Question

how to insert a combobox with values ​​from the data base
I want to select from the database and add in the combobox
I have two class:

constructor Database() first class

//constructeur 
Database()   
                  {  

 void remplir_Jcomb() {
           Connection conn = null;
           Statement  st = null;
           String rq1 = "SELECT region  FROM  rg";
           String rq2 = "SELECT ACTELS  FROM  rg";

        // - Paramètres de connexion à la base de données
           String url = "jdbc:mysql://localhost/réseau";
           String login = "root";
           String password = "";
           String driver = "com.mysql.jdbc.Driver";

           try {

            //comboBox_gouver.removeAllItems();
               try {
                    Class.forName(driver);
                    conn = DriverManager.getConnection(url,login,password);
                    } catch (ClassNotFoundException e) {
                        e.printStackTrace();
                         }

               st = conn.createStatement();
               ResultSet rs1 = st.executeQuery(rq1);
               ResultSet rs2 = st.executeQuery(rq2);

               while ((rs1.next())&&(rs2.next())) {
                comboBox_gouver.addItem(rs1.getString(1));
                comboBox_ACTELS.addItem(rs2.getString(1));
               }
               st.close();
               rs1.close();
               rs2.close();
               conn.close();

           }  catch (SQLException e) { 
                e.printStackTrace();}

            }
        }

interface swing second class which contains two JComboBox
call constructor Database()

private Database BD= new Database();  
    public Region() {


//first JComboBox
    comboBox_gouver = new JComboBox<String>();
            BD.remplir_Jcomb();
                sl_contentPanel.putConstraint(SpringLayout.NORTH, lbl_gouver, 5, SpringLayout.NORTH, comboBox_gouver);
                sl_contentPanel.putConstraint(SpringLayout.NORTH, comboBox_gouver, 5, SpringLayout.NORTH, contentPanel);
                sl_contentPanel.putConstraint(SpringLayout.WEST, comboBox_gouver, 16, SpringLayout.EAST, lbl_gouver);
                sl_contentPanel.putConstraint(SpringLayout.EAST, comboBox_gouver, -26, SpringLayout.EAST, contentPanel);
                contentPanel.add(comboBox_gouver);

comboBox_ACTELS = new JComboBox<String>();
        sl_contentPanel.putConstraint(SpringLayout.NORTH, lbl_ACTELS, 5, SpringLayout.NORTH, comboBox_ACTELS);
        sl_contentPanel.putConstraint(SpringLayout.NORTH, comboBox_ACTELS, 6, SpringLayout.SOUTH, comboBox_gouver);
        sl_contentPanel.putConstraint(SpringLayout.WEST, comboBox_ACTELS, 90, SpringLayout.EAST, lbl_ACTELS);
        sl_contentPanel.putConstraint(SpringLayout.SOUTH, comboBox_ACTELS, -29, SpringLayout.SOUTH, contentPanel);
        sl_contentPanel.putConstraint(SpringLayout.EAST, comboBox_ACTELS, -26, SpringLayout.EAST, contentPanel);
        contentPanel.add(comboBox_ACTELS);
}}

erreur:

java.sql.SQLException: Operation not allowed after ResultSet closed
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1073)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:987)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:982)
    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:927)
    at com.mysql.jdbc.ResultSetImpl.checkClosed(ResultSetImpl.java:794)
    at com.mysql.jdbc.ResultSetImpl.next(ResultSetImpl.java:7139)
    at tn.pack.ACTEL.Database.remplir_Jcomb(Database.java:94)
    at tn.pack.ACTEL.Region.<init>(Region.java:75)
    at tn.pack.ACTEL.Region.main(Region.java:41)

Answer 1

1) fill data from Db (use finally block for closing opened Objects, because this code is executed in all cases)

void whatever {

   Connection conn = null;
   Statement  st1 = null;

   try {
       st1 = conn.createStatement();

   }  catch (SQLException e) { 
      e.printStackTrace();
   }  finally { 
      try {
          st1.close();
          rs1.close();
          rs2.close();
          conn.close();
      } catch (SQLException ex) {
          //
      } 
   }
} 

2) inside Db Statement fill data to the (notice difference in API betweens Java6 / Java7),

• to the ComboBoxModel – JComboBox(ComboBoxModel aModel)/JComboBox(ComboBoxModel<E> aModel)

• to the arrays of Objects/Elements – JComboBox(Object[] items)/JComboBox(E[] items)

• to the Vector of Objects/Elements – JComboBox(Vector items)/JComboBox(Vector<E> items)

if Sql block ended then add array type to the JComboBox

EDIT:

import java.awt.*;
import java.awt.event.*;
import java.util.ArrayList;
import javax.swing.*;

public class ComboBoxListeners {

    private JFrame f;
    private JComboBox comboBox;
    private JLabel label = new JLabel();
    private DefaultComboBoxModel comboBoxModel = new DefaultComboBoxModel();

    public ComboBoxListeners() {
        comboBox = new JComboBox(comboBoxModel);
        comboBox.addItemListener(new ItemListener() {

            @Override
            public void itemStateChanged(ItemEvent e) {
                if ((e.getStateChange() == ItemEvent.SELECTED)) {
                    String str = comboBox.getSelectedItem().toString();
                    label.setText("Selected Value From JComboBox is :   " + str);
                }
            }
        });
        label.setPreferredSize(new Dimension(400, 30));
        JButton btnAdd = new JButton(new AbstractAction("Append Items") {

            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e) {
                addNewItemsFromDatabase();
            }
        });
        JButton btnRefresh = new JButton(new AbstractAction("Refresh Items") {

            private static final long serialVersionUID = 1L;

            @Override
            public void actionPerformed(ActionEvent e) {
                refreshItemsFromDatabase();
            }
        });
        f = new JFrame("ComboBox ItemListeners");
        f.setLayout(new GridLayout(0, 1, 15, 15));
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.add(comboBox);
        f.add(label);
        f.add(btnAdd);
        f.add(btnRefresh);
        f.setLocation(150, 150);
        f.pack();
        f.setVisible(true);
    }

    public void addNewItemsFromDatabase() {
        ArrayList<Integer> ageList = new ArrayList<Integer>();
        for (int i = 1; i <= 5; ++i) {
            ageList.add(i);
        }
        for (final Integer i : ageList) {
            EventQueue.invokeLater(new Runnable() {

                public void run() {// updates to the Swing GUI must be done on EDT
                    comboBoxModel.addElement(i);
                }
            });
        }
    }

    public void refreshItemsFromDatabase() {
        comboBoxModel = new DefaultComboBoxModel();
        ArrayList<Integer> ageList = new ArrayList<Integer>();
        for (int i = 1; i <= 5; ++i) {
            ageList.add(i);
        }
        for (final Integer i : ageList) {
            EventQueue.invokeLater(new Runnable() {

                public void run() {// updates to the Swing GUI must be done on EDT
                    comboBoxModel.addElement(i);
                }
            });
        }
        comboBox.setModel(comboBoxModel);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                ComboBoxListeners comboBoxListeners = new ComboBoxListeners();
            }
        });
    }
}