用Java 和MySQL做一个学生选课系统

好的，以下是一个使用 Java 和 MySQL 的学生选课系统的代码示例：

首先，需要建立一个名为 `student_course_system` 的数据库，并在其中创建两个表格：`students` 和 `courses`。

`students` 表格包含 `id`、`name`、`age` 和 `gender` 四个字段，其中 `id` 是主键；`courses` 表格包含 `id`、`name`、`teacher` 和 `capacity` 四个字段，其中 `id` 是主键。这两个表格可以通过以下 SQL 语句创建：

CREATE DATABASE student_course_system;

USE student_course_system;

CREATE TABLE students (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    age INT,
    gender VARCHAR(10)
);

CREATE TABLE courses (
    id INT PRIMARY KEY,
    name VARCHAR(50),
    teacher VARCHAR(50),
    capacity INT
);

接下来，可以使用 Java 中的 JDBC API 连接到 MySQL 数据库，并实现学生选课系统的功能。以下是一个简单的代码示例：

import java.sql.*;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Scanner;

public class StudentCourseSystem {
    // 数据库连接信息
    private static final String URL = "jdbc:mysql://localhost:3306/student_course_system";
    private static final String USERNAME = "root";
    private static final String PASSWORD = "password";

    // 数据库连接对象
    private Connection connection;

    public StudentCourseSystem() {
        try {
            // 建立数据库连接
            connection = DriverManager.getConnection(URL, USERNAME, PASSWORD);
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 关闭数据库连接
    public void close() {
        try {
            connection.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 添加学生
    public void addStudent(Student student) {
        try {
            PreparedStatement statement = connection.prepareStatement("INSERT INTO students VALUES (?, ?, ?, ?)");
            statement.setInt(1, student.getId());
            statement.setString(2, student.getName());
            statement.setInt(3, student.getAge());
            statement.setString(4, student.getGender());
            statement.executeUpdate();
            System.out.println("添加学生成功！");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 添加课程
    public void addCourse(Course course) {
        try {
            PreparedStatement statement = connection.prepareStatement("INSERT INTO courses VALUES (?, ?, ?, ?)");
            statement.setInt(1, course.getId());
            statement.setString(2, course.getName());
            statement.setString(3, course.getTeacher());
            statement.setInt(4, course.getCapacity());
            statement.executeUpdate();
            System.out.println("添加课程成功！");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 学生选课
    public void selectCourse(Student student, Course course) {
        try {
            // 检查课程容量是否已满
            PreparedStatement capacityStatement = connection.prepareStatement("SELECT COUNT(*) FROM selections WHERE course_id = ?");
            capacityStatement.setInt(1, course.getId());
            ResultSet capacityResult = capacityStatement.executeQuery();
            capacityResult.next();
            int selectedCount = capacityResult.getInt(1);
            if (selectedCount == course.getCapacity()) {
                System.out.println("课程容量已满！");
                return;
            }

            // 添加选课记录
            PreparedStatement statement = connection.prepareStatement("INSERT INTO selections VALUES (?, ?)");
            statement.setInt(1, student.getId());
            statement.setInt(2, course.getId());
            statement.executeUpdate();
            System.out.println("学生选课成功！");
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 查询学生的选课情况
    public void querySelections(Student student) {
        try {
            // 查询学生选课记录
            PreparedStatement statement = connection.prepareStatement("SELECT * FROM selections WHERE student_id = ?");
            statement.setInt(1, student.getId());
            ResultSet result = statement.executeQuery();
            ArrayList<Course> selectedCourses = new ArrayList<>();
            while (result.next()) {
                int courseId = result.getInt("course_id");
                PreparedStatement courseStatement = connection.prepareStatement("SELECT * FROM courses WHERE id = ?");
                courseStatement.setInt(1, courseId);
                ResultSet courseResult = courseStatement.executeQuery();
                if (courseResult.next()) {
                    String courseName = courseResult.getString("name");
                    String teacher = courseResult.getString("teacher");
                    int capacity = courseResult.getInt("capacity");
                    Course course = new Course(courseId, courseName, teacher, capacity);
                    selectedCourses.add(course);
                }
            }

            // 输出选课情况
            System.out.println(student.getName() + "选了以下课程：");
            for (Course course : selectedCourses) {
                System.out.println(course.getName() + "（" + course.getTeacher() + "）");
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        StudentCourseSystem system = new StudentCourseSystem();
        Scanner scanner = new Scanner(System.in);

        // 添加学生
        System.out.print("请输入学生姓名：");
        String name = scanner.nextLine();
        System.out.print("请输入学生年龄：");
        int age = scanner.nextInt();
        scanner.nextLine();
        System.out.print("请输入学生性别：");
        String gender = scanner.nextLine();
        Student student = new Student(0, name, age, gender);
        system.addStudent(student);

        // 添加课程
        System.out.print("请输入课程名称：");
        String courseName = scanner.nextLine();
        System.out.print("请输入授课教师：");
        String teacher = scanner.nextLine();
        System.out.print("请输入课程容量：");
        int capacity = scanner.nextInt();
        Course course = new Course(0, courseName, teacher, capacity);
        system.addCourse(course);

        // 学生选课
        system.selectCourse(student, course);

        // 查询选课情况
        system.querySelections(student);

        // 关闭数据库连接
        system.close();
    }
}

class Student {
    private int id;
    private String name;
    private int age;
    private String gender;

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String getGender() {
        return gender;
    }
}

class Course {
    private int id;
    private String name;
    private String teacher;
    private int capacity;

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

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getTeacher() {
        return teacher;
    }

    public int getCapacity() {
        return capacity;
    }
}

以上代码中使用了 `PreparedStatement` 来防止 SQL 注入攻击。同时，为了防止课程容量超限，还需要先查询选课记录中已经选了该课程的人数。如果已经选课的人数达到了课程容量，就不能再选这门课程了。