用java与数据库连接前端写出个增删查改的功能

好的，这里我提供一个简单的示例代码：

1. 首先，我们需要在 Java 中连接数据库。这里以 MySQL 数据库为例，使用 JDBC 进行连接：

import java.sql.*;

public class DatabaseConnection {
    static final String JDBC_DRIVER = "com.mysql.jdbc.Driver";
    static final String DB_URL = "jdbc:mysql://localhost:3306/mydatabase";
    static final String USER = "username";
    static final String PASS = "password";

    public static Connection getConnection() {
        Connection conn = null;
        try {
            Class.forName(JDBC_DRIVER);
            conn = DriverManager.getConnection(DB_URL, USER, PASS);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
        return conn;
    }
}

2. 接下来，我们定义一个 User 类，表示数据库中的一个用户：

public class User {
    private int id;
    private String name;
    private int age;
    private String gender;

    // 构造函数
    public User(int id, String name, int age, String gender) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.gender = gender;
    }

    // getter 和 setter 方法
    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }
}

3. 现在，我们可以编写增删查改功能了。以下代码实现了四个方法：addUser、deleteUser、getUserById 和 updateUser，分别实现了添加用户、删除用户、根据 ID 查询用户和更新用户信息的功能：

import java.sql.*;

public class UserDao {
    // 添加用户
    public void addUser(User user) {
        Connection conn = DatabaseConnection.getConnection();
        String sql = "INSERT INTO user (id, name, age, gender) VALUES (?, ?, ?, ?)";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, user.getId());
            pstmt.setString(2, user.getName());
            pstmt.setInt(3, user.getAge());
            pstmt.setString(4, user.getGender());
            pstmt.executeUpdate();
            pstmt.close();
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 删除用户
    public void deleteUser(int id) {
        Connection conn = DatabaseConnection.getConnection();
        String sql = "DELETE FROM user WHERE id = ?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            pstmt.executeUpdate();
            pstmt.close();
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }

    // 根据 ID 查询用户
    public User getUserById(int id) {
        Connection conn = DatabaseConnection.getConnection();
        String sql = "SELECT * FROM user WHERE id = ?";
        User user = null;
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setInt(1, id);
            ResultSet rs = pstmt.executeQuery();
            if (rs.next()) {
                int userId = rs.getInt("id");
                String name = rs.getString("name");
                int age = rs.getInt("age");
                String gender = rs.getString("gender");
                user = new User(userId, name, age, gender);
            }
            rs.close();
            pstmt.close();
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return user;
    }

    // 更新用户信息
    public void updateUser(User user) {
        Connection conn = DatabaseConnection.getConnection();
        String sql = "UPDATE user SET name = ?, age = ?, gender = ? WHERE id = ?";
        try {
            PreparedStatement pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getName());
            pstmt.setInt(2, user.getAge());
            pstmt.setString(3, user.getGender());
            pstmt.setInt(4, user.getId());
            pstmt.executeUpdate();
            pstmt.close();
            conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

4. 最后，我们可以在前端调用这些方法来实现增删查改的功能。例如，以下代码演示了如何添加一个用户：

UserDao userDao = new UserDao();
User user = new User(1, "张三", 20, "男");
userDao.addUser(user);

以上代码只是一个简单的示例，实际应用中还需要考虑异常处理、安全性、性能优化等问题。