                    case 4:
                        System.out.println("已退出系统。");
                        return;
                    default:
                        System.out.println("无效的输入，请重新输入。");
                }
            }
        }
    }

    private void listAllCourses() {
        System.out.println("查看全部课程");
        System.out.println("课程编号\t课程名称\t授课教师\t已选人数\t可选人数");
        for (Course course : CourseDB.getInstance().getCourses()) {
            System.out.println(course.getId() + "\t" + course.getName() + "\t" + course.getTeacher() + "\t" + course.getSelectedNum() + "\t" + course.getAvailableNum());
        }
    }
}

Course.java

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

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

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getSelectedNum() {
        return selectedNum;
    }

    public void setSelectedNum(int selectedNum) {
        this.selectedNum = selectedNum;
    }

    public int getAvailableNum() {
        return availableNum;
    }

    public void setAvailableNum(int availableNum) {
        this.availableNum = availableNum;
    }

    public String getTeacher() {
        return teacher;
    }
}

Student.java

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

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

    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;
        selectedCourses = new ArrayList<Course>();
    }

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getGender() {
        return gender;
    }

    public String getGrade() {
        return grade;
    }

    public String getPassword() {
        return password;
    }

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

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

    public boolean hasCourse(Course course) {
        for (Course selectedCourse : selectedCourses) {
            if (selectedCourse.getId().equals(course.getId())) {
                return true;
            }
        }
        return false;
    }
}

以上是一个简单的学生选课系统的Java代码，包括了所需的七个文件。需要注意的是，这只是一个简单的实现，可能需要根据实际情况进行修改和补充。