public static void main(String[] args){
        
        
         // --------ВЫВОД ДАННЫХ--------
     try {
     String query = "SELECT operator_Name FROM operators " ;// получаем данные из таблицы
        
     try {
                // opening database connection to MySQL server
                con = DriverManager.getConnection(url, user, password);
 
                // getting Statement object to execute query
                stmt = con.createStatement();
 
               // executing SELECT query
                rs = stmt.executeQuery(query);
                    
               while (rs.next()) {
                    
                    textArea.append (rs.getString(1));
                    textArea.setEditable(false);
                    
                
                }
 
            } catch (SQLException sqlEx) {
                sqlEx.printStackTrace();
            } finally {
                //close connection ,stmt and resultset here
                try { con.close(); } catch(SQLException se) { /*can't do anything */ }
                try { stmt.close(); } catch(SQLException se) { /*can't do anything */ }
                try { rs.close(); } catch(SQLException se) { /*can't do anything */ }
            }
            }catch (Exception e){ JOptionPane.showMessageDialog(null, "Ошибка получения данных.");
        }
    
    JFrame frame = new JFrame("Combo Box Demo");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(500, 200);
    frame.setResizable(false);
    frame.setLocationRelativeTo(null);
    frame.setLayout(new FlowLayout());
 
    Vector<String> comboBoxItems=new Vector<String>();
    
   
    
    @SuppressWarnings({ "rawtypes" })
    final DefaultComboBoxModel model = new DefaultComboBoxModel(comboBoxItems);
    
    @SuppressWarnings({ "rawtypes" })
    JComboBox comboBox = new JComboBox(model);
    frame.add(comboBox);
  
    
   
    model.addElement(textArea.getText());
    
    
    /*JButton button = new JButton("Add new element in combo box");
    frame.add(button);
    button.addActionListener(new ActionListener() {
        
        @SuppressWarnings("unchecked")
        public void actionPerformed(ActionEvent ae) {
            model.addElement(textArea.getText());
        }
   });*/
 
    frame.setVisible(true);
            }
    
}

final DefaultComboBoxModel model = new DefaultComboBoxModel(comboBoxItems);

 model.addElement(textArea.getText());