public static User existsUser(String username, String password){
        if(Utils.isNullOrEmpty(username) || Utils.isNullOrEmpty(password)){
            return null;
        }
        try {
            Connection con = DB.getConnection();
            PreparedStatement s = con.prepareStatement(CHECKUSER);
            s.setString(1, username);
            s.setString(2, password);
            ResultSet result = s.executeQuery();
            if(result.first()){
                int id = result.getInt("id");
                String fullname = result.getString("fullname");
                int usertype = result.getInt("usertype");
                User user = new User(id, username, password, fullname, usertype);
                return user;
            }
        } catch(Exception e){ }
        return null;
    }