Race Condition: The Code

For the second example, I created another class that extends JFrame.

This time, however, I put in two JButtons and a JProgressBar. One of the buttons starts a background timer that updates the progress bar, and the second button stops the background timer. The error in this code is quite a bit more subtle than the first.

See if you can spot the issues with it. Again: do your best to identify the problem before you peek at the answer!

package com.zarrastudios.example;

import javax.swing.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.awt.*;

public class FrameExample2 extends JFrame {
  private JButton activate, shutdown;
  private JProgressBar progressBar;
  private Ticker ticker;

  public FrameExample2() {
    super("Frame Example 2");
    initAndLayoutComponents();
    initListeners();
    pack();
  }

  private void initListeners() {
    activate.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        ticker = new Ticker();
        ticker.start();
        activate.setEnabled(false);
        shutdown.setEnabled(true);
      }
    });
    shutdown.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent evt) {
        ticker.shutdown();
        activate.setEnabled(true);
        shutdown.setEnabled(false);
      }
    });
    addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent event) {
        System.exit(0);
      }
    });
  }

  private void initAndLayoutComponents() {
    JPanel main = new JPanel(new BorderLayout());
    JPanel north = new JPanel(new FlowLayout());
    JPanel south = new JPanel(new FlowLayout());
    activate = new JButton("Activate");
    shutdown = new JButton("Shutdown");
    shutdown.setEnabled(false);
    progressBar = new JProgressBar();

    north.add(activate);
    north.add(shutdown);

    south.add(progressBar);

    main.add(north, BorderLayout.NORTH);
    main.add(south, BorderLayout.SOUTH);

    setContentPane(main);
  }

  public static void main(String args[]) {
    FrameExample2 fe2 = new FrameExample2();
    fe2.addWindowListener(new WindowAdapter() {
      public void windowClosing(WindowEvent event) {
        System.exit(0);
      }
    });
    fe2.setVisible(true);
  }

  class Ticker extends Thread {
    private boolean running = true;

    public void shutdown() {
      running = false;
      interrupt();
    }

    public void run() {
      System.out.println("Starting counter");
      for (int counter = 1; counter <= 10; counter++) {
        if (!running) break;
        progressBar.setValue(counter * 10);
        try {
          Thread.sleep(1000);
        } catch (Exception e) {
          //Expected exception
          System.out.println("Sleep interrupted");
        }
      }
      System.out.println("Counter is finished");
    }
  }
}