user(用户登录表) 字段名 类型 注释 其他 Username Varchar(50) 用户名 主键，非空 password varchar(50) 密码 空 根据以上内容使用Java写出增删改查的方法类

假设使用JDBC连接数据库，可以按照以下方式编写增删改查的方法类：

import java.sql.*;

public class UserDao {
    private Connection conn;
    private PreparedStatement pstmt;
    private ResultSet rs;

    public UserDao() {
        try {
            // 连接数据库
            Class.forName("com.mysql.jdbc.Driver");
            conn = DriverManager.getConnection("jdbc:mysql://localhost:3306/test", "root", "password");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

    // 增加用户
    public boolean addUser(User user) {
        try {
            String sql = "INSERT INTO user(username, password) VALUES(?, ?)";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getUsername());
            pstmt.setString(2, user.getPassword());
            int result = pstmt.executeUpdate();
            if (result > 0) {
                return true;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return false;
    }

    // 删除用户
    public boolean deleteUser(String username) {
        try {
            String sql = "DELETE FROM user WHERE username=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            int result = pstmt.executeUpdate();
            if (result > 0) {
                return true;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return false;
    }

    // 修改用户密码
    public boolean updateUser(User user) {
        try {
            String sql = "UPDATE user SET password=? WHERE username=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, user.getPassword());
            pstmt.setString(2, user.getUsername());
            int result = pstmt.executeUpdate();
            if (result > 0) {
                return true;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return false;
    }

    // 查询用户
    public User getUser(String username) {
        try {
            String sql = "SELECT * FROM user WHERE username=?";
            pstmt = conn.prepareStatement(sql);
            pstmt.setString(1, username);
            rs = pstmt.executeQuery();
            if (rs.next()) {
                User user = new User();
                user.setUsername(rs.getString("username"));
                user.setPassword(rs.getString("password"));
                return user;
            }
        } catch (SQLException e) {
            e.printStackTrace();
        }
        return null;
    }

    public void close() {
        try {
            if (rs != null) rs.close();
            if (pstmt != null) pstmt.close();
            if (conn != null) conn.close();
        } catch (SQLException e) {
            e.printStackTrace();
        }
    }
}

其中，User是一个JavaBean类，包含username和password两个属性，以及对应的getters和setters方法。以上代码中使用了预编译SQL语句，可以有效避免SQL注入攻击。