How can I tell when another class has been updated?

Question

I am writing a Java program that displays a combo box that get's it's information from a properties file. There is a settings class that will allow the user to update the names of the fields that are in the combo box. My problem is when the settings class is called and modified, I cannot figure out how to update the combo box with the new settings. I have a method that will repaint the entire panel and re-load the combo box. But I do not know how to activate that method when the "apply" button is pressed in the setting class.

Here is a rough and tough example of what I am trying to accomplish.

Main Class:

package testing;

import java.awt.*;
import java.awt.event.*;
import java.io.*;
import java.util.Properties;

import javax.swing.*;

public class testConfigLoad extends JFrame
{
    JButton apply = new JButton("Apply");
JButton set   = new JButton("Settings");

    Properties          config      = new Properties();
    FileInputStream     fis         = null;
    FileOutputStream    fos         = null;
    final String        configFile  = "config.properties";

    OptPanel opt;

    public testConfigLoad() throws IOException
    {
        super("Test Program");
        setSize(200, 200);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLayout(new FlowLayout());

        config();
        opt = new OptPanel(config);
        buildFrame();
    }

    public void buildFrame()
    {
        set.addActionListener(new setListener());

        add(opt);
        add(apply);
        add(set);

        setVisible(true);
    }

    public void config() throws IOException
    {
        try
        {
            fis = new FileInputStream(configFile);
            config.load(fis);
        }
        catch (FileNotFoundException e)
        {
            System.out.println("File not found");
        }
        finally
        {
            if (fis != null)
            {
                fis.close();
            }
        }       
    }

    private class setListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e)
        {
            settings set = new settings(config);
        }
    }

    public static void main(String[] args) throws IOException
    {
        new testConfigLoad();
    }
} 

Panel that needs to be Refreshed:

package testing;

import java.util.Properties;
import javax.swing.*;

public class OptPanel extends JPanel
{
    String[] opts;
    JLabel optLabel = new JLabel("Available Options");
    Properties config;

    public OptPanel(Properties p)
    {
        config = p;
        opts = new String[3];
        buildPanel();
    }

    public void buildPanel()
    {
        for (int i = 0; i < opts.length; i++)
        {
            opts[i] = config.getProperty("option." + i + ".name");
        }

        JComboBox optBox = new JComboBox(opts);

        add(optLabel);
        add(optBox);
    }

    public void refPanel()
    {
        removeAll();
        this.buildPanel();
        ((JPanel) this).revalidate();
        repaint();
    }
}

And the settings class:

package testing;

import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.Properties;
import javax.swing.*;

public class settings 
{
    Properties config;
    final String        configFile  = "config.properties";

    JFrame setFrame = new JFrame("Settings");
    JLabel opt1 = new JLabel("Option 1");
    JLabel opt2 = new JLabel("Option 2");
    JLabel opt3 = new JLabel("Option 3");
    JTextField text1 = new JTextField(15);
    JTextField text2 = new JTextField(15);
    JTextField text3 = new JTextField(15);
    JButton apply = new JButton("Apply");

    public settings(Properties p)
    {
        config = p;
        setFrame.setSize(275, 200);
        setFrame.setDefaultCloseOperation(JFrame.HIDE_ON_CLOSE);
        setFrame.setLayout(new FlowLayout());

        buildSetFrame();
    }

    public void buildSetFrame()
    {
        text1.setText(config.getProperty("option.0.name"));
        text2.setText(config.getProperty("option.1.name"));
        text3.setText(config.getProperty("option.2.name"));

        apply.addActionListener(new applyListener());

        setFrame.add(opt1);
        setFrame.add(text1);
        setFrame.add(opt2);
        setFrame.add(text2);
        setFrame.add(opt3);
        setFrame.add(text3);
        setFrame.add(apply);

        setFrame.setVisible(true);
    }

    private class applyListener implements ActionListener
    {
        public void actionPerformed(ActionEvent e) 
        {
            config.setProperty("option.0.name", text1.getText());
            config.setProperty("option.1.name", text2.getText());
            config.setProperty("option.2.name", text3.getText());

            FileOutputStream fos = null;
            try
            {
                fos = new FileOutputStream(configFile);
                config.store(fos, null);
            }
            catch (IOException f)
            {
                System.out.println("Error");
            }
            finally
            {
                if (fos != null)
                {
                    try
                    {
                        fos.close();
                    }
                    catch (IOException g)
                    {
                        System.out.println("Problem");
                    }
                }
            }
            setFrame.setVisible(false);

            //  This is where I need to pass something back to the 
                    //  testConfigLoad class to tell it to 
            //  run the refPanel method in the OptPanel class.
        }
    }
}   

The config file is named config.properties and looks like this:

option.2.name=two
option.1.name=one
option.0.name=zero

Answer 1

One approach would be to use a callback between the OptPanel and the settings class. You could extract out the functionality to add the elements to the properties JComboBox into its own method:

public void updateProperties(Properties p) {
    model.removeAllElements();
    for (String s: p.stringPropertyNames()) {
        model.addElement(p.getProperty(s));
    }
}

where model is a DefaultComboBoxModel. Then you could simply call

optPanel.updateProperties(config);

after successfully storing the properties.

---

Some Points:

• The preferred approach is not to use multiple JFrames. One option is to use a single JFrame with a modal dialog. See this discussion.
• Don't extend JFrame, rather use an instance directly.
• Java Naming conventions indicate that class names start with an uppercase letter, so settings would become Settings