Centering an application (window, frame or dialog) can be done with the following codes:

 // Get the size of the screen
Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
    
// Determine the new location of the window
int w = window.getSize().width;
int h = window.getSize().height;
int x = (dim.width-w)/2;
int y = (dim.height-h)/2;
    
// Move the window
window.setLocation(x, y);