How to add a progress bar?

问题:

I have been trying to understand how to add a progress bar, I can create one within the GUI I am implementing and get it to appear but even after checking through http://docs.oracle.com/javase/tutorial/uiswing/components/progress.html I am still no clearer on how I can set a method as a task so that I can create a progress bar for running a method. Please can someone try to explain this to me or post an example of a progress bar being used in the GUI with a task being set as a method. Thanks.

回答1:

Maybe i can help you with some example code:

public class SwingProgressBarExample extends JPanel {

  JProgressBar pbar;

  static final int MY_MINIMUM = 0;

  static final int MY_MAXIMUM = 100;

  public SwingProgressBarExample() {
    // initialize Progress Bar
    pbar = new JProgressBar();
    pbar.setMinimum(MY_MINIMUM);
    pbar.setMaximum(MY_MAXIMUM);
    // add to JPanel
    add(pbar);
  }

  public void updateBar(int newValue) {
    pbar.setValue(newValue);
  }

  public static void main(String args[]) {

    final SwingProgressBarExample it = new SwingProgressBarExample();

    JFrame frame = new JFrame("Progress Bar Example");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setContentPane(it);
    frame.pack();
    frame.setVisible(true);

    // run a loop to demonstrate raising
    for (int i = MY_MINIMUM; i <= MY_MAXIMUM; i++) {
      final int percent = i;
      try {
        SwingUtilities.invokeLater(new Runnable() {
          public void run() {
            it.updateBar(percent);
          }
        });
        java.lang.Thread.sleep(100);
      } catch (InterruptedException e) {
        ;
      }
    }
  }
}

回答2:

Your question is a bit vague, but it sounds to me like you want the progress bar to show progress for a specific running method, which I'll call the "work()" method. Unfortunately, there's no way to just pass a reference to your method to a progress bar - your method needs to explicitly tell the progress bar what to display. Here's what I would do:

1. Make the progress bar's reference available to work() - either pass it in as an argument to work(), or provide an accessor method that your code in work() can call to get a reference to the progress bar.

2. Inside work(), after you've obtained a reference to the progress bar (which I'll call "pb", call pb.setMinimum(0) and pb.setMaximum(n) where n is the number of steps your method has to get through.

3. As your method completes each step, call pb.setValue(pb.getValue()+1);

4. At the end of your method, call pb.setValue(0); to reset the progress bar prior to returning.

Also, if you want your progress bar to display a String message, you first have to call pb.setStringPainted(true), then subsequent calls to pb.setString(string) will show up on the progress bar.

回答3:

See my answer on another SO question which includes an example of a JProgressBar which gets updated by using a SwingWorker. The SwingWorker is used to execute a long running task in the background (in case of the example it is just a regular Thread.sleep) and report on progress at certain intervals.

I would also strongly suggest to take a look at the Swing concurrency tutorial for more background info on why you should use a SwingWorker when performing long-running tasks which interfere with the UI.

A similar example as the one I posted is available in the Swing tutorial about JProgressBars, which it also worth looking at