Thread: Is there a way to specify where on screen a JFrame or a JDialog should appear?

The title says it all.

I've got a program that uses a few JFrames and JDialogs. The main window is a JFrame and always appears in the top left corner of the screen. The same applies to all other JFrames I use. In contrast, a JDialog that I use appears in the middle of the screen (both X and Y). I think this is rather "ugly". How can I change this.

The nicest would be if I could specify that every JFrame and JDialog that are not the main window should appear in the middle of the main window JFrame (both X and Y).

If that is not possible, I would like to specify that every JFrame appears in the middle of the screen.

Your first resource to answer these kind of "How do I . . . ?" questions should always be the APIs. Review the methods, think of the possibilities as you read their descriptions, then explore how to use them in code. As you look at the API pages for JDialog, JFrame, probably all of the top-level containers, do you see any methods that control placement of the container on the screen? You should.

Thanks. That was useful advice. It was a bit of searching between all the different methods of JFrame and its friends, but I found it.

The location of a JOptionpane (which I use quite a lot) can be set in its constructor. The location of a JFrame can be set using the methods setLocation and setLocationRelativeTo.

However, I've still got a question. setLocationRelativeTo can be used to center a JFrame relative to another component (or to the screen). However, what happens is that the top-left corner of the JFrame is centered relative to the other component, not the center of the JFrame.

So I need the following code to center the center of the JFrame relative to another component:

    public class DemoFrame extends JFrame  {
 
        DemoFrame(Component c) {
            super();
            setLocationRelativeTo(c);
            // ...
            // now construct the main frame
            // ...
            setVisible(true);
            Point p = getLocation();
            int xPos = (int) p.getX() - ( getWidth() / 2 );
            int yPos = (int) p.getY() - ( getHeight() / 2 );
            setLocation(xPos, yPos);
       }
  }

This works. But it looks a bit odd. I need to set the location twice, first in order to determine where the window would be using the normal setLocationRelativeTo (i.e. centered with its top-left corner) and then to properly set it centered (i.e. centered with its center). Isn't there a more simple solution?

Then I discovered an oddity. If I call "setLocationRelativeTo(c)" followed by "p = getLocation()" directly at the beginning of the constructor, it returns different values than when I call "setLocationRelativeTo(c)" at the end (i.e. directly before the statement "Point p = getLocation()". For example, for a JFrame in the program I am writing:

at the beginning of the constructor: p.getX(): 960.0 , p.getY(): 525.0
at the end of the constructor: p.getX(): X: 875.0 , p.getY(): 465.0

Does anyone have an idea why?

Great follow-up post.

It seems the first setLocation call puts it roughly where you want it, and the second call fine tunes it or shifts it to where you really want it. That's my sense, but I haven't verified it by running variations of your code.

Well, I discovered the reason...

One can easily center a JFrame such that its center coincides with the center of the component c that creates it. Simply use the statement "setLocationRelativeTo(c)" or "setLocationRelativeTo(null)" for the first JFrame displayed.

However... if setLocationRelativeTo is called in the beginning of the JFrame code, the JFrame has not been created yet. It then has a size of 0 x 0 and its center coincides with its top-left corner. If setLocationRelativeTo is called at the end of the JFrame code, when the JFrame has been created (or at least has its final size) it has a different size and a different center. So only now its center will be correctly overlapping with the center of c.

Moral of the lesson? Only call setLocationRelativeTo once the JFrame has its final size.

Of course. One of the many tidbits buried too deep in my head to surface right away. Thanks for explaining.