query a database and generate an arraylist object

Your updated code looks much better. You still moved where the code gets the Connection. I moved that back, and tweaked it a tiny bit. Get the id, first_name and last_name from the ResultSet. Then instantiate a Person instance and add it to a List. Finally, return the List. Something like,

public ArrayList<Person> getPeople() throws Exception {
    Connection conn = this.getCandidateConnection();
     /* ---------- DO NOT CHANGE THE CODE ABOVE THIS LINE, IT WILL BE RESET ON RUN ---------- */
    final String selectSql = "SELECT id, first_name, last_name " //
            + "FROM people ORDER BY last_name, first_name";
    ArrayList<Person> al = new ArrayList<>();
    try (Statement statement = conn.createStatement(); //
            ResultSet resultSet = statement.executeQuery(selectSql)) {
        while (resultSet.next()) {
            int id = resultSet.getInt("id");
            String firstName = resultSet.getString("first_name");
            String lastName = resultSet.getString("last_name");
            al.add(new Person(id, firstName, lastName));
        }
    } finally {
        if (conn != null) {
            conn.close();
        }
    }
    return al;
    /* ---------- DO NOT CHANGE THE CODE BELOW THIS LINE, IT WILL BE RESET WHEN RUNNING ---------- */
}