Implementing Custom Event - non-static referencing in static context error

Question

Hi,
I'm implementing a custom event, and I have problems adding my custom listeners to objects. I can't compile because I'm referencing a non-static method (my custom addListener method ) from a static context (a JFrame which contains static main).
However, the same error occurs even if I try to add the custom listener to another class without the main function.
Q1. Is the way I'm adding the listener wrong? Is there a way to resolve the non-static referencing error?
Q2. From the examples online, I don't see people adding the Class name in front of addListener.
Refering to the code below, if I remove "Data." in front of addDataUpdatelistener, I get the error:
cannot resolve symbol method addDataUpdateListener (<anonymous DataUpdateListener>)
I'm wondering if this is where the error is coming from.
Below is a simplified version of my code. Thanks in advance!
Cindy
//dividers indicate contents are in separate source files
//DataUpdateEvent Class
public class DataUpdateEvent extends java.util.EventObject
     public DataUpdateEvent(Object eventSource)
          super(eventSource);
//DataUpdateListener Interface
public interface DataUpdateListener extends java.util.EventListener
  public void dataUpdateOccured(DataUpdateEvent event);
//Data Class: contains data which is updated periodically. Needs to notify Display frame.
class Data
//do something to data
//fire an event to notify listeners data has changed
fireEvent(new DataUpdateEvent(this));
  private void fireEvent(DataUpdateEvent event)
       // Make a copy of the list and use this list to fire events.
       // This way listeners can be added and removed to and from
       // the original list in response to this event.
       Vector list;
       synchronized(this)
            list = (Vector)listeners.clone();
       for (int i=0; i < list.size(); i++)
            // Get the next listener and cast the object to right listener type.
           DataUpdateListener listener = (DataUpdateListener) list.elementAt(i);
           // Make a call to the method implemented by the listeners
              // and defined in the listener interface object.
              listener.dataUpdateOccured(event);
           System.out.println("event fired");
public synchronized void addDataUpdateListener(DataUpdateListener listener)
     listeners.addElement(listener);
  public synchronized void removeDataUpdateListener(DataUpdateListener listener)
     listeners.removeElement(listener);
//Display Class: creates a JFrame to display data
public class Display extends javax.swing.JFrame
     public static void main(String args[])
          //display frame
          new Display().show();
     public Display()
     //ERROR OCCURS HERE:
     // Non-static method addDataUpdateListener (DataUpdateListener) cannot be referenced from a static context
     Data.addDataUpdateListener(new DataUpdateListener()
         public void dataUpdateOccured(DataUpdateEvent e)
             System.out.println("Event Received!");
//-----------------------------------------------------------

Answer 1

Calling
    Data.someMethodName()is referencing a method in the Data class whose signature includes the 'static' modifier and
might look something like this:
class Data
    static void someMethodName() {}What you want is to add the listener to an instance of the Data class. It's just like adding
an ActionListener to a JButton:
    JButton.addActionListener(new ActionListener()    // won't work
    JButton button = new JButton("button");           // instance of JButton
    button.addActionListener(new ActionListener()     // okay
import java.awt.*;
import java.awt.event.*;
import java.util.Vector;
import javax.swing.*;
public class EventTest extends JFrame
    Data data;
    JLabel label;
    public EventTest()
        label = getLabel();
        data = new Data();
        // add listener to instance ('data') of Data
        data.addDataUpdateListener(new DataUpdateListener()
            public void dataUpdateOccured(DataUpdateEvent e)
                System.out.println("Event Received!");
                label.setText("count = " + e.getValue());
        getContentPane().add(label, "South");
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setSize(300,200);
        setLocation(200,200);
        setVisible(true);
    private JLabel getLabel()
        label = new JLabel("  ", JLabel.CENTER);
        Dimension d = label.getPreferredSize();
        d.height = 25;
        label.setPreferredSize(d);
        return label;
    public static void main(String[] args)
        new EventTest();
* DataUpdateEvent Class
class DataUpdateEvent extends java.util.EventObject
    int value;
    public DataUpdateEvent(Object eventSource, int value)
        super(eventSource);
        this.value = value;
    public int getValue()
        return value;
* DataUpdateListener Interface
interface DataUpdateListener extends java.util.EventListener
    public void dataUpdateOccured(DataUpdateEvent event);
* Data Class: contains data which is updated periodically.
* Needs to notify Display frame.
class Data
    Vector listeners;
    int count;
    public Data()
        listeners = new Vector();
        count = 0;
        new Thread(runner).start();
    private void increaseCount()
        count++;
        fireEvent(new DataUpdateEvent(this, count));
    private void fireEvent(DataUpdateEvent event)
        // Make a copy of the list and use this list to fire events.
        // This way listeners can be added and removed to and from
        // the original list in response to this event.
        Vector list;
        synchronized(this)
            list = (Vector)listeners.clone();
        for (int i=0; i < list.size(); i++)
            // Get the next listener and cast the object to right listener type.
            DataUpdateListener listener = (DataUpdateListener) list.elementAt(i);
            // Make a call to the method implemented by the listeners
            // and defined in the listener interface object.
            listener.dataUpdateOccured(event);
        System.out.println("event fired");
    public synchronized void addDataUpdateListener(DataUpdateListener listener)
        listeners.addElement(listener);
    public synchronized void removeDataUpdateListener(DataUpdateListener listener)
        listeners.removeElement(listener);
    private Runnable runner = new Runnable()
        public void run()
            boolean runit = true;
            while(runit)
                increaseCount();
                try
                    Thread.sleep(1000);
                catch(InterruptedException ie)
                    System.err.println("interrupt: " + ie.getMessage());
                if(count > 100)
                    runit = false;
}