利用java程序输出实现学生选课系统实现： 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；

好的，以下是一个简单的Java程序实现学生选课系统的示例代码：

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 String getPassword() {
        return password;
    }
}

Course.java：

public class Course {
    private String id;
    private String name;
    private int selectedNumber;
    private int totalNumber;
    private String teacher;

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

    public String getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getSelectedNumber() {
        return selectedNumber;
    }

    public void setSelectedNumber(int selectedNumber) {
        this.selectedNumber = selectedNumber;
    }

    public int getTotalNumber() {
        return totalNumber;
    }

    public String getTeacher() {
        return teacher;
    }
}

CourseDB.java：

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

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

    private CourseDB() {
        courses = new ArrayList<>();
        courses.add(new Course("C001", "Java Programming", 0, 50, "Tom"));
        courses.add(new Course("C002", "Data Structures and Algorithms", 0, 40, "John"));
        courses.add(new Course("C003", "Database Systems", 0, 60, "Mary"));
        courses.add(new Course("C004", "Computer Networks", 0, 30, "Bob"));
        courses.add(new Course("C005", "Operating Systems", 0, 50, "Alice"));
        courses.add(new Course("C006", "Web Development", 0, 40, "David"));
        courses.add(new Course("C007", "Artificial Intelligence", 0, 30, "Peter"));
        courses.add(new Course("C008", "Computer Graphics", 0, 50, "Maggie"));
        courses.add(new Course("C009", "Mobile Application Development", 0, 40, "Jason"));
        courses.add(new Course("C010", "Software Engineering", 0, 60, "Lucy"));
    }

    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;
    private List<Student> students;

    private StudentDB() {
        students = new ArrayList<>();
        students.add(new Student("S001", "Alice", "Female", "Sophomore", "123456"));
        students.add(new Student("S002", "Bob", "Male", "Junior", "123456"));
        students.add(new Student("S003", "Mary", "Female", "Senior", "123456"));
        students.add(new Student("S004", "John", "Male", "Freshman", "123456"));
        students.add(new Student("S005", "Lucy", "Female", "Sophomore", "123456"));
    }

    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 Student student;

    public void 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();
        StudentDB studentDB = StudentDB.getInstance();
        for (Student s : studentDB.getStudents()) {
            if (s.getId().equals(id) && s.getPassword().equals(password)) {
                student = s;
                System.out.println("登录成功");
                break;
            }
        }
        if (student == null) {
            System.out.println("学号或密码错误");
        }
    }

    public Student getStudent() {
        return student;
    }
}

SelectCourse.java：

import java.util.Scanner;

public class SelectCourse {
    private Student student;

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

    public void select() {
        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 2.选课 3.查看已选课程 4.退出");
            int choice = scanner.nextInt();
            if (choice == 1) {
                viewAllCourses();
            } else if (choice == 2) {
                selectCourse();
            } else if (choice == 3) {
                viewSelectedCourses();
            } else if (choice == 4) {
                break;
            } else {
                System.out.println("请选择正确的操作");
            }
        }
    }

    private void viewAllCourses() {
        System.out.println("全部课程信息如下：");
        CourseDB courseDB = CourseDB.getInstance();
        for (Course c : courseDB.getCourses()) {
            System.out.println("课程编号：" + c.getId());
            System.out.println("课程名称：" + c.getName());
            System.out.println("授课教师：" + c.getTeacher());
            System.out.println("已选人数：" + c.getSelectedNumber());
            System.out.println("可选人数：" + (c.getTotalNumber() - c.getSelectedNumber()));
            System.out.println("--------------");
        }
    }

    private void selectCourse() {
        System.out.print("请输入要选的课程编号：");
        Scanner scanner = new Scanner(System.in);
        String id = scanner.nextLine();
        CourseDB courseDB = CourseDB.getInstance();
        Course selectedCourse = null;
        for (Course c : courseDB.getCourses()) {
            if (c.getId().equals(id)) {
                selectedCourse = c;
                break;
            }
        }
        if (selectedCourse == null) {
            System.out.println("没有该课程");
            return;
        }
        if (selectedCourse.getSelectedNumber() >= selectedCourse.getTotalNumber()) {
            System.out.println("该课程已选满");
            return;
        }
        for (Course c : student.getCourses()) {
            if (c != null && c.getId().equals(id)) {
                System.out.println("该课程无法重复选择");
                return;
            }
        }
        selectedCourse.setSelectedNumber(selectedCourse.getSelectedNumber() + 1);
        for (int i = 0; i < student.getCourses().length; i++) {
            if (student.getCourses()[i] == null) {
                student.getCourses()[i] = selectedCourse;
                System.out.println("选课成功");
                return;
            }
        }
        System.out.println("您已选了最多的课程");
    }

    private void viewSelectedCourses() {
        System.out.println("已选课程信息如下：");
        for (Course c : student.getCourses()) {
            if (c != null) {
                System.out.println("课程编号：" + c.getId());
                System.out.println("课程名称：" + c.getName());
                System.out.println("授课教师：" + c.getTeacher());
                System.out.println("已选人数：" + c.getSelectedNumber());
                System.out.println("可选人数：" + (c.getTotalNumber() - c.getSelectedNumber()));
                System.out.println("--------------");
            }
        }
    }
}

MainProcess.java：

public class MainProcess {
    public static void main(String[] args) {
        Login login = new Login();
        login.login();
        Student student = login.getStudent();
        if (student == null) {
            return;
        }
        SelectCourse selectCourse = new SelectCourse(student);
        selectCourse.select();
    }
}

这样就实现了一个简单的学生选课系统，其中包括登录、查看全部课程、选课、查看已选课程等功能，同时也考虑了学生选择“已选”课程、选课人数等重要细节。