Java: How to return JDBC queries from JcomboBox

Question

package userProfile;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;

import javax.swing.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

public class Database extends JFrame implements ActionListener   {

        JFrame frame;
        JButton search, mainMenu;
        JComboBox fromLoc, toLoc, fromDate, fromTime;
        JLabel fromLabel, toLabel, fromDateLabel, fromTimeLabel;
        PreparedStatement ps = null;
        Connection link;
        ResultSet rs = null;

    public  Database() {

        frame = new JFrame("Make a Reservation");
        frame.getContentPane().setLayout(null);     

        //Arrival date/time comboBoxes and labels
        fromDateLabel = new JLabel("Departure Date");
        fromDateLabel.setBounds(50,230,100,30);
        fromDate = new JComboBox(new String[]{"12/15/2015", 
                  "12/21/2015", "12/21/2015", "12/24/2015"});
        fromDate.addActionListener(this);
        fromDate.setBounds(50,200,100,30);
        /*
        fromTimeLabel = new JLabel("Departure Time");
        fromTimeLabel.setBounds(160,230,100,30);
        fromTime = new JComboBox(new String[]{"13:00","15:00", "15:30", "08:00"});
        fromTime.addActionListener(this);
        fromTime.setBounds(160,200,100,30);
        */  
        //Departure label and comboBox
        fromLabel = new JLabel("Departure");
        fromLabel.setBounds(50,300,100,30);
        fromLoc = new JComboBox(new String[]{"Atlanta", "Charleston", "New York", "Los Angeles", "Orlando", "San Francisco"});
        fromLoc.addActionListener(this);
        fromLoc.setBounds(50,270,100,30);

        toLabel = new JLabel("Arrival");
        toLabel.setBounds(160,300,100,30);
        toLoc = new JComboBox(new String[]{"Atlanta", "Charleston", "New York", "Los Angeles", "Orlando", "San Francisco"});
        toLoc.addActionListener(this);
        toLoc.setBounds(160,270,100,30);

        search = new JButton("Ok");
        search.addActionListener(this);
        search.setBounds(270,270,100,30);

        //adding the buttons in frame
        frame.getContentPane().add(fromDateLabel);
        frame.getContentPane().add(fromDate);
        frame.getContentPane().add(fromTimeLabel);
        frame.getContentPane().add(fromTime);
        frame.getContentPane().add(fromLabel);
        frame.getContentPane().add(fromLoc);
        frame.getContentPane().add(toLabel);
        frame.getContentPane().add(toLoc);
        frame.getContentPane().add(search);

        frame.setSize(400,400);
        frame.setVisible(true);

          try {
                // Driver for mysql
                Class.forName("com.mysql.jdbc.Driver");
                 // connection link obj
                 link = DriverManager.getConnection("jdbc:mysql://localhost:3306/world", "root", "root");   
                 // query statement obj
            } catch (SQLException sqle) {
                System.out.println("An error occurred.Maybe user/password is invalid");
                sqle.printStackTrace();
            } catch (ClassNotFoundException cfne) {
                cfne.printStackTrace();
            }
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        JComboBox fromLoc = (JComboBox)e.getSource();
        String fromL = fromLoc.getSelectedItem().toString();
        JComboBox fromDate = (JComboBox)e.getSource();
        String fromD = fromDate.getSelectedItem().toString();
        JComboBox toLoc = (JComboBox)e.getSource();
        String toL = toLoc.getSelectedItem().toString();

        ArrayList<Flights> results = new ArrayList<>();

        try {
         ps = link.prepareStatement("select * from flights where from_date = ? and from_loc = ? and to_loc = ?");
         ps.setString(1, fromD);
         ps.setString(2, fromL);
         ps.setString(3, toL);
         rs = ps.executeQuery();

             while(rs.next()) { 

             Flights flight = new Flights(); 
             flight.setFromD(rs.getString("from_date"));
             flight.setFromL(rs.getString("from_loc"));
             flight.setToL(rs.getString("to_loc"));

             results.add(flight);
             }

        } catch (SQLException sqle) {
            System.out.println("An error occurred.Maybe user/password is invalid");
            sqle.printStackTrace();
        }


    }

}

I'm getting an NullPointerException when running this. This program should query database using hard-coded strings from an array in 3 comboBoxes and save returned strings as objects into an ArrayList. Please help, again Thanks

Answer 1

I would create a Flight model class and use it as a DTO. So, in your while(rs.next) i would use the the result to instantiate Flight objects and add them to an ArrayList<Flight> which could be returned to where ever you need to use it. Your 3rd question I'm not quite sure about since i havn't worked a lot with Swing. Hope my response is helpful to you. If I misunderstood your question please let me know.

EDIT:

A DTO is a Data Transfer Object used for contain and transfer data. So a Flight class could look like this:

public class Flight{  
public String fromD;  
public String fromL; 
public String toL;... 
//And so on 
//You also need setters and getters
}

Then in your Database class:

ArrayList<Flight> results = new ArrayList<>();
while(rs.next){ 
Flight flight = new Flight(); 
flight.setFromD(rs.getString("columnname"));
//Same method for the other variables you need to set. 
results.add(flight);
}