利用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，以下是利用Java代码实现上述系统的示例：

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基础", 0, 2, "张三"));
        courses.add(new Course("C002", "Java高级", 0, 3, "李四"));
        courses.add(new Course("C003", "数据库基础", 0, 2, "王五"));
        courses.add(new Course("C004", "操作系统", 0, 2, "赵六"));
        courses.add(new Course("C005", "计算机网络", 0, 2, "钱七"));
        courses.add(new Course("C006", "数据结构与算法", 0, 2, "孙八"));
        courses.add(new Course("C007", "软件工程", 0, 2, "周九"));
        courses.add(new Course("C008", "人工智能", 0, 2, "吴十"));
        courses.add(new Course("C009", "计算机组成原理", 0, 2, "郑一"));
        courses.add(new Course("C010", "编译原理", 0, 2, "钱二"));
    }

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

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

    public void setCourses(List<Course> courses) {
        this.courses = 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", "张三", "男", "大一", "123456"));
        students.add(new Student("S002", "李四", "女", "大一", "234567"));
        students.add(new Student("S003", "王五", "男", "大二", "345678"));
        students.add(new Student("S004", "赵六", "女", "大二", "456789"));
        students.add(new Student("S005", "钱七", "男", "大三", "567890"));
    }

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

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

    public void setStudents(List<Student> students) {
        this.students = students;
    }
}

Login.java

import java.util.Scanner;

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

        for (Student student : StudentDB.getInstance().getStudents()) {
            if (student.getId().equals(id) && student.getPassword().equals(password)) {
                System.out.println("登录成功！");
                return student;
            }
        }

        System.out.println("学号或密码错误，请重新输入！");
        return null;
    }
}

SelectCourse.java

import java.util.Scanner;

public class SelectCourse {
    public static void selectCourse(Student student) {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请选择要选的课程编号：");
        String courseId = scanner.nextLine();

        for (Course course : CourseDB.getInstance().getCourses()) {
            if (course.getId().equals(courseId)) {
                if (course.getSelectedNum() < course.getLimitNum()) {
                    if (student.getSelectedCourses().contains(course)) {
                        System.out.println("该课程已经选择过了！");
                    } else {
                        student.getSelectedCourses().add(course);
                        course.setSelectedNum(course.getSelectedNum() + 1);
                        System.out.println("选课成功！");
                    }
                } else {
                    System.out.println("该课程已经选满！");
                }
                return;
            }
        }

        System.out.println("没有找到该课程！");
    }
}

MainProcess.java

public class MainProcess {
    public static void main(String[] args) {
        Student student = Login.login();
        if (student != null) {
            System.out.println("欢迎您，" + student.getName() + "！");

            while (true) {
                System.out.println("请选择要进行的操作：");
                System.out.println("1. 选课");
                System.out.println("2. 查看已选课程");
                System.out.println("3. 退出");
                int choice = Integer.parseInt(System.console().readLine());

                if (choice == 1) {
                    SelectCourse.selectCourse(student);
                } else if (choice == 2) {
                    System.out.println("您已经选了以下课程：");
                    for (Course course : student.getSelectedCourses()) {
                        System.out.println(course.getName());
                    }
                } else if (choice == 3) {
                    System.out.println("成功退出系统！");
                    break;
                } else {
                    System.out.println("无效的选择！");
                }
            }
        }
    }
}

以上是一个简单的选课系统的Java代码示例，其中实现了学生登录、选课、查看已选课程等功能。注意，这只是一个示例代码，实际应用中可能需要更多的功能和优化。