用Java写出学生选课系统

好的，下面是一个简单的学生选课系统的Java代码示例。

首先，我们需要设计一个数据库，用于存储学生和课程的信息。这里以MySQL数据库为例，创建一个名为“course_selection”的数据库，其中包含两个表格：学生表格“students”和课程表格“courses”。

CREATE DATABASE course_selection;

USE course_selection;

CREATE TABLE students (
  id INT NOT NULL AUTO_INCREMENT,
  name VARCHAR(50) NOT NULL,
  gender VARCHAR(10) NOT NULL,
  grade INT NOT NULL,
  major VARCHAR(50) NOT NULL,
  PRIMARY KEY (id)
);

CREATE TABLE courses (
  id INT NOT NULL AUTO_INCREMENT,
  name VARCHAR(50) NOT NULL,
  teacher VARCHAR(50) NOT NULL,
  credit INT NOT NULL,
  capacity INT NOT NULL,
  PRIMARY KEY (id)
);

然后，我们可以使用Java JDBC技术连接数据库，并且创建一个名为“CourseSelection”的类，用于处理学生选课系统的业务逻辑。

import java.sql.*;

public class CourseSelection {
  private Connection conn;
  
  public CourseSelection() {
    try {
      Class.forName("com.mysql.jdbc.Driver");
      conn = DriverManager.getConnection("jdbc:mysql://localhost/course_selection", "root", "password");
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
  
  public void addStudent(String name, String gender, int grade, String major) {
    try {
      PreparedStatement stmt = conn.prepareStatement("INSERT INTO students (name, gender, grade, major) VALUES (?, ?, ?, ?)");
      stmt.setString(1, name);
      stmt.setString(2, gender);
      stmt.setInt(3, grade);
      stmt.setString(4, major);
      stmt.executeUpdate();
      stmt.close();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
  
  public void addCourse(String name, String teacher, int credit, int capacity) {
    try {
      PreparedStatement stmt = conn.prepareStatement("INSERT INTO courses (name, teacher, credit, capacity) VALUES (?, ?, ?, ?)");
      stmt.setString(1, name);
      stmt.setString(2, teacher);
      stmt.setInt(3, credit);
      stmt.setInt(4, capacity);
      stmt.executeUpdate();
      stmt.close();
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
  
  public void selectCourse(int studentId, int courseId) {
    try {
      PreparedStatement stmt = conn.prepareStatement("SELECT COUNT(*) FROM selections WHERE student_id = ? AND course_id = ?");
      stmt.setInt(1, studentId);
      stmt.setInt(2, courseId);
      ResultSet rs = stmt.executeQuery();
      rs.next();
      int count = rs.getInt(1);
      rs.close();
      stmt.close();
      if (count > 0) {
        System.out.println("You have already selected this course.");
      } else {
        stmt = conn.prepareStatement("SELECT COUNT(*) FROM selections WHERE course_id = ?");
        stmt.setInt(1, courseId);
        rs = stmt.executeQuery();
        rs.next();
        int selectedCount = rs.getInt(1);
        rs.close();
        stmt.close();
        stmt = conn.prepareStatement("SELECT capacity FROM courses WHERE id = ?");
        stmt.setInt(1, courseId);
        rs = stmt.executeQuery();
        rs.next();
        int capacity = rs.getInt(1);
        rs.close();
        stmt.close();
        if (selectedCount >= capacity) {
          System.out.println("This course is full.");
        } else {
          stmt = conn.prepareStatement("INSERT INTO selections (student_id, course_id) VALUES (?, ?)");
          stmt.setInt(1, studentId);
          stmt.setInt(2, courseId);
          stmt.executeUpdate();
          stmt.close();
          System.out.println("You have selected this course successfully.");
        }
      }
    } catch (Exception e) {
      e.printStackTrace();
    }
  }
}

最后，我们可以编写一个测试程序，用于演示学生选课系统的使用方法。

import java.util.Scanner;

public class TestCourseSelection {
  public static void main(String[] args) {
    CourseSelection cs = new CourseSelection();
    Scanner scanner = new Scanner(System.in);
    while (true) {
      System.out.println("Please select an operation:");
      System.out.println("1. Add student");
      System.out.println("2. Add course");
      System.out.println("3. Select course");
      System.out.println("4. Exit");
      int option = scanner.nextInt();
      if (option == 1) {
        System.out.println("Please enter student's name:");
        String name = scanner.next();
        System.out.println("Please enter student's gender:");
        String gender = scanner.next();
        System.out.println("Please enter student's grade:");
        int grade = scanner.nextInt();
        System.out.println("Please enter student's major:");
        String major = scanner.next();
        cs.addStudent(name, gender, grade, major);
      } else if (option == 2) {
        System.out.println("Please enter course's name:");
        String name = scanner.next();
        System.out.println("Please enter course's teacher:");
        String teacher = scanner.next();
        System.out.println("Please enter course's credit:");
        int credit = scanner.nextInt();
        System.out.println("Please enter course's capacity:");
        int capacity = scanner.nextInt();
        cs.addCourse(name, teacher, credit, capacity);
      } else if (option == 3) {
        System.out.println("Please enter student's id:");
        int studentId = scanner.nextInt();
        System.out.println("Please enter course's id:");
        int courseId = scanner.nextInt();
        cs.selectCourse(studentId, courseId);
      } else if (option == 4) {
        break;
      } else {
        System.out.println("Invalid option.");
      }
    }
    scanner.close();
  }
}

以上就是一个简单的学生选课系统的Java代码示例。当然，这只是一个基础版的系统，实际开发中还需要考虑更多的因素，如安全性、性能、可扩展性等。