java代码：学生选课系统。 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。创建学生和课程类。 (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；

以下是一个简单的学生选课系统的Java代码示例：

Course.java

public class Course {
    private int id;
    private String name;
    private int selectedCount;
    private int maxCount;
    private String teacher;

    public Course(int id, String name, int maxCount, String teacher) {
        this.id = id;
        this.name = name;
        this.maxCount = maxCount;
        this.teacher = teacher;
        this.selectedCount = 0;
    }

    public int getId() { return id; }
    public String getName() { return name; }
    public int getSelectedCount() { return selectedCount; }
    public int getMaxCount() { return maxCount; }
    public String getTeacher() { return teacher; }

    public boolean isAvailable() {
        return selectedCount < maxCount;
    }

    public void select() {
        selectedCount++;
    }

    public void deselect() {
        selectedCount--;
    }
}

Student.java

public class Student {
    private int id;
    private String name;
    private String gender;
    private int grade;
    private String password;

    public Student(int id, String name, String gender, int grade, String password) {
        this.id = id;
        this.name = name;
        this.gender = gender;
        this.grade = grade;
        this.password = password;
    }

    public int getId() { return id; }
    public String getName() { return name; }
    public String getGender() { return gender; }
    public int getGrade() { return grade; }
    public String getPassword() { return password; }
}

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(1, "Java Programming", 50, "Tom"));
        courses.add(new Course(2, "Data Structures", 40, "Jerry"));
        courses.add(new Course(3, "Web Development", 30, "Bob"));
        courses.add(new Course(4, "Database Systems", 35, "Alice"));
        courses.add(new Course(5, "Algorithms", 45, "David"));
        courses.add(new Course(6, "Network Security", 25, "Peter"));
        courses.add(new Course(7, "Operating Systems", 30, "Mary"));
        courses.add(new Course(8, "Artificial Intelligence", 20, "John"));
        courses.add(new Course(9, "Computer Graphics", 25, "Sara"));
        courses.add(new Course(10, "Software Engineering", 35, "Steve"));
    }

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

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

    public Course getCourseById(int id) {
        for (Course course : courses) {
            if (course.getId() == id) {
                return course;
            }
        }
        return null;
    }
}

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(1, "张三", "男", 1, "123456"));
        students.add(new Student(2, "李四", "女", 2, "654321"));
        students.add(new Student(3, "王五", "男", 3, "111111"));
        students.add(new Student(4, "赵六", "女", 4, "222222"));
        students.add(new Student(5, "钱七", "男", 5, "333333"));
    }

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

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

    public Student getStudentById(int id) {
        for (Student student : students) {
            if (student.getId() == id) {
                return student;
            }
        }
        return null;
    }
}

Login.java

import java.util.Scanner;

public class Login {
    public static Student login() {
        Scanner scanner = new Scanner(System.in);
        System.out.print("请输入学号：");
        int id = scanner.nextInt();
        System.out.print("请输入密码：");
        String password = scanner.next();

        Student student = StudentDB.getInstance().getStudentById(id);
        if (student != null && student.getPassword().equals(password)) {
            return student;
        } else {
            System.out.println("学号或密码错误！");
            return null;
        }
    }
}

SelectCourse.java

import java.util.Scanner;

public class SelectCourse {
    public static void select(Student student) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("可选课程列表：");
        for (Course course : CourseDB.getInstance().getCourses()) {
            if (course.isAvailable()) {
                System.out.println(course.getId() + " - " + course.getName() + " - " + course.getTeacher());
            }
        }
        System.out.print("请输入要选择的课程编号：");
        int id = scanner.nextInt();

        Course course = CourseDB.getInstance().getCourseById(id);
        if (course != null && course.isAvailable()) {
            course.select();
            System.out.println("选课成功！");
        } else if (course != null && !course.isAvailable()) {
            System.out.println("该课程已选满，无法选择！");
        } else {
            System.out.println("课程编号错误！");
        }
    }
}

MainProcess.java

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