用java设计一个教材类，一个课程类，及一个为某门课程指定参考教材的类。一门课程可以有多本参考教材，再设计一个测试类。 设计一个简单的学生选课系统，并可以完成添加、删除、修改、查询相关信息 。 说明：设计以下类： 描述课程基本信息类(课程编号，课程名，学分 )； 描述学生基本信息类(学号，姓名，专业方向 )； 记录选课结果的类(课程，选课的学生，课程最多人数，当前人数 )； 测试类(设定或输入最多人数，创建课程对象，创建选课学生对象，创建选课对象，显示选课情况 )

好的，以下是Java代码实现：

Course.java

public class Course {
    private String courseId;
    private String courseName;
    private int credit;

    public Course(String courseId, String courseName, int credit) {
        this.courseId = courseId;
        this.courseName = courseName;
        this.credit = credit;
    }

    public String getCourseId() {
        return courseId;
    }

    public void setCourseId(String courseId) {
        this.courseId = courseId;
    }

    public String getCourseName() {
        return courseName;
    }

    public void setCourseName(String courseName) {
        this.courseName = courseName;
    }

    public int getCredit() {
        return credit;
    }

    public void setCredit(int credit) {
        this.credit = credit;
    }
}

Textbook.java

public class Textbook {
    private Course course;
    private String textbookName;

    public Textbook(Course course, String textbookName) {
        this.course = course;
        this.textbookName = textbookName;
    }

    public Course getCourse() {
        return course;
    }

    public void setCourse(Course course) {
        this.course = course;
    }

    public String getTextbookName() {
        return textbookName;
    }

    public void setTextbookName(String textbookName) {
        this.textbookName = textbookName;
    }
}

Student.java

public class Student {
    private String studentId;
    private String name;
    private String major;

    public Student(String studentId, String name, String major) {
        this.studentId = studentId;
        this.name = name;
        this.major = major;
    }

    public String getStudentId() {
        return studentId;
    }

    public void setStudentId(String studentId) {
        this.studentId = studentId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getMajor() {
        return major;
    }

    public void setMajor(String major) {
        this.major = major;
    }
}

Enrollment.java

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

public class Enrollment {
    private Course course;
    private List<Student> students = new ArrayList<>();
    private int maxNumOfStudents;
    private int currentNumOfStudents;

    public Enrollment(Course course, int maxNumOfStudents) {
        this.course = course;
        this.maxNumOfStudents = maxNumOfStudents;
    }

    public Course getCourse() {
        return course;
    }

    public void setCourse(Course course) {
        this.course = course;
    }

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

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

    public int getMaxNumOfStudents() {
        return maxNumOfStudents;
    }

    public void setMaxNumOfStudents(int maxNumOfStudents) {
        this.maxNumOfStudents = maxNumOfStudents;
    }

    public int getCurrentNumOfStudents() {
        return currentNumOfStudents;
    }

    public void setCurrentNumOfStudents(int currentNumOfStudents) {
        this.currentNumOfStudents = currentNumOfStudents;
    }

    public void addStudent(Student student) {
        if (currentNumOfStudents < maxNumOfStudents) {
            students.add(student);
            currentNumOfStudents++;
            System.out.println("Add student " + student.getName() + " to course " + course.getCourseName() + " successfully.");
        } else {
            System.out.println("Failed to add student " + student.getName() + " to course " + course.getCourseName()
                    + ". The course is full.");
        }
    }

    public void removeStudent(Student student) {
        if (students.remove(student)) {
            currentNumOfStudents--;
            System.out.println("Remove student " + student.getName() + " from course " + course.getCourseName() + " successfully.");
        } else {
            System.out.println("Failed to remove student " + student.getName() + " from course " + course.getCourseName()
                    + ". The student is not in the course.");
        }
    }

    public void modifyCourse(Course course) {
        this.course = course;
        System.out.println("Modify course " + course.getCourseName() + " successfully.");
    }

    public void queryEnrollment() {
        System.out.println("Course " + course.getCourseName() + " currently has " + currentNumOfStudents + " students:");
        for (Student student : students) {
            System.out.println(student.getName() + " (" + student.getStudentId() + ")");
        }
    }
}

Test.java

public class Test {
    public static void main(String[] args) {
        int maxNumOfStudents = 3;
        Course javaCourse = new Course("C001", "Java Programming", 3);
        Student student1 = new Student("S001", "Tom", "Computer Science");
        Student student2 = new Student("S002", "Jerry", "Computer Science");
        Student student3 = new Student("S003", "Kate", "Computer Science");

        Enrollment enrollment = new Enrollment(javaCourse, maxNumOfStudents);
        enrollment.addStudent(student1);
        enrollment.addStudent(student2);
        enrollment.addStudent(student3);
        enrollment.queryEnrollment();

        enrollment.removeStudent(student2);
        enrollment.queryEnrollment();

        Course pythonCourse = new Course("C002", "Python Programming", 4);
        enrollment.modifyCourse(pythonCourse);
        enrollment.queryEnrollment();

        Textbook javaTextbook1 = new Textbook(javaCourse, "Thinking in Java");
        Textbook javaTextbook2 = new Textbook(javaCourse, "Java: A Beginner's Guide");
        Textbook pythonTextbook = new Textbook(pythonCourse, "Learning Python");

        System.out.println("Course " + javaCourse.getCourseName() + " has the following textbooks:");
        System.out.println(javaTextbook1.getTextbookName());
        System.out.println(javaTextbook2.getTextbookName());

    }
}