以下是一个简单的Java选课系统的源代码，包括小标题和单元表格：

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;
class Course {
    private String courseName;
    private int courseId;
    public Course(String courseName, int courseId) {
        this.courseName = courseName;
        this.courseId = courseId;
    }
    public String getCourseName() {
        return courseName;
    }
    public int getCourseId() {
        return courseId;
    }
}
class Student {
    private String studentName;
    private int studentId;
    private List<Course> courses;
    public Student(String studentName, int studentId) {
        this.studentName = studentName;
        this.studentId = studentId;
        this.courses = new ArrayList<>();
    }
    public void addCourse(Course course) {
        courses.add(course);
    }
    public void removeCourse(Course course) {
        courses.remove(course);
    }
    public void displayCourses() {
        System.out.println("学生 " + studentName + " 的课程：");
        for (Course course : courses) {
            System.out.println("课程ID：" + course.getCourseId() + "，课程名称：" + course.getCourseName());
        }
    }
}
public class CourseSelectionSystem {
    public static void main(String[] args) {
        List<Course> courses = new ArrayList<>();
        courses.add(new Course("计算机科学", 1));
        courses.add(new Course("数学", 2));
        courses.add(new Course("物理", 3));
        courses.add(new Course("化学", 4));
        Student student = new Student("张三", 1001);
        Scanner scanner = new Scanner(System.in);
        while (true) {
            System.out.println("请选择操作：1. 选课 2. 退课 3. 查看已选课程 4. 退出");
            int operation = scanner.nextInt();
            if (operation == 1) {
                System.out.println("请输入要选的课程ID：");
                int courseId = scanner.nextInt();
                Course selectedCourse = null;
                for (Course course : courses) {
                    if (course.getCourseId() == courseId) {
                        selectedCourse = course;
                        break;
                    }
                }
                if (selectedCourse != null) {
                    student.addCourse(selectedCourse);
                    System.out.println("选课成功！");
                } else {
                    System.out.println("课程不存在，请重新选择！");
                }
            } else if (operation == 2) {
                System.out.println("请输入要退的课程ID：");
                int courseId = scanner.nextInt();
                Course removedCourse = null;
                for (Course course : student.courses) {
                    if (course.getCourseId() == courseId) {
                        removedCourse = course;
                        break;
                    }
                }
                if (removedCourse != null) {
                    student.removeCourse(removedCourse);
                    System.out.println("退课成功！");
                } else {
                    System.out.println("未找到该课程，请重新选择！");
                }
            } else if (operation == 3) {
                student.displayCourses();
            } else if (operation == 4) {
                System.out.println("退出系统");
                break;
            } else {
                System.out.println("无效的操作，请重新选择！");
            }
        }
        scanner.close();
    }
}

代码解析：

1、定义了一个Course类，用于表示课程，包含课程名称和课程ID两个属性。

2、定义了一个Student类，用于表示学生，包含学生姓名、学生ID和一个存储已选课程的列表，提供了添加课程、删除课程和显示已选课程的方法。

3、在main方法中，首先创建了一些课程对象和一个学生对象，然后使用while循环不断接收用户输入的操作，根据操作执行相应的功能。