How to take String of a JComboBox Event handler into a variable for query?

Question

In the JCombobox are possible Routes for a train.I want to get the value of JCombobox based on the event handler and then use it in a class that uses a Database. This value will be a parameter for a Mysql query.I have succeeded in getting it,but can not use it. I'm not very experienced in java, I am doing something wrong. I have searched the site for similar problems, seen them but not understood them clearly.What am I missing?

//imports...

public class Cashier extends JFrame{

      //here is main...
 public Cashier(){

      //some Window code...

     final  String[] Routes = {"--Select--", "value1",....."valueN" };
     final JComboBox comboBox = new JComboBox(Routes);

     comboBox.addActionListener(new ActionListener() {
       /*-->*/ public String d; 
            public void WhereTo(String dest){

                 this.d=dest;                 
                   System.out.println(d); 

          // comes out correct!           
      /*I want d for use in DBaccess class as query parameter, by invoking 
        GetRoute()*/         
}            

         public void actionPerformed(ActionEvent e) {
                 int val = comboBox.getSelectedIndex();
                         this.d= Routes[val];    
              WhereTo(d);

        }         
     });
            comboBox.setBounds(165, 124, 130, 23);     
    contentPane.add(comboBox);

   //this method will be used by DBaccess

   public String GetRoute(){

     return d;                    
    }

    //More Objects...
  }
}

This is my DBaccess class where i want to use the string d, probably by invoking Get Route() of Cashier.

 public DBaccess extends Cashier{

  //connection code....

 // Executing the query
  System.out.println("Creating statement...");
  stmt = conn.createStatement();
  String sql; 

 //probably like this...   
 String go = Cashier.GetRoute();

  sql = "SELECT FROM reservations WHERE destination='"+go+"'";
  ResultSet rs = stmt.executeQuery(sql);
  }

Please try to describe your exact problem. Does your code compile? Does it run? Does it produce a result? Is this result different from what you expected? — PM 77-1, Jan 11 '14 at 19:01
neoprez --- I declared as you said but also had to make it static, inluding GetRoute. Yes it compiles, now! but the value is null in DBaccess though. Cannot make the query! — KRF, Jan 11 '14 at 19:28
All in all, I am trying to pass d to DBaccess to make the query — KRF, Jan 11 '14 at 19:35
Java GUIs might have to work on a number of platforms, on different screen resolutions & using different PLAFs. As such they are not conducive to exact placement of components. To organize the components for a robust GUI, instead use layout managers, or [combinations of them](http://stackoverflow.com/a/5630271/418556), along with layout padding & borders for [white space](http://stackoverflow.com/q/17874717/418556). — Andrew Thompson, Jan 11 '14 at 19:40

score 2 · Accepted Answer · edited May 23 '17 at 12:03

Here:

String go = Cashier.GetRoute();

This method is not static and it can't be invoked in this way. In any case it's a poor design choice. Consider providing DBaccess class with a setter for the desired route. The actionPerformed() implementation should look like this:

@override
public void actionPerformed(ActionEvent e) {
    JComboBox comboBox = (JComboBox)e.getSource();
    String selectedRoute = (String)comboBox.getSelectedItem();
    DBaccess dbAccess = new DBaccess();
    dbAccess.setRoute(selectedRoute);
    dbAccess.setVisible(true);
}

Some tips to help you:

Cashier extends JFrame: don't extend from swing components if you won't add some Swing related functionality. You can use a simple variable instead.
DBaccess extends Cashier (which extends from JFrame): a tipical Swing application should have only one JFrame. You should use JDialog instead. See The Use of Multiple JFrames, Good/Bad Practice?
Provide DBaccess class with a method to set the desired route from another class (Cashier).
The query you're trying to execute is vulnerable to SQL Injection attacks. Take a look to PreparedStatement to avoid this.
If DBaccess class will display the query results using Swing components, then you may want to take a look to SwingWorker class to do the database call in a background thread and update Swing components in the Event Dispatch Thread. Take a look to Concurrency in Swing trail for more details.

neoprez · Answer 2 · 2014-01-11T19:44:39.707

Make d a Global class variable. By saying this I mean, declare it right after the class name.

public class Cashier extends JFrame{
private String d;
  //here is main...
 public Cashier(){
//some Window code...

 final  String[] Routes = {"--Select--", "value1",....."valueN" };
 final JComboBox comboBox = new JComboBox(Routes);

 comboBox.addActionListener(new ActionListener() {
   /*-->*/ public String d; 
        public void WhereTo(String dest){

             Cashier.this.d=dest;   //to change d value              
               System.out.println(d); 

      // comes out correct!           
  /*I want d for use in DBaccess class as query parameter, by invoking 
    GetRoute()*/

}

     public void actionPerformed(ActionEvent e) {
             int val = comboBox.getSelectedIndex();
                     Cashier.this.d= Routes[val];   //to change d value 
          WhereTo(d);

    }         
 });
        comboBox.setBounds(165, 124, 130, 23);     
contentPane.add(comboBox);

//this method will be used by DBaccess

public String GetRoute(){

 return d;                    
}

//More Objects...

} }

Had to change it to (private static d) and the method to (private static String GetRoute()) ,ok. But then i have the d declared in the ActionListener and the static one ,have to connect them some how... — KRF, Jan 11 '14 at 19:57
I;m trying to find the way to pass d from one class to the other, Cashier to DBaccess to make the query. — KRF, Jan 11 '14 at 20:01

How to take String of a JComboBox Event handler into a variable for query?

2 Answers2