Return Values & JProgressbar

Question

I got a problem with setting a value to a JProgressBar which i got from inside a type returning method. I know I have to do multi threading but I´m really new to this topic and really don´t know how to implement this.

I try to briefly explain my dilemma in code:

This is my data returning method to count lines in a String (e.g. from a JTextArea or whatsoever).

    public static int countLines(String s) {
        int count = 0;

        String[] words = s.split("\n");

        for(String i: words) {
            count++;
        }
        return count;
    }

What I want to add is a method inside this method that sets my JProgressBar that I created in my JFrame to the value count. E.g. setProgress(count);

Is this even possible? Because i tried several ways of doing this and no matter what, the progressbar only updates AFTER the return value has been sent out.

Do I have to run this method in an own Thread or just the progress setting method? or both?

Cheers!

Answer 1

Do I have to run this method in an own Thread or just the progress setting method? or both?

Both tasks should run in separate threads: business logic (in this case word's count) must be performed in a background thread while progress bar update must run in the Swing thread, also known as Event Dispatch Thread (EDT).

IMO the best way to accomplish this is by using a SwingWorker:

• Move all count logic to doInBackground() implementation.
• Attach a PropertyChangeListener to the swing worker to listen for progress property and update the progress bar within this listener.
• Use setProgress() inside doInBackground to set the worker's progress and fire a property change event notifying your listener.

There are a lot of examples here in SO, just take a look to swingworker tag. Also, consider this snippet:

SwingWorker<Void, Void> worker = new SwingWorker<Void, Void>() {

    // Following code is performed in a background thread

    @Override
    protected Void doInBackground() throws Exception {

        String[] words = string.split(System.lineSeparator());
        int totalWords = words.length;

        for (int count = 0; count < totalWords; count++) {
            int progress = (count + 1) * 100 / totalWords;
            setProgress(progress);
        }
        return null;
    }
};

worker.addPropertyChangeListener(new PropertyChangeListener() {

    // Following code is performed in the EDT, as all event handling code is.
    // Progress bar update must be done here, NOT within doInBackground()

    @Override
    public void propertyChange(PropertyChangeEvent evt) {
        if ("progress".equals(evt.getPropertyName())) {
            Integer progress = (Integer)evt.getNewValue();
            progressBar.setValue(progress); 
        }
    }
});

Answer 2

Using a SwingWorker you can accomplish it this way:

public class GuiTest extends JFrame
{
   private static final long serialVersionUID = 1L;

   private JProgressBar progressBar;
   private int stringLength;

   GuiTest()
   {
      // Length of your string
      stringLength = yourString.length();

      progressBar = new JProgressBar(0, stringLength);
      ProgressBarWorker worker = new ProgressBarWorker(); 
      worker.addPropertyChangeListener(new PropertyChangeListener() {

         @Override
         public void propertyChange(PropertyChangeEvent evt) {
            if ("progress".equals(evt.getPropertyName())) {
               Integer progress = (Integer)evt.getNewValue();
               progressBar.setValue(progress); 
            }
         }
      });

      worker.execute();

      this.getContentPane().add(progressBar);
      this.setVisible(true);
      this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   }

   private class ProgressBarWorker extends SwingWorker<Void, Void>
   {
       private void countLines(String s) {
          int count = 0;

          String[] words = s.split("\n");

          for(String i: words) {
              count++;
              setProgress(count);
           }
      }

      @Override
      protected Void doInBackground() throws Exception {
        countLines();
        return null;
      }
    }

Answer 3

If you call paintImmediately on the JProgressBar, then you can update it while your code is running on the event dispatch thread. The other solution is to start another thread.

If the user is supposed to interact with the GUI while your counting code is running, then starting a worker thread (with or without SwingWorker) is the only solution. If the user must wait anyway, and blocking the GUI is not a problem, then the (much simpler) paintImmediately is fine.

Answer 4

I got stuck with this too couple of weeks ago. I found this site, which helped me a lot on understanding the topic.

All I can say, is that is recommended to start your GUI Applications with the Swing-Library like this:

    public static void main(String[] args) {
    SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        MyApplication app = new MyApplication();
        app.setVisible(true);
      }
    });
  }