start jFrame in the center of the screen

// 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);