Question

I currently have the following code that retrieves data from the database and then create a User. This code is used in many of my classe to create other objects such as News, Commentsetc…

It uses apache commons dbutils.

final ResultSetHandler<User> handler = new ResultSetHandler<User>() {

            @Override
            public User handle(ResultSet rs) throws SQLException {

                User user = null;
                if (rs.next()) {
                    user = new User();
                    user.setId(rs.getInt("id"));
                    user.setUsername(rs.getString("username"));
                    user.setPassword(rs.getString("password"));
                }
                return user;
            }
        };

        final User user = run.query(
                "SELECT id, username, password FROM users WHERE username = ? AND active = 2 LIMIT 1;", handler,
                username);

Would it be possible to wrap the QueryRunner in a generic class and override the query method so the handler instanciate the generic T with the ResultSet. I would make sure any Ttype would ahve a constructor accepting a ResultSet.

Like so :

        public class QueryExecuter<T> extends QueryRunner {
    private ResultSetHandler<T> _handler;

    public QueryExecuter(){//The T type was for testing haha
        super();
        handler = new ResultSetHandler<T>() {

            @Override
            public T handle(ResultSet rs) throws SQLException {

                T object = null;
                if (rs.next()) {
                    object = new T(rs);
                }
                return object;
            }
        };
    }
}

I don’t know if you’ll understand, but I hope so, ask me if you want more details or a better explanation.

EDIT

I thought I could use a AbstractClass instead of the generic type that all of the differents objects would extends but it seems like I can’t write an abstract constructor. Will I have to make a static method that return an instance of the object like:

public abstract class DatabaseEntity {
    public static abstract DatabaseEntity create(ResultSet rs);//even this doesn't work...
}

Answer 1

Possible, yes? But its a bad idea.

You could do:

class ResultSetHandler<T> {
  ResultSetHandler<T>(Class<T> clazz) {
    this.clazz = clazz;
  }

  public T handle(ResultSet rs) throws SQLException {
    T object = null;
    if (rs.next()) {
      object = clazz.getConstructor(ResultSet.class).newInstance(rs)
    }
    return object;
  }
}

Mixing domain and database is a bad idea, however. What would be better, however, would be to define an abtract method that creates the object based on the resultset:

abstract class ResultSetHandler<T> {

  protected abstract T create(ResultSet rs);

  public T handle(ResultSet rs) throws SQLException {
    T object = null;
    if (rs.next()) {
      object = create(rs);
    }
    return object;
  }
}

Then, in your implementing class, you only need to provide a create() method instead of handling the result set yourself, for example:

h = new ResultSetHandler<Person>() {
  protected Person create(ResultSet rs) {
    return new Person(rs.getString("name"));
  }
}