java - (Java) Appearing/Disappearing JLabel in a JPanel only disappears on resize

Question

I'm working in Java, and I have a JPanel in a JFrame. In that JPanel, among other things, I have a JLabel that I want to make appear and disappear at will. I've tried setting visibility to true/false, adding and removing it from the JFrame and JPanel, and, having looked online, I tried validate()ing and revalidate()ing ad infinitum. What can be done here to solve this problem?

Answer 1

In general, calling the setVisible method is sufficient to make a Swing component to be shown or hidden.

Just to be sure that it works, I tried the following:

public class Visibility {
  private void makeGUI() {
    JFrame f = new JFrame();
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    final JLabel l = new JLabel("Hello");
    final JButton b = new JButton("Hide Label");
    b.addActionListener(new ActionListener() {
      public void actionPerformed(ActionEvent e) {
        l.setVisible(false);
      }
    });

    f.getContentPane().setLayout(new BorderLayout());
    f.getContentPane().add(l, BorderLayout.CENTER);
    f.getContentPane().add(b, BorderLayout.SOUTH);
    f.setSize(200, 200);
    f.setLocation(200, 200);
    f.validate();
    f.setVisible(true);
  }

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

The above program is able to affect the visibility by clicking on a JButton.

Could it be a Threading Issue?

My next suspicion was that perhaps a Thread that is not on the event dispatch thread (EDT) may not be affecting the display immediately, so I added the following after initializing the JLabel and JButton.

Thread t = new Thread(new Runnable() {
  public void run() {
    while (true) {
      b.setVisible(!b.isVisible());

      try {
        Thread.sleep(100);
      } catch (InterruptedException e) { /* Handle exception /* }
    }
  }
});

t.start();

With the new Thread running, it changed the toggled the visibility of the JLabel every 100 ms, and this also worked without a problem.

Calling a Swing component off the event dispatch thread (EDT) is a bad thing, as Swing is not thread-safe. I was a little surprised it worked, and the fact that it works may just be a fluke.

Repaint the JPanel?

If the JLabel's visibility is only being affected on resizing, it probably means that the JLabel is being drawn only when the JPanel is being repainted.

One thing to try is to call the JPanel's repaint method to see if the visibility of the JLabel will change.

But this method seems to be just a band-aid to a situation, if the main cause is due to a thread off the EDT is attempting to make changes to the GUI. (Just as a note, the repaint method is thread-safe, so it can be called by off-EDT threads, but relying on repaint is a workaround than a solution.)

Try using SwingUtilities.invokeLater

Finally, probably the thing I would try is the SwingUtilities.invokeLater method, which can be called (and should only be called) from a thread running separate from the EDT, if it wants to affect the GUI.

So, the earlier Thread example should be written as:

Thread t = new Thread(new Runnable() {
  public void run() {
    while (true) {
      try {
        SwingUtilities.invokeLater(new Runnable() {
          public void run() {
            b.setVisible(!b.isVisible());
          }
        });
      } catch (Exception e1) { /* Handle exception */ }

      try  {
        Thread.sleep(100);
      } catch (InterruptedException e) { /* Handle exception */ }
    }
  }
});

t.start();

If the change to the GUI is indeed occurring on a separate thread, then I would recommend reading Lesson: Concurrency in Swing from The Java Tutorials in order to find out more information on how to write well-behaving multi-threaded code using Swing.

Answer 2

setVisible() or removing it should work fine, make sure you are doing it from the event dispatch thread though. There are utility methods in EventQueue for running blocks in that thread.

http://helpdesk.objects.com.au/java/how-do-i-update-my-gui-from-any-thread

You would need to call revalidate() on the parent JPanel if you need its components to be re-laid out.

If you can post an example that demonstrates the problem I can have a look at it for you.