Home » Tip Bank » Application Design » User Interface Design

Language: Java
Expertise: Intermediate

Sep 28, 1999

You can find many tips on how to center a window within the screen, but how about centering a window in its parent container? The following code shows you how to do that. Say you have a JFrame (or Frame), and you launch a JDialog (or Dialog) like this:

 
JDialog jd = new JDialog(this); 
//you have to pass the this pointer of Frame to establish
//parent-child (or owner-owned) relationship
jd.setVisible(true);

In the code for jd you can have the following to center the dialog with respect to its ownere frame:

 
public void centerInParent () 
{
    int x;
    int y;
    Container parent = this.getParent();
    Point topLeft = parent.getLocationOnScreen();
    Dimension parentSize = parent.getSize();
    Dimension ownSize = this.getSize();
    if (parentSize.width < ownSize.width) 
        {x = ((parentSize.width - ownSize.width)/2) +
 topLeft.x;}
   else 
        {x = topLeft.x;}
   if (parentSize.height < ownSize.height) 
       {y = ((parentSize.height - ownSize.height)/2) + 
topLeft.y;}
   else 
       {y = topLeft.y;}
    this.setLocation (x, y);
    this.requestFocus();
}  

For the sake of completeness, the following code centers a Dialog or Frame within the screen:

 
public void centerInScreen() 
{
    Dimension dim = this.getToolkit().getScreenSize();
    Rectangle bounds = this.getBounds();
    this.setLocation((dim.width - bounds.width) / 2, 
(dim.height -
bounds.height) / 2);
    this.requestFocus();
}