Java SwingWorker create

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.List;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JProgressBar;
import javax.swing.SwingUtilities;
import javax.swing.SwingWorker;

public class Main extends JFrame implements ActionListener {
   private final JProgressBar bar = new JProgressBar(0, 100);
   private final JButton btn = new JButton("Start Calculation");
   private SwingWorker<Boolean, Integer> backgroundTask = null;
   /*from   w  w  w . j  a  v  a  2 s .  c  om*/
   public Main() {
      super();
      setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      setTitle("Progress Tracker");
      setLayout(new FlowLayout());
      btn.addActionListener(this);
      add(bar);
      add(btn);
      pack();
      setVisible(true);
   }
   
   public SwingWorker<Boolean, Integer> makeBackgroundTask(final long total) {
      SwingWorker<Boolean, Integer> task = new SwingWorker<Boolean, Integer>(){
         @Override
         protected Boolean doInBackground() throws Exception {
            btn.setText("Stop Calculation");
            for (long i = 0; i < total; i++) {
               if (isCancelled()) {
                  bar.setValue(0);
                  return false;
               }
               int perc = (int) 
                     (i * (bar.getMaximum() - bar.getMinimum())
                           / total);
               publish(perc);
            }
            return true;
         }
         @Override
         protected void process(List<Integer> percs) {
            for (int perc : percs)
               if (bar.getValue() < perc)
                  bar.setValue(perc);
         }
         @Override
         public void done() {
            btn.setText("Start Calculation");
            backgroundTask = null;
         }
      };
      return task;
   }

   
   @Override
   public void actionPerformed(ActionEvent arg0) {
      if (backgroundTask == null) {
         backgroundTask = makeBackgroundTask(1000000000);
         backgroundTask.execute();
      } else {
         backgroundTask.cancel(true);
      }
   }
   
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() { new Main(); }
      });
   }
}