JComboBox oddity

[Tim Manchester]

I've noticed something really quite odd about the JComboBox class. For background, let me say that I'm populating a JComboBox (let's say the instance name is cbMyCombo) with an array of objects (let's call them dataObj class) that have data members: String name, int id and a toString() method that returns the name. This is working ducky. What irks me is the way the the JComboBox behaves in the container's actionPerfomed() method.
If I do the following:
========================================

public void actionPerformed(ActionEvent e) {
    // WHO FIRED THIS EVENT
    System.out.println("** "+e.getSource.getActionCommand());
    // WHAT TYPE OF OBJECT DO WE GET FROM THE COMBO BOX?
    System.out.println("** "+cbMyCombo.getSelectedItem().getClass().getName());
}

========================================
I see that when I trigger an event on cbMyCombo, cbMyCombo.getSelectedItem() returns a dataObj object. In any other case (when any other control fires an event), cbMyCombo.getSelectedItem() returns a string! How can I ensure that I always get a dataObj returned from cbMyCombo.getSelectedItem()? This is really troubling. Thanks for looking at this issue.

Regards,

- DM_Tim

[jefff willis]

I'm not sure I'm following you on this one.

Here is a quick and dirty example of a combo class:

 
package testing;
 
import java.awt.*;
import javax.swing.*;
import java.awt.event.*;
import java.util.*;
 
public class comboTest extends JFrame
{
  private Vector v;
  JComboBox cbMyCombo;
 
  public comboTest()
  {
    super("duper");
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
    JMenuBar menuBar = createMenuBar();
    getContentPane().add(menuBar, BorderLayout.NORTH);
 
    JPanel panel = myPanel();
 
    getContentPane().add( panel );
    setSize(400, 200);
    show();
  }
 
  private JMenuBar createMenuBar()
  {
    JMenuItem addItem = null;
    JMenu menu        = null;
    // create the menu bar...
    JMenuBar bar = new JMenuBar();
 
    menu   = new JMenu("File");
 
    addItem = new JMenuItem("Add ", KeyEvent.VK_A);
 
    addItem.setAccelerator(KeyStroke.getKeyStroke(
        KeyEvent.VK_A, ActionEvent.ALT_MASK));
 
    JMenuItem exitItem = new JMenuItem("Exit", KeyEvent.VK_X);
 
    exitItem.setAccelerator(KeyStroke.getKeyStroke(
        KeyEvent.VK_X, ActionEvent.ALT_MASK));
 
    menu.add(addItem);
    menu.add(exitItem);
 
    bar.add(menu);
 
    exitItem.addActionListener(new java.awt.event.ActionListener()
    {
      public void actionPerformed(ActionEvent e) {
        System.exit(0);
      }
    });
 
    addItem.addActionListener(new java.awt.event.ActionListener()
    {
      public void actionPerformed(ActionEvent e)
      {
        // do nothing
      }
    });
    return bar;
  }
 
  public JPanel myPanel()
  {
    JPanel panel = new JPanel();
 
    Object[] comboData = new Object[10];
 
    for (int i = 0; i < 10; i++)
    {
      comboData[i] = new myClass(i);
    }
 
    cbMyCombo = new JComboBox( comboData );
 
    cbMyCombo.addActionListener( new ActionListener()
    {
      public void actionPerformed(ActionEvent e)
      {
        // WHO FIRED THIS EVENT
        System.out.println("** "+e.getSource());
        // WHAT TYPE OF OBJECT DO WE GET FROM THE COMBO BOX?
        System.out.println("** "+cbMyCombo.getSelectedItem().getClass().getName());
      }
    }
    );
 
    panel.add(cbMyCombo);
 
    return panel;
  }
 

 
  class myClass
  {
    String name = null;
    int id;
 
    public myClass(int val)
    {
      id = val;
      name = "hello world " + id;
    }
 
    public String toString() {  return name; }
 
  }
 

  public static void main(String[] args)
  {
    comboTest ct = new comboTest();
  }
}

How is this particular code exhibiting the problems you are having?

[Tim Manchester]

I see one major difference between my code and yours. My JPanel class implements ActionListener and all of my controls use that action listener:

cbMyCombo.addActionListener(this);

You on the other hand, create a unique action listener for each control:

cbMyCombo.addActionListener( new ActionListener() 
        {      
            public void actionPerformed(ActionEvent e) 
            {        
                // WHO FIRED THIS EVENT        
                System.out.println("** "+e.getSource());        
                // WHAT TYPE OF OBJECT DO WE GET FROM THE COMBO BOX?
        System.out.println("** "+cbMyCombo.getSelectedItem().getClass().getName());
            }    
        }    
    );

So I'm guessing that the behavior has to do with the fact that a JPanel that implements actionPerformed() for all it's controls is the cause. Does that sound right to you?

Regards,

- DM_Tim

[jefff willis]

So I'm guessing that the behavior has to do with the fact that a JPanel that implements actionPerformed() for all it's controls is the cause. Does that sound right to you?

Yeah, that sounds right to me too.

If it's not introducing a great deal of work, try doing it with an actionPerformed() method for each component.

[Tim Manchester]

Ya, I did duplicate your process in my code. That did the trick. That was a great tip. What torques me is that I could find no mention of this behavior on the Sun site or any of the books I've perused. You have been incredibly helpful. Thanks!

Regards,

- Tim Manchester