IllegalMonitorStateException?

Following program is based on Sun's Flipper2.java example
(http://java.sun.com/docs/books/tutorial/uiswing/examples/QandE/Flipper2Project/src/QandE/Flipper2.java).

Instead of slowing the background task, I want to suspend the background
task until I get a signal from the user interface to flip another coin.
In the background task I replaced sleep() with wait(). Added a JButton
(wakeupBotton) and extended the actionPerformed method to issue a
notifyAll() statement to wakeup the background task, when the
wakeupBotton is clicked.

Instead of waking up the wait(), notifyAll() causes an
IllegalMonitorStateException.

I looked up the API and found notifyAll() should only be called by a
thread that is the owner of this object's monitor. A thread becomes the
owner of the object's monitor in one of three ways:

* By executing a synchronized instance method of that object.
* By executing the body of a synchronized statement that synchronizes on
the object.
* For objects of type Class, by executing a synchronized static method
of that class.

Only one thread at a time can own an object's monitor.

Throws: IllegalMonitorStateException - if the current thread is not the
owner of this object's monitor.

In the context of my program, I don't understand how to do that. What
modifications do I need to make this work?

Here is the program:

package concurrency;

import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.List;
import java.util.Random;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JTextField;
import javax.swing.SwingUtilities;
import javax.swing.SwingWorker;
import javax.swing.border.Border;

public class Flipper extends JFrame implements
          ActionListener {
    private static final long serialVersionUID = 1L;
    private final Border border = BorderFactory
             .createLoweredBevelBorder();
    private final GridBagConstraints constraints;
    private FlipTask flipTask;
    private final JTextField headsText, totalText, devText;
    private final JButton startButton, stopButton,
             wakeupButton;

    public void actionPerformed(final ActionEvent e) {
       final String eventActionCommand = ((JButton) e
                .getSource()).getActionCommand();
       if (startButton.getActionCommand().equals(
                eventActionCommand))
       /* Handle Start Button */
       {
          startButton.setEnabled(false);
          stopButton.setEnabled(true);
          wakeupButton.setEnabled(true);
          (flipTask = new FlipTask()).execute();
       } else if (stopButton.getActionCommand().equals(
                eventActionCommand))
       /* Handle Stop Button */
       {
          startButton.setEnabled(true);
          stopButton.setEnabled(false);
          wakeupButton.setEnabled(false);
          flipTask.cancel(true);
          flipTask = null;
       } else
          /* Handle Wake Up Button (signal flipTask to wakeup) */
          notifyAll();
    }

    private class FlipTask extends
             SwingWorker<Void, FlipPair> {
       @Override
       protected Void doInBackground() {
          long heads = 0;
          long total = 0;
          final Random random = new Random();
          while (!isCancelled()) {
             total++;
             if (random.nextBoolean())
                heads++;
             publish(new FlipPair(heads, total));
             /* Wait for signal from wakeupButton to continue */
             try {
                wait();
             } catch (final InterruptedException e) {
                /* Ignore */
             }
          }
          return null;
       }

       @Override
       protected void process(final List<FlipPair> pairs) {
          final FlipPair pair = pairs.get(pairs.size() - 1);
          headsText.setText(String.format("%d", pair.heads));
          totalText.setText(String.format("%d", pair.total));
          devText.setText(String.format("%.10g",
                   (double) pair.heads / (double) pair.total
                            - 0.5));
       }
    }

    private static class FlipPair {
       private final long heads, total;

       FlipPair(final long heads, final long total) {
          this.heads = heads;
          this.total = total;
       }
    }

    public Flipper() {
       super("Flipper");
       setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
       // Make text boxes
       getContentPane().setLayout(new GridBagLayout());
       constraints = new GridBagConstraints();
       constraints.insets = new Insets(3, 10, 3, 10);
       headsText = makeText();
       totalText = makeText();
       devText = makeText();
       // Make buttons
       startButton = makeButton("Start");
       wakeupButton = makeButton("Wake Up");
       wakeupButton.setEnabled(false);
       stopButton = makeButton("Stop");
       stopButton.setEnabled(false);
       // Display the window.
       pack();
       setVisible(true);
    }

    public static void main(final String[] args) {
       SwingUtilities.invokeLater(new Runnable() {
          public void run() {
             new Flipper();
          }
       });
    }

    private JButton makeButton(final String caption) {
       final JButton b = new JButton(caption);
       b.setActionCommand(caption);
       b.addActionListener(this);
       getContentPane().add(b, constraints);
       return b;
    }

    private JTextField makeText() {
       final JTextField t = new JTextField(10);
       t.setEditable(false);
       t.setHorizontalAlignment(JTextField.RIGHT);
       t.setBorder(border);
       getContentPane().add(t, constraints);
       return t;
    }
}

--
Dan Maloney