How to add scrollbars on JPanel using null layout manager?

Question

I'm working on a class extending JDialog. I have a JPanel field named "panel" inside it, which is added to the contentPane (another JPanel), and I add the components that are intended to be displayed to "panel".

"Panel" always has the same size as the window itself. (It's practically a duplicate of contentPane.) But the window's size is different by every run, its size is counted in the constructor of the class based on the value of some specific fields that come from the program's business logic. (This size is static through one run, but when writing the code I don't know the exact numbers yet, only the method how to count it.)

This size could sometimes be very big, but I never want my window to be bigger than a specific size, e.g. (1300,800). When the window would be not larger than this size, I don't want the scrollbars to appear. When it would be larger than this on one dimension only, I only want the appropriate scrollbar to appear (vertical / horizontal). And when it's larger on both dimensions, then both scrollbars should appear.

I have read at least 50 tutorials and questions on this topic, here and on other similar forums. And I tried every idea that I found, in every different combination I could only think of. But none of them worked, and now I'm already very desperate.

It might be because neither my contentPane, nor "panel" uses a layout manager. They both use null. I read by another question that we have to set the preferred size of the component we want to be scrolled, but setPreferredSize leans on a layout manager. They there wrote that they don't really have an idea how to solve this issue, else than starting using a layout manager.

But if I start using one, it confuses the layout that I have designed, it ruins the x, y values, which I have set manually by each component. Layout is important in my exercise. It's not right if a layout manager confuses it, and I don't really like for this excercise how the different layout managers set the layout.

Could you give me any ideas on how I could make scrollbars work keeping using null layout manager?... :/

Here's my class Kimenet (Kimenet is the word for "output" in my mother tongue):

public class Kimenet extends JDialog {
    private JPanel contentPane, panel;
    private int window_width, window_height;

    public static Kimenet showDialog(...) {...}

    public Kimenet(...){
        window_width = ...; //some counting here
        window_height = ...; //some counting here
        GUI();
    }

    private void GUI(){
        setBounds(0, 0, window_width, window_height);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
        contentPane.setLayout(null);
        setContentPane(contentPane);

        panel = new JPanel();
        panel.setLayout(null);
        panel.setBounds(0, 0, window_width, window_height);
        contentPane.add(panel);

        //here is where I try to add the scrollbars in every desperate way......

        ... components that I wanna add: panel.add(component);
    }

Here's the part of the Main class from where I make an instance of Kimenet:

EventQueue.invokeLater(new Runnable() {
    public void run() {
        try {
            Kimenet dialog = Kimenet.showDialog();
            dialog.setDefaultCloseOperation(JDialog.HIDE_ON_CLOSE);
            dialog.setModal(true);
            dialog.setVisible(true);
            System.exit(0);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
});

I have tried adding the scrollbars from Main before making the dialog visible, and from Kimenet's GUI() method as well.

I have tried creating JScrollPane in many different combinations, but this mostly resulted that the scrollbars still didn't appear, but every component that I added to "panel" disappeared.

panel.setPreferredSize(new Dimension(1000, 600));
JScrollPane scrollpane = new JScrollPane(panel);
panel.setAutoscrolls(true);
scrollpane.setPreferredSize(new Dimension(800, 300));
this.add(scrollpane);

I've tried here this.add(scrollpane), contentPane.add(scrollpane), panel.add(scrollpane), scrollpane.add(contentPane) and many combinations.

I have tried creating JScrollBars separately in many different combinations too, but this mostly resulted that the scrollbars simply didn't appear (I have tried much more combinations than what I copy here, e.g. vertical and horizontal scrollbar policy.)

vertikális = new JScrollBar(JScrollBar.VERTICAL, 0, 10, 0, 100);
vertikális.setPreferredSize(new Dimension(700, 600));
contentPane.add(vertikális);

This didn't work in no way either.