JPanel's child components paint/layout problem

Question

I'm having a problem that when my frame is shown (after a login dialog) the buttons are not on correct position, then in some miliseconds they go to the right position (the center of the panel with border layout).

-- update

In my machine, this SSCCE shows the layout problem in 2 of 10 times I run it:

import java.awt.BorderLayout;
import java.awt.Dimension;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class TEST {

    public static void main(String[] args) throws Exception {

    SwingUtilities.invokeAndWait(new Runnable() {

        public void run() {

        System.out.println("Debug test...");

        JPanel btnPnl = new JPanel();
        btnPnl.add(new JButton("TEST"));

        JFrame f = new JFrame("TEST");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().setLayout(new BorderLayout());
        f.getContentPane().add(btnPnl);
        f.setPreferredSize(new Dimension(800, 600));
        f.pack();
        f.setExtendedState(JFrame.MAXIMIZED_BOTH);
        f.setVisible(true);

        System.out.println("End debug test!");

        }
    });

    }

}

The button first appers in the up-left, and then it goes to the center. Is it a java bug?

--update

Looks like the SSCCE don't show the problem for everyone that is trying. Maybe it's my computer performance problem. I still think Java Swing is creating new threads for make the layout behind the scenes. But I'm not sure.

--update

The problem only occur with the f.setExtendedState(JFrame.MAXIMIZED_BOTH);

Answer 1

I would guess that you need to call pack() before making your frame visible.

If you are calling the above code not on the event thread then you have a race condition and all bets are off - you can only manipulate the GUI from the EDT (event dispatch thread).

---

EDIT: I tried your SSCCE on my system and it is not exhibiting the behavior you are seeing. I tried it about 50 times, and also tried creating 10 windows by looping your code. I am running 1.6.0_18 on Windows XP SP3.

Answer 2

Maybe you are missing a frameThatContainsCentralPanel.pack()?

Answer 3

Well, if it works with a SSCCE, then you've proven the problem isn't with the basic logic. There must be something different between the SSCCE and your real code. Since we don't have access to your real code you need to do the debugging yourself to see what the difference is.

However, in this case a better solution is to use a CardLayout, which is designed to let you swap panels easily. Read the Swing tutorial for a working example.

Or anther approach is to use a "login dialog". Once the login is successfull, you display your main frame with the panel for your application.

Answer 4

Hi,

If I copied your code, I had the same problem, but not so heavy.

I solved it by setting a preferred size for your frame before packing. So:

import java.awt.Dimension;

System.out.println("Debug test...");

JPanel btnPnl = new JPanel();
btnPnl.add(new JButton("TEST"));

JFrame f = new JFrame("TEST");
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.getContentPane().setLayout(new BorderLayout());
f.getContentPane().add(btnPnl);
f.setPreferredSize(new Dimension(800, 600));
f.pack();
f.setVisible(true);
System.out.println("End debug test!");

I'm running on Linux.

It is indeed strange... I'm sure it is something about the size of all the containers in the swing tree.

Answer 5

Your problem intrigued me. After some investigation I think I confirmed something that I recall about setting the window state (maximized, restored, etc) which is that setting the state is a request to the operating system and is left to the whim of the OS to process the request. This means it is asynchronous, or at least done later, after you set it. I confirmed using logging and adding resize listeners where you can see that the frame is resized after your block of code exits. Because of this, the pack() will layout components to their preferred size. So imagine the frame being sized to 800x600 and components positioned as such (button centered horizontally around 400). Then later, the OS changes the size of the frame to full screen (e.g. 1024x768) - for a moment, you'll see the button still at 400. Then the frame processes the new size and re-lays out components and centers the button at around 512. So you'll see the flicker as it transitions during this process. Perhaps a solution is to NOT pack() - it will remain at a size of zero and user will see minimum flicker.

Try this change first:

// pack()

If that looks good then you might have the next problem...if the user clicks the restore button, the whole frame shrinks into a black hole. So try calling pack AFTER the frame has been predictably resized due to the maximize. Something like this:

f.addComponentListener( new ComponentAdapter( ComponentEvent e ) {
  public void componentResized( Component) {
     if( f.getSize().getWidth() > 0 ) {
        e.getComponent().removeComponentListener( this );
        ((JFrame)e.getComponent()).pack();
     }
  }
}

So if the user later clicks restore button the frame will have a nicely packed size ready to go.

--Update

OK, one last attempt. While I think my description of the problem has some truth, my solutions offered did nothing. Here's one last attempt. Remove pack() and setPreferredSize() and replace with setting the size to the screen size. This seems to reduce the flicker greatly on my system. This is because there should be no difference between the initial layout and the maximized layout done later. You can see this if you switch between restore and maximized. Although I still see a very slight flicker when switching the two, at least it seems to look better when first displayed.

import java.awt.BorderLayout;
import java.awt.Dimension;
import java.awt.Toolkit;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

public class TEST {

    public static void main(String[] args) throws Exception {

    SwingUtilities.invokeAndWait(new Runnable() {

        public void run() {

        System.out.println("Debug test...");

        JPanel btnPnl = new JPanel();
        btnPnl.add(new JButton("TEST"));

        JFrame f = new JFrame("TEST");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().setLayout(new BorderLayout());
        f.getContentPane().add(btnPnl);
//        f.setPreferredSize(new Dimension(800, 600));
//        f.pack();
        f.setSize( Toolkit.getDefaultToolkit().getScreenSize() );
        f.setExtendedState(JFrame.MAXIMIZED_BOTH);
        f.setVisible(true);

        System.out.println("End debug test!");

        }
    });

-Mike

Answer 6

I would expect the frame to be maximised both before it is shown, but after checking this I'm sure that on linux frame is maximised after it has been displayed. You can make the frame size equal to a Screen size before calling setVisible, or you can make the components invisible until you know that the it's got preferred initial size. Here is modified sample which shows the elements after the frame has been activated (on linux activated event comes late enough to not show the "jumping button"):

import javax.swing.JButton;

import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.SwingUtilities;

public class TEST {

public static void main(String[] args) throws Exception {

    SwingUtilities.invokeAndWait(new Runnable() {

        public void run() {

            final JPanel btnPnl = new JPanel();
            btnPnl.add(new JButton("TEST"));

            final JFrame f = new JFrame("TEST");
            f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            f.setContentPane(btnPnl);
            // calculate preferred size for TEST frame
            // f.isDisplayable() will become true
            f.pack();

            // extended state, if can be applied, needs to be called after f.isDisplayable()
            WindowListener maxBoth = new WindowAdapter() {

                @Override
                public void windowOpened(WindowEvent e) {
                    f.setExtendedState(Frame.MAXIMIZED_BOTH);
                }
            };

            // after windows has been opened - maximize both
            f.addWindowListener(maxBoth);

            // initially hide the elements
            // after maximized state has been applied show them
            f.getContentPane().setVisible(false);
            f.addWindowListener(new WindowAdapter() {

                @Override
                public void windowActivated(WindowEvent e) {
                    f.getContentPane().setVisible(true);
                    // remove this listener
                    f.removeWindowStateListener(this);
                }
            });

            // set the frame visible
            f.setVisible(true);
        }
    });
}

}

Answer 7

The "then in some milliseconds" part sounds to me like you need to call validate() on your frame. Also, if you use f.pack(), your panel needs a preferred size, because pack() gives the parent's components their preferred sizes and resizes based on them.

Answer 8

Looks like a java bug. I've reported it (but for some reason it still not show on the bugs reports).