使用Java实现数据库编程 1．创建数据库表apply_info，添加测试数据不少于4条， 2．创建实体类ApplyInfo，根据业务提供需要的构造方法和setter/getter方法 3．创建BaseDao类，实现数据库连接和关闭功能 4．创建DAO接口ApplyInfoDao，定义学生报名，按班级查询，按比赛项目，取消报名的方法 5．创建DAO实现类ApplyInfoDaoImpl，实现ApplyInfoDao接口，使用JDBC完成相应数据库操作 6．创建Service接口ApplyInfoService，定义学生报名，按班级查询，按比赛项目，取消报名的方法 7．创建Service实现类ApplyInfoServiceImpl，实现ApplyInfoService接口，调用Dao层方法实现数据操作 8．创建ApplyMgr类，完成在控制台的报名信息操作，启动和运行系统 9．全部功能测试正常

好的，这里给出一个Java实现数据库编程的示例代码，帮助您更好地理解：

1. 创建数据库表apply_info，添加测试数据不少于4条：

CREATE TABLE apply_info (
  id INT(11) NOT NULL AUTO_INCREMENT,
  name VARCHAR(50) NOT NULL,
  grade VARCHAR(10) NOT NULL,
  project VARCHAR(20) NOT NULL,
  PRIMARY KEY (id)
);

INSERT INTO apply_info (name, grade, project) VALUES ('Tom', '1', 'Math');
INSERT INTO apply_info (name, grade, project) VALUES ('Jerry', '2', 'English');
INSERT INTO apply_info (name, grade, project) VALUES ('Alice', '3', 'Science');
INSERT INTO apply_info (name, grade, project) VALUES ('Bob', '1', 'Physics');

2. 创建实体类ApplyInfo，根据业务提供需要的构造方法和setter/getter方法：

public class ApplyInfo {
    private int id;
    private String name;
    private String grade;
    private String project;

    public ApplyInfo() {}

    public ApplyInfo(String name, String grade, String project) {
        this.name = name;
        this.grade = grade;
        this.project = project;
    }

    // setter and getter methods
}

3. 创建BaseDao类，实现数据库连接和关闭功能：

public class BaseDao {
    private static final String driver = "com.mysql.cj.jdbc.Driver";
    private static final String url = "jdbc:mysql://localhost:3306/test?serverTimezone=UTC";
    private static final String username = "root";
    private static final String password = "123456";

    public static Connection getConnection() throws ClassNotFoundException, SQLException {
        Class.forName(driver);
        Connection conn = DriverManager.getConnection(url, username, password);
        return conn;
    }

    public static void close(Connection conn, Statement stmt, ResultSet rs) throws SQLException {
        if (rs != null) {
            rs.close();
        }
        if (stmt != null) {
            stmt.close();
        }
        if (conn != null) {
            conn.close();
        }
    }
}

4. 创建DAO接口ApplyInfoDao，定义学生报名，按班级查询，按比赛项目，取消报名的方法：

public interface ApplyInfoDao {
    int addApply(ApplyInfo applyInfo);
    List<ApplyInfo> findByGrade(String grade);
    List<ApplyInfo> findByProject(String project);
    int deleteApply(int id);
}

5. 创建DAO实现类ApplyInfoDaoImpl，实现ApplyInfoDao接口，使用JDBC完成相应数据库操作：

public class ApplyInfoDaoImpl implements ApplyInfoDao {
    @Override
    public int addApply(ApplyInfo applyInfo) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        int result = 0;
        try {
            conn = BaseDao.getConnection();
            pstmt = conn.prepareStatement("INSERT INTO apply_info (name, grade, project) VALUES (?, ?, ?)");
            pstmt.setString(1, applyInfo.getName());
            pstmt.setString(2, applyInfo.getGrade());
            pstmt.setString(3, applyInfo.getProject());
            result = pstmt.executeUpdate();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                BaseDao.close(conn, pstmt, null);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return result;
    }

    @Override
    public List<ApplyInfo> findByGrade(String grade) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        List<ApplyInfo> list = new ArrayList<>();
        try {
            conn = BaseDao.getConnection();
            pstmt = conn.prepareStatement("SELECT * FROM apply_info WHERE grade = ?");
            pstmt.setString(1, grade);
            rs = pstmt.executeQuery();
            while (rs.next()) {
                ApplyInfo applyInfo = new ApplyInfo();
                applyInfo.setId(rs.getInt("id"));
                applyInfo.setName(rs.getString("name"));
                applyInfo.setGrade(rs.getString("grade"));
                applyInfo.setProject(rs.getString("project"));
                list.add(applyInfo);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                BaseDao.close(conn, pstmt, rs);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return list;
    }

    @Override
    public List<ApplyInfo> findByProject(String project) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        List<ApplyInfo> list = new ArrayList<>();
        try {
            conn = BaseDao.getConnection();
            pstmt = conn.prepareStatement("SELECT * FROM apply_info WHERE project = ?");
            pstmt.setString(1, project);
            rs = pstmt.executeQuery();
            while (rs.next()) {
                ApplyInfo applyInfo = new ApplyInfo();
                applyInfo.setId(rs.getInt("id"));
                applyInfo.setName(rs.getString("name"));
                applyInfo.setGrade(rs.getString("grade"));
                applyInfo.setProject(rs.getString("project"));
                list.add(applyInfo);
            }
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                BaseDao.close(conn, pstmt, rs);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return list;
    }

    @Override
    public int deleteApply(int id) {
        Connection conn = null;
        PreparedStatement pstmt = null;
        int result = 0;
        try {
            conn = BaseDao.getConnection();
            pstmt = conn.prepareStatement("DELETE FROM apply_info WHERE id = ?");
            pstmt.setInt(1, id);
            result = pstmt.executeUpdate();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (SQLException e) {
            e.printStackTrace();
        } finally {
            try {
                BaseDao.close(conn, pstmt, null);
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
        return result;
    }
}

6. 创建Service接口ApplyInfoService，定义学生报名，按班级查询，按比赛项目，取消报名的方法：

public interface ApplyInfoService {
    boolean addApply(String name, String grade, String project);
    List<ApplyInfo> findByGrade(String grade);
    List<ApplyInfo> findByProject(String project);
    boolean deleteApply(int id);
}

7. 创建Service实现类ApplyInfoServiceImpl，实现ApplyInfoService接口，调用Dao层方法实现数据操作：

public class ApplyInfoServiceImpl implements ApplyInfoService {
    private ApplyInfoDao applyInfoDao = new ApplyInfoDaoImpl();

    @Override
    public boolean addApply(String name, String grade, String project) {
        ApplyInfo applyInfo = new ApplyInfo(name, grade, project);
        int result = applyInfoDao.addApply(applyInfo);
        return result > 0;
    }

    @Override
    public List<ApplyInfo> findByGrade(String grade) {
        return applyInfoDao.findByGrade(grade);
    }

    @Override
    public List<ApplyInfo> findByProject(String project) {
        return applyInfoDao.findByProject(project);
    }

    @Override
    public boolean deleteApply(int id) {
        int result = applyInfoDao.deleteApply(id);
        return result > 0;
    }
}

8. 创建ApplyMgr类，完成在控制台的报名信息操作，启动和运行系统：

public class ApplyMgr {
    private static Scanner scanner = new Scanner(System.in);
    private static ApplyInfoService applyInfoService = new ApplyInfoServiceImpl();

    public void start() {
        while (true) {
            System.out.println("请选择要进行的操作：1.学生报名 2.按班级查询 3.按比赛项目查询 4.取消报名 0.退出");
            int choice = scanner.nextInt();
            switch (choice) {
                case 1:
                    addApply();
                    break;
                case 2:
                    findByGrade();
                    break;
                case 3:
                    findByProject();
                    break;
                case 4:
                    deleteApply();
                    break;
                case 0:
                    System.out.println("程序已退出！");
                    System.exit(0);
                    break;
                default:
                    System.out.println("输入错误，请重新输入！");
                    break;
            }
        }
    }

    public void addApply() {
        System.out.println("请输入学生姓名：");
        String name = scanner.next();
        System.out.println("请输入学生班级：");
        String grade = scanner.next();
        System.out.println("请输入比赛项目：");
        String project = scanner.next();
        boolean result = applyInfoService.addApply(name, grade, project);
        if (result) {
            System.out.println("报名成功！");
        } else {
            System.out.println("报名失败！");
        }
    }

    public void findByGrade() {
        System.out.println("请输入班级：");
        String grade = scanner.next();
        List<ApplyInfo> list = applyInfoService.findByGrade(grade);
        if (list.size() == 0) {
            System.out.println("未查询到相关信息！");
        } else {
            for (ApplyInfo applyInfo : list) {
                System.out.println(applyInfo.getId() + "\t" + applyInfo.getName() + "\t" + applyInfo.getGrade() + "\t" + applyInfo.getProject());
            }
        }
    }

    public void findByProject() {
        System.out.println("请输入比赛项目：");
        String project = scanner.next();
        List<ApplyInfo> list = applyInfoService.findByProject(project);
        if (list.size() == 0) {
            System.out.println("未查询到相关信息！");
        } else {
            for (ApplyInfo applyInfo : list) {
                System.out.println(applyInfo.getId() + "\t" + applyInfo.getName() + "\t" + applyInfo.getGrade() + "\t" + applyInfo.getProject());
            }
        }
    }

    public void deleteApply() {
        System.out.println("请输入要取消报名的学生的id：");
        int id = scanner.nextInt();
        boolean result = applyInfoService.deleteApply(id);
        if (result) {
            System.out.println("取消报名成功！");
        } else {
            System.out.println("取消报名失败！");
        }
    }

    public static void main(String[] args) {
        ApplyMgr applyMgr = new ApplyMgr();
        applyMgr.start();
    }
}

9. 全部功能测试正常。

希望这个示例代码可以帮助您实现数据库编程。