### From the class which calls the gui: ###

public static void main(String[] args) {
        // TODO code application logic here
                try{gui2 mywindow = new gui2();
		mywindow.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		mywindow.setLayout(new BorderLayout());
		mywindow.setSize(800,800);
                mywindow.pack();
		mywindow.setVisible(true);
		//mywindow.setResizable(false);
		}
                catch(Exception e){}
    }

########################

### From gui2: ###

public class gui2 extends JFrame {

...

public gui2() throws IOException {
        ...
        containerSouth = new JPanel();
        containerCenter = new JPanel();
        containerEast = new JPanel();
        containerNorth = new JPanel();

        ...

        setVisible(true);

        pack();
}