Mapping a row from a SQL data to a Java object

Question

I have a Java class with instance fields (and matching setter methods) that match the column names of a SQL database table. I would like to elegantly fetch a row from the table

Answer 1

You can do it generically by doing the following simple methods:

Interface to use as a method pointer:

public interface I_DBtoJavaObjectConvertable
{
    public T createFromDB(ResultSet i_rs) throws SQLException;
}

Generic class to handle every mapping from SQL to java Object:

public class DBManager
{

    static volatile Connection conn;

    //set here a static c'tor to handle the connection to the database

    //The General generic method:    
    public static  List GetObjectsFromDB(String i_Query, I_DBtoJavaObjectConvertable i_Converter)
    {
        List ResList = new ArrayList<>();

        try
        {
            Statement st = conn.createStatement();
            for (ResultSet rs = st.executeQuery(i_Query); rs.next();)
            {
                ResList.add((T) i_Converter.createFromDB(rs));
            }
        }
        catch (SQLException ex)
        {
            _LOG_ERROR(ex.getMessage());
        }

        return ResList;
    }
}

Now By using Lanbda expression use can easlly convert an sql row to object, by given your convertion method, for example:

public static User FetchUserFromDB(ResultSet i_rs)
{
    User userToCreate = null;
    try
    {
        String FirstName = i_rs.getString("FirstName");
        String LastName = i_rs.getString("LastName");
        String Password = i_rs.getString("Password");

        userToCreate = new User(FirstName, LastName, Password);

    }
    catch (SQLException ex)
    {
        _LOG_ERROR("Error in fetching user from DB: \n" + ex.getMessage());
    }
    return userToCreate;
}

And now you can use this this method to bring any Users you want:

public static List GetAllUsersFromDB() throws SQLException
{
    String Query = "select * "
            + "from UsersTable";

    return DBManager.GetObjectsFromDB(Query, rs -> FetchUserFromDB(rs));
}

Or:

public static List GetAllNamesFromDB() throws SQLException
{
    String Query = "select FirstName "
            + "from UsersTable";

    return DBManager.GetObjectsFromDB(Query, rs -> rs.getString("FirstName"));
}